Centrum voor Wiskunde en Informatica

REPORTRAPPORT

Python reference manual
G. van Rossum
Computer Science/Department of Algorithmics and Architecture

CS-R9525 1995

Report CS-R9525
ISSN 0169-118X
CWI
P.O. Box 94079
1090 GB Amsterdam
The Netherlands

CWI is the National Research Institute for Mathematics
and Computer Science. CWI is part of the Stichting
Mathematisch Centrum (SMC), the Dutch foundation
for promotion of mathematics and computer scienceand their applications.
SMC is sponsored by the Netherlands Organization for
Scientific Research (NWO). CWI is a member of
ERCIM, the European Research Consortium for
Informatics and Mathematics.

Copyright © Stichting Mathematisch Centrum
P.O. Box 94079, 1090 GB Amsterdam (NL)
Kruislaan 413, 1098 SJ Amsterdam (NL)
Telephone +31 20 592 9333
Telefax +31 20 592 4199

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Contents
1

Introduction
1.1 Notation \]\ \ \0\ \]\ \ \]\ \ \0\ \ \]\ \ \0\ \ \]\ \]\ \ \0\ \ \]\ \ \0\ \ \]\ \ \]\1
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Lexical analysis
2.1 Line structure \0\ \]\ \ \]\2.1.1 Comments \]\ \ \]\2.1.2 Explicit line joining
2.1.3 Implicit line joining
2.1.4 Blank lines \]\ \ \]\2.1.5 Indentation \]\ \ \]\2.2 Other tokens \ \0\ \]\ \ \]\2.3 Identifiers \ \ \0\ \]\ \ \]\2.3.1 Keywords \]\ \ \]\2.4 Literals \ \]\ \ \0\ \]\ \ \]\2.4.1 String literals \ \]\2.4.2 Numeric literals \]\2.5 Operators \ \ \0\ \]\ \ \]\2.6 Delimiters \ \ \0\ \]\ \ \]\

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3.1 Objects, values and types \ \ \0\ \ \]\ \ \0\ \ \]\ \]\ \ \0\3.2 The standard type hierarchy \ \0\ \ \]\ \ \0\ \ \]\ \]\ \ \0\
3.3 Special method names \ \]\ \ \0\ \ \]\ \ \0\ \ \]\ \]\ \ \0\3.3.1 Special methods for any type \ \ \0\ \ \]\ \]\ \ \0\3.3.2 Special methods for attribute access \ \ \]\ \]\ \ \0\3.3.3 Special methods for sequence and mapping types \0\3.3.4 Special methods for sequence types \ \ \]\ \]\ \ \0\3.3.5 Special methods for numeric types \0\ \ \]\ \]\ \ \0\

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Execution model
4.1 Code blocks, execution frames, and name spaces \ ]\ \ \ \0\ \ \]\ \ \0\ \ \]\ \ \]\\ \ \ ]
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Expressions and conditions
5.1 Arithmetic conversions \ \]\ \ 0\ \ \ ]
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5.2.2 Literals \0\ \]\ \ \]\ \ \0\ \5.2.3 Parenthesized forms \ \0\ \5.2.4 List displays \ \ \]\ \ \0\ \5.2.5 Dictionary displays \ \0\ \5.2.6 String conversions \ \ \0\ \5.3 Primaries \]\ \ \0\ \]\ \ \]\ \ \0\ \5.3.1 Attribute references \ \0\ \5.3.2 Subscriptions \ \]\ \ \0\ \5.3.3 Slicings \ \]\ \ \]\ \ \0\ \5.3.4 Calls \ \0\ \]\ \ \]\ \ \0\ \5.4 Unary arithmetic operations \ \0\ \5.5 Binary arithmetic operations \ \0\ \5.6 Shifting operations \]\ \ \]\ \ \0\ \5.7 Binary bit-wise operations \ \ \0\ \5.8 Comparisons \ \0\ \]\ \ \]\ \ \0\ \5.9 Boolean operations \]\ \ \]\ \ \0\ \5.10 Expression lists and condition lists \5.11 Summary \]\ \ \0\ \]\ \ \]\ \ \0\ \
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Simple statements
6.1 Expression statements \ \]\6.2 Assignment statements \ \]\6.3 The pass statement \ \ \]\6.4 The del statement \]\ \ \]\6.5 The print statement \ \]\6.6 The return statement \]\6.7 The raise statement \ \]\6.8 The break statement \ \]\6.9 The continue statement \6.10 The import statement \]\6.11 The global statement \]\6.12 The access statement \]\6.13 The exec statement \ \ \]\

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Compound statements
7.1 The if statement \]\7.2 The while statement
7.3 The for statement \]\7.4 The try statement \]\7.5 Function definitions \7.6 Class definitions \ \]\

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\ \0\
\ \0\
\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\
\ \]\
\ \]\

\]\
\ \
]
\ \
]
\ \
]
\ \
]
\ \
]

\ \0\
\ \0\
\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\
\ \]\
\ \]\

\ \0\
\ \0\
\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\
\ \]\
\ \]\

\ \]\
\ \]\
\ \]\
\ \]\
\ \]\
\ \]\

38
39
39
39
40
41
43

Top-level components
8.1 Complete Python programs
8.2 File input \]\ \ \0\ \]\ \ \]\8.3 Interactive input \ \]\ \ \]\8.4 Expression input \ \]\ \ \]\

\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\

\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\

\]\
\ \
]
\ \
]
\ \
]

\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\

\ \0\
\ \0\
\ \0\
\ \0\

\ \]\
\ \]\
\ \]\
\ \]\

\ \]\
\ \]\
\ \]\
\ \]\

44
44
44
45
45

ii

Chapter 1

Introduction
This reference manual describes the Python programming language. It is not intended as a tutorial.
While I am trying to be as precise as possible, I chose to use English rather than formal specifications foreverything except syntax and lexical analysis. This should make the document more understandable
to the average reader, but will leave room for ambiguities. Consequently, if you were coming from
Mars and tried to re-implement Python from this document alone, you might have to guess things andin fact you would probably end up implementing quite a different language. On the other hand, if you
are using Python and wonder what the precise rules about a particular area of the language are, you
should definitely be able to find them here.
It is dangerous to add too many implementation details to a language reference document — theimplementation may change, and other implementations of the same language may work differently.
On the other hand, there is currently only one Python implementation, and its particular quirks
are sometimes worth being mentioned, especially where the implementation imposes additional
limitations. Therefore, you’ll find short “implementation notes” sprinkled throughout the text.
Every Python implementation comes with a number of built-in and standard modules. These are notdocumented here, but in the separate Python Library Reference document. A few built-in modules
are mentioned when they interact in a significant way with the language definition.

1.1 Notation
The descriptions of lexical analysis and syntax use a modified BNF grammar notation. This uses thefollowing style of definition:
name:lc_letter:lc_letter (lc_letter | "_")*"a"..."z"The first line says that a name is an lc_letter followed by a sequence of zero or more
lc_letters and underscores. An lc_letter in turn is any of the single characters ‘a’ through‘z’. (This rule is actually adhered to for the names defined in lexical and grammar rules in thisdocument.)1

Each rule begins with a name (which is the name defined by the rule) and a colon. A vertical bar(|) is used to separate alternatives; it is the least binding operator in this notation. A star (*) means
zero or more repetitions of the preceding item; likewise, a plus (+) means one or more repetitions,and a phrase enclosed in square brackets ([ ]) means zero or one occurrences (in other words, theenclosed phrase is optional). The * and + operators bind as tightly as possible; parentheses are used
for grouping. Literal strings are enclosed in quotes. White space is only meaningful to separate
tokens. Rules are normally contained on a single line; rules with many alternatives may be formatted
alternatively with each line after the first beginning with a vertical bar.
In lexical definitions (as the example above), two more conventions are used: Two literal charactersseparated by three dots mean a choice of any single character in the given (inclusive) range of ASCII
characters. A phrase between angular brackets (<...>) gives an informal description of the symbol
defined; e.g. this could be used to describe the notion of ‘control character’ if needed.
Even though the notation used is almost the same, there is a big difference between the meaning oflexical and syntactic definitions: a lexical definition operates on the individual characters of the input
source, while a syntax definition operates on the stream of tokens generated by the lexical analysis.
All uses of BNF in the next chapter (“Lexical Analysis”) are lexical definitions; uses in subsequent
chapters are syntactic definitions.

2Chapter 2

Lexical analysis
A Python program is read by a parser. Input to the parser is a stream of tokens, generated by thelexical analyzer. This chapter describes how the lexical analyzer breaks a file into tokens.

2.1 Line structure
A Python program is divided in a number of logical lines. The end of a logical line is represented
by the token NEWLINE. Statements cannot cross logical line boundaries except where NEWLINE isallowed by the syntax (e.g. between statements in compound statements).

2.1.1

Comments

A comment starts with a hash character (#) that is not part of a string literal, and ends at the end of
the physical line. A comment always signifies the end of the logical line. Comments are ignored by
the syntax.

2.1.2

Explicit line joining

Two or more physical lines may be joined into logical lines using backslash characters (\), as follows:when a physical line ends in a backslash that is not part of a string literal or comment, it is joinedwith the following forming a single logical line, deleting the backslash and the following end-of-line
character. For example:
if 1900 < year < 2100 and 1 <= month <= 12 \and 1 <= day <= 31 and 0 <= hour < 24 \and 0 <= minute < 60 and 0 <= second < 60:
return 1# Looks like a valid date

A line ending in a backslash cannot carry a comment; a backslash does not continue a comment (butit does continue a string literal, see below).

32.1.3

Implicit line joining

Expressions in parentheses, square brackets or curly braces can be split over more than one physicalline without using backslashes. For example:
month_names = [’Januari’, ’Februari’, ’Maart’,’April’,’Mei’,’Juni’,’Juli’,’Augustus’, ’September’,’Oktober’, ’November’, ’December’]# These are the# Dutch names# for the months# of the year

Implicitly continued lines can carry comments. The indentation of the continuation lines is notimportant. Blank continuation lines are allowed.

2.1.4

Blank lines

A logical line that contains only spaces, tabs, and possibly a comment, is ignored (i.e., no NEWLINE
token is generated), except that during interactive input of statements, an entirely blank logical line
terminates a multi-line statement.

2.1.5

Indentation

Leading whitespace (spaces and tabs) at the beginning of a logical line is used to compute theindentation level of the line, which in turn is used to determine the grouping of statements.
First, tabs are replaced (from left to right) by one to eight spaces such that the total number ofcharacters up to there is a multiple of eight (this is intended to be the same rule as used by UNIX). The
total number of spaces preceding the first non-blank character then determines the line’s indentation.
Indentation cannot be split over multiple physical lines using backslashes.
The indentation levels of consecutive lines are used to generate INDENT and DEDENT tokens, using
a stack, as follows.
Before the first line of the file is read, a single zero is pushed on the stack; this will never be popped
off again. The numbers pushed on the stack will always be strictly increasing from bottom to top.
At the beginning of each logical line, the line’s indentation level is compared to the top of the stack.
If it is equal, nothing happens. If it is larger, it is pushed on the stack, and one INDENT token is
generated. If it is smaller, it must be one of the numbers occurring on the stack; all numbers on thestack that are larger are popped off, and for each number popped off a DEDENT token is generated.
At the end of the file, a DEDENT token is generated for each number remaining on the stack that islarger than zero.
Here is an example of a correctly (though confusingly) indented piece of Python code:def perm(l):# Compute the list of all permutations of l
if len(l) <= 1:4

return [l]r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)for x in p:
r.append(l[i:i+1] + x)
return r
The following example shows various indentation errors:

def perm(l):# error: first line indented
for i in range(len(l)):# error: not indenteds = l[:i] + l[i+1:]
p = perm(l[:i] + l[i+1:])# error: unexpected indent
for x in p:
r.append(l[i:i+1] + x)
return r# error: inconsistent dedent(Actually, the first three errors are detected by the parser; only the last error is found by the lexical
analyzer — the indentation of return r does not match a level popped off the stack.)

2.2 Other tokens
Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist: identifiers,
keywords, literals, operators, and delimiters. Spaces and tabs are not tokens, but serve to delimit
tokens. Where ambiguity exists, a token comprises the longest possible string that forms a legal
token, when read from left to right.

2.3 Identifiers
Identifiers (also referred to as names) are described by the following lexical definitions:
identifier:letter:lowercase:uppercase:digit:(letter|"_") (letter|digit|"_")*
lowercase | uppercase"a"..."z""A"..."Z""0"..."9"Identifiers are unlimited in length. Case is significant.

52.3.1

Keywords

The following identifiers are used as reserved words, or keywords of the language, and cannot be usedas ordinary identifiers. They must be spelled exactly as written here:
access
and
break
class
continue
def

del
elif
else
exceptfinallyforfromglobalif
importinislambda
not
or
pass
print
raise

returntry
while

2.4 Literals
Literals are notations for constant values of some built-in types.

2.4.1

String literals

String literals are described by the following lexical definitions:stringliteral:
shortstring | longstring
shortstring:"’" shortstringitem* "’" | ’"’ shortstringitem* ’"’longstring:"’’’" longstringitem* "’’’" | ’"""’ longstringitem* ’"""’
shortstringitem: shortstringchar | escapeseq
shortstringchar: <any ASCII character except "\" or newline or the quote>longstringchar: <any ASCII character except "\">
escapeseq:"\" <any ASCII character>In “long strings” (strings surrounded by sets of three quotes), unescaped newlines and quotes areallowed (and are retained), except that three unescaped quotes in a row terminate the string. (A“quote” is the character used to open the string, i.e. either ’ or ".)Escape sequences in strings are interpreted according to rules similar to those used by Standard C.
The recognized escape sequences are:6

\newline
\\
\’
\"
\a
\b
\f
\n
\r
\t
\v
\ooo
\xxx...

IgnoredBackslash (\)
Single quote (’)
Double quote (")
ASCII Bell (BEL)
ASCII Backspace (BS)ASCII Formfeed (FF)ASCII Linefeed (LF)
ASCII Carriage Return (CR)ASCII Horizontal Tab (TAB)
ASCII Vertical Tab (VT)ASCII character with octal value ooo
ASCII character with hex value xx...

In strict compatibility with Standard C, up to three octal digits are accepted, but an unlimited numberof hex digits is taken to be part of the hex escape (and then the lower 8 bits of the resulting hex number
are used in all current implementations...).
All unrecognized escape sequences are left in the string unchanged, i.e., the backslash is left in thestring. (This behavior is useful when debugging: if an escape sequence is mistyped, the resulting
output is more easily recognized as broken. It also helps a great deal for string literals used as regularexpressions or otherwise passed to other modules that do their own escape handling.)

2.4.2

Numeric literals

There are three types of numeric literals: plain integers, long integers, and floating point numbers.
Integer and long integer literals are described by the following lexical definitions:longinteger:
integer ("l"|"L")
integer:decimalinteger | octinteger | hexinteger
decimalinteger: nonzerodigit digit* | "0"octinteger:"0" octdigit+
hexinteger:"0" ("x"|"X") hexdigit+
nonzerodigit:
octdigit:
hexdigit:"1"..."9""0"..."7"
digit|"a"..."f"|"A"..."F"

Although both lower case ‘l’ and upper case ‘L’ are allowed as suffix for long integers, it is strongly
recommended to always use ‘L’, since the letter ‘l’ looks too much like the digit ‘1’.
Plain integer decimal literals must be at most 2147483647 (i.e., the largest positive integer, using
32-bit arithmetic). Plain octal and hexadecimal literals may be as large as 4294967295, but values
larger than 2147483647 are converted to a negative value by subtracting 4294967296. There is nolimit for long integer literals apart from what can be stored in available memory.
Some examples of plain and long integer literals:
7

7
3L

2147483647
79228162514264337593543950336L

0177
0377L

0x80000000
0x100000000L

Floating point literals are described by the following lexical definitions:floatnumber:pointfloat:
exponentfloat:
intpart:fraction:
exponent:

pointfloat | exponentfloat[intpart] fraction | intpart "."(intpart | pointfloat) exponent
digit+"." digit+("e"|"E") ["+"|"-"] digit+

The allowed range of floating point literals is implementation-dependent.
Some examples of floating point literals:
3.14

10.

.001

1e100

3.14e-10

Note that numeric literals do not include a sign; a phrase like -1 is actually an expression composedof the operator - and the literal 1.

2.5 Operators
The following tokens are operators:
+
<<
<

>>
==

*
&
>

/
|
<=

%
ˆ
<>

˜
!=

>=

The comparison operators <> and != are alternate spellings of the same operator.

2.6 Delimiters
The following tokens serve as delimiters or otherwise have a special meaning:
(
,
= ;

)
:

[
.

]
"

{
‘

}
’

The following printing ASCII characters are not used in Python. Their occurrence outside string literals
and comments is an unconditional error:@$?

They may be used by future versions of the language though!
8

Chapter 3

Data model
3.1 Objects, values and types
Objects are Python’s abstraction for data. All data in a Python program is represented by objects orby relations between objects. (In a sense, and in conformance to Von Neumann’s model of a “stored
program computer”, code is also represented by objects.)
Every object has an identity, a type and a value. An object’s identity never changes once it has beencreated; you may think of it as the object’s address in memory. An object’s type is also unchangeable.
It determines the operations that an object supports (e.g. “does it have a length?”) and also defines
the possible values for objects of that type. The value of some objects can change. Objects whose
value can change are said to be mutable; objects whose value is unchangeable once they are createdare called immutable. The type determines an object’s (im)mutability.
Objects are never explicitly destroyed; however, when they become unreachable they may be garbagecollected. An implementation is allowed to delay garbage collection or omit it altogether — it is amatter of implementation quality how garbage collection is implemented, as long as no objects arecollected that are still reachable. (Implementation note: the current implementation uses a referencecounting scheme which collects most objects as soon as they become unreachable, but never collects
garbage containing circular references.)
Note that the use of the implementation’s tracing or debugging facilities may keep objects alive thatwould normally be collectable.
Some objects contain references to “external” resources such as open files or windows. It is understoodthat these resources are freed when the object is garbage-collected, but since garbage collection is
not guaranteed to happen, such objects also provide an explicit way to release the external resource,
usually a close method. Programs are strongly recommended to always explicitly close such objects.
Some objects contain references to other objects; these are called containers. Examples of containers
are tuples, lists and dictionaries. The references are part of a container’s value. In most cases, when
we talk about the value of a container, we imply the values, not the identities of the contained objects;
however, when we talk about the (im)mutability of a container, only the identities of the immediately
contained objects are implied. (So, if an immutable container contains a reference to a mutable object,its value changes if that mutable object is changed.)9Types affect almost all aspects of objects’ lives. Even the meaning of object identity is affected insome sense: for immutable types, operations that compute new values may actually return a reference
to any existing object with the same type and value, while for mutable objects this is not allowed. E.g.
after
a = 1; b = 1; c = []; d = []a and b may or may not refer to the same object with the value one, depending on the implementation,
but c and d are guaranteed to refer to two different, unique, newly created empty lists.

3.2 The standard type hierarchy
Below is a list of the types that are built into Python. Extension modules written in C can define
additional types. Future versions of Python may add types to the type hierarchy (e.g. rational orcomplex numbers, efficiently stored arrays of integers, etc.).
Some of the type descriptions below contain a paragraph listing ‘special attributes’. These are
attributes that provide access to the implementation and are not intended for general use. Theirdefinition may change in the future. There are also some ‘generic’ special attributes, not listed with
the individual objects: __methods__ is a list of the method names of a built-in object, if it has any;__members__ is a list of the data attribute names of a built-in object, if it has any.
None This type has a single value. There is a single object with this value. This object is accessed
through the built-in name None. It is returned from functions that don’t explicitly return an
object.
Numbers These are created by numeric literals and returned as results by arithmetic operators andarithmetic built-in functions. Numeric objects are immutable; once created their value neverchanges. Python numbers are of course strongly related to mathematical numbers, but subject
to the limitations of numerical representation in computers.
Python distinguishes between integers and floating point numbers:
Integers These represent elements from the mathematical set of whole numbers.
There are two types of integers:
Plain integers These represent numbers in the range -2147483648 through 2147483647.
(The range may be larger on machines with a larger natural word size, but not
smaller.) When the result of an operation falls outside this range, the exception
OverflowError is raised. For the purpose of shift and mask operations, integers
are assumed to have a binary, 2’s complement notation using 32 or more bits, andhiding no bits from the user (i.e., all 4294967296 different bit patterns correspond todifferent values).
Long integers These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2’s complement
which gives the illusion of an infinite string of sign bits extending to the left.
10The rules for integer representation are intended to give the most meaningful interpretationof shift and mask operations involving negative integers and the least surprises whenswitching between the plain and long integer domains. For any operation except left shift,
if it yields a result in the plain integer domain without causing overflow, it will yield thesame result in the long integer domain or when using mixed operands.
Floating point numbers These represent machine-level double precision floating point numbers. You are at the mercy of the underlying machine architecture and C implementation
for the accepted range and handling of overflow.
Sequences These represent finite ordered sets indexed by natural numbers. The built-in function
len() returns the number of elements of a sequence. When this number is n, the index set
contains the numbers 0, 1, \)\\ , n-1. Element i of sequence a is selected by a[i].
Sequences also support slicing: a[i:j] selects all elements with index k such that i <= k <j. When used as an expression, a slice is a sequence of the same type — this implies that theindex set is renumbered so that it starts at 0 again.
Sequences are distinguished according to their mutability:
Immutable sequences An object of an immutable sequence type cannot change once it iscreated. (If the object contains references to other objects, these other objects may bemutable and may be changed; however the collection of objects directly referenced by an
immutable object cannot change.)
The following types are immutable sequences:
Strings The elements of a string are characters. There is no separate character type; acharacter is represented by a string of one element. Characters represent (at least)
8-bit bytes. The built-in functions chr() and ord() convert between charactersand nonnegative integers representing the byte values. Bytes with the values 0-127
represent the corresponding ASCII values. The string data type is also used to represent
arrays of bytes, e.g. to hold data read from a file.
(On systems whose native character set is not ASCII, strings may use EBCDIC intheir internal representation, provided the functions chr() and ord() implementa mapping between ASCII and EBCDIC, and string comparison preserves the ASCII
order. Or perhaps someone can propose a better rule?)
Tuples The elements of a tuple are arbitrary Python objects. Tuples of two or moreelements are formed by comma-separated lists of expressions. A tuple of one element(a ‘singleton’) can be formed by affixing a comma to an expression (an expression
by itself does not create a tuple, since parentheses must be usable for grouping of
expressions). An empty tuple can be formed by enclosing ‘nothing’ in parentheses.
Mutable sequences Mutable sequences can be changed after they are created. The subscription
and slicing notations can be used as the target of assignment and del (delete) statements.
There is currently a single mutable sequence type:
Lists The elements of a list are arbitrary Python objects. Lists are formed by placing
a comma-separated list of expressions in square brackets. (Note that there are no
special cases needed to form lists of length 0 or 1.)
11

Mapping types These represent finite sets of objects indexed by arbitrary index sets. The subscript
notation a[k] selects the element indexed by k from the mapping a; this can be used in
expressions and as the target of assignments or del statements. The built-in function len()returns the number of elements in a mapping.
There is currently a single mapping type:
Dictionaries These represent finite sets of objects indexed by almost arbitrary values. The
only types of values not acceptable as keys are values containing lists or dictionaries orother mutable types that are compared by value rather than by object identity — the reason
being that the implementation requires that a key’s hash value be constant. Numeric types
used for keys obey the normal rules for numeric comparison: if two numbers compare
equal (e.g. 1 and 1.0) then they can be used interchangeably to index the same dictionary
entry.
Dictionaries are mutable; they are created by the {...} notation (see section 5.2.5).
Callable types These are the types to which the function call (invocation) operation, written asfunction(argument, argument, ...), can be applied:
User-defined functions A user-defined function object is created by a function definition (seesection 7.5). It should be called with an argument list containing the same number of
items as the function’s formal parameter list.
Special read-only attributes: func_code is the code object representing the compiled
function body, and func_globals is (a reference to) the dictionary that holds thefunction’s global variables — it implements the global name space of the module in which
the function was defined.
User-defined methods A user-defined method (a.k.a. object closure) is a pair of a classinstance object and a user-defined function. It should be called with an argument list
containing one item less than the number of items in the function’s formal parameter list.
When called, the class instance becomes the first argument, and the call arguments are
shifted one to the right.
Special read-only attributes: im_self is the class instance object, im_func is thefunction object.
Built-in functions A built-in function object is a wrapper around a C function. Examples ofbuilt-in functions are len and math.sin. There are no special attributes. The number
and type of the arguments are determined by the C function.
Built-in methods This is really a different disguise of a built-in function, this time containing
an object passed to the C function as an implicit extra argument. An example of a built-in
method is list.append if list is a list object.
Classes Class objects are described below. When a class object is called as a function, a newclass instance (also described below) is created and returned. This implies a call to theclass’s __init__ method if it has one. Any arguments are passed on to the __init__
method — if there is __init__ method, the class must be called without arguments.
Modules Modules are imported by the import statement (see section 6.10). A module object is acontainer for a module’s name space, which is a dictionary (the same dictionary as referenced by
the func_globals attribute of functions defined in the module). Module attribute references12are translated to lookups in this dictionary. A module object does not contain the code object
used to initialize the module (since it isn’t needed once the initialization is done).
Attribute assignment update the module’s name space dictionary.
Special read-only attributes: __dict__ yields the module’s name space as a dictionary object;__name__ yields the module’s name as a string object.
Classes Class objects are created by class definitions (see section 7.6). A class is a container for adictionary containing the class’s name space. Class attribute references are translated to lookups
in this dictionary. When an attribute name is not found there, the attribute search continues inthe base classes. The search is depth-first, left-to-right in the order of their occurrence in thebase class list.
Class attribute assignments update the class’s dictionary, never the dictionary of a base class.
A class can be called as a function to yield a class instance (see above).
Special read-only attributes: __dict__ yields the dictionary containing the class’s namespace; __bases__ yields a tuple (possibly empty or a singleton) containing the base classes,
in the order of their occurrence in the base class list.
Class instances A class instance is created by calling a class object as a function. A class instancehas a dictionary in which attribute references are searched. When an attribute is not found there,
and the instance’s class has an attribute by that name, and that class attribute is a user-defined
function (and in no other cases), the instance attribute reference yields a user-defined method
object (see above) constructed from the instance and the function.
Attribute assignments update the instance’s dictionary.
Class instances can pretend to be numbers, sequences, or mappings if they have methods withcertain special names. These are described in section 3.3.
Special read-only attributes: __dict__ yields the attribute dictionary; __class__ yields
the instance’s class.
Files A file object represents an open file. (It is a wrapper around a C stdio file pointer.) Fileobjects are created by the open() built-in function, and also by posix.popen() and the
makefile method of socket objects. sys.stdin, sys.stdout and sys.stderr are
file objects corresponding to the interpreter’s standard input, output and error streams. See thePython Library Reference for methods of file objects and other details.
Internal types A few types used internally by the interpreter are exposed to the user. Their definition
may change with future versions of the interpreter, but they are mentioned here for completeness.

Code objects Code objects represent “pseudo-compiled” executable Python code. The difference between a code object and a function object is that the function object contains an
explicit reference to the function’s context (the module in which it was defined) while acode object contains no context.
Special read-only attributes: co_code is a string representing the sequence of instructions; co_consts is a list of literals used by the code; co_names is a list of names(strings) used by the code; co_filename is the filename from which the code was13compiled. (To find out the line numbers, you would have to decode the instructions; thestandard library module dis contains an example of how to do this.)Frame objects Frame objects represent execution frames. They may occur in traceback objects(see below).
Special read-only attributes: f_back is to the previous stack frame (towards the caller),or None if this is the bottom stack frame; f_code is the code object being executed inthis frame; f_globals is the dictionary used to look up global variables; f_locals
is used for local variables; f_lineno gives the line number and f_lasti gives theprecise instruction (this is an index into the instruction string of the code object).
Traceback objects Traceback objects represent a stack trace of an exception. A traceback
object is created when an exception occurs. When the search for an exception handler
unwinds the execution stack, at each unwound level a traceback object is inserted in frontof the current traceback. When an exception handler is entered (see also section 7.4),
the stack trace is made available to the program as sys.exc_traceback. When the
program contains no suitable handler, the stack trace is written (nicely formatted) to thestandard error stream; if the interpreter is interactive, it is also made available to the user
as sys.last_traceback.
Special read-only attributes: tb_next is the next level in the stack trace (towards theframe where the exception occurred), or None if there is no next level; tb_frame points
to the execution frame of the current level; tb_lineno gives the line number where theexception occurred; tb_lasti indicates the precise instruction. The line number andlast instruction in the traceback may differ from the line number of its frame object if
the exception occurred in a try statement with no matching except clause or with afinally clause.

3.3 Special method names
A class can implement certain operations that are invoked by special syntax (such as subscription
or arithmetic operations) by defining methods with special names. For instance, if a class defines
a method named __getitem__, and x is an instance of this class, then x[i] is equivalent to
x.__getitem__(i). (The reverse is not true — if x is a list object, x.__getitem__(i) is
not equivalent to x[i].)Except for __repr__, __str__ and __cmp__, attempts to execute an operation raise an exception
when no appropriate method is defined. For __repr__, the default is to return a string describing theobject’s class and address. For __cmp__, the default is to compare instances based on their address.
For __str__, the default is to use __repr__.

3.3.1

Special methods for any type__init__(self, args...) Called when the instance is created. The arguments are thosepassed to the class constructor expression. If a base class has an __init__ method thederived class’s __init__ method must explicitly call it to ensure proper initialization of thebase class part of the instance.

14

__del__(self) Called when the instance is about to be destroyed. If a base class has an __del__method the derived class’s __del__ method must explicitly call it to ensure proper deletion of
the base class part of the instance. Note that it is possible for the __del__ method to postpone
destruction of the instance by creating a new reference to it. It may then be called at a later time
when this new reference is deleted. It is not guaranteed that __del__ methods are called for
objects that still exist when the interpreter exits.
Note that del x doesn’t directly call x.__del__ — the former decrements the reference
count for x by one, but x,__del__ is only called when its reference count reaches zero.
__repr__(self) Called by the repr() built-in function and by string conversions (reverse or
backward quotes) to compute the string representation of an object.
__str__(self) Called by the str() built-in function and by the print statement compute thestring representation of an object.
__cmp__(self, other) Called by all comparison operations.
Should return -1 if
self < other, 0 if self == other, +1 if self > other. If no __cmp__ operation is defined, class instances are compared by object identity (“address”). (Implementation
note: due to limitations in the interpreter, exceptions raised by comparisons are ignored, andthe objects will be considered equal in this case.)__hash__(self) Called for the key object for dictionary operations, and by the built-in functionhash(). Should return a 32-bit integer usable as a hash value for dictionary operations. The
only required property is that objects which compare equal have the same hash value; it isadvised to somehow mix together (e.g. using exclusing or) the hash values for the componentsof the object that also play a part in comparison of objects. If a class does not define a__cmp__ method it should not define a __hash__ operation either; if it defines __cmp__
but not __hash__ its instances will not be usable as dictionary keys. If a class defines mutableobjects and implements a __cmp__ method it should not implement __hash__, since thedictionary implementation assumes that a key’s hash value is a constant.
__call__(self, *args) Called when the instance is “called” as a function.

3.3.2

Special methods for attribute access

The following methods can be used to change the meaning of attribute access for class instances.
__getattr__(self, name) Called when an attribute lookup has not found the attribute in theusual places (i.e. it is not an instance attribute nor is it found in the class tree for self). nameis the attribute name.
Note that if the attribute is found through the normal mechanism, __getattr__ is not called.
(This is an asymmetry between __getattr__ and __setattr__.) This is done both for
efficiency reasons and because otherwise __getattr__ would have no way to access otherattributes of the instance. Note that at least for instance variables, __getattr__ can faketotal control by simply not inserting any values in the instance attribute dictionary.

15

__setattr__(self, name, value) Called when an attribute assignment is attempted. Thisis called instead of the normal mechanism (i.e. store the value as an instance attribute). nameis the attribute name, value is the value to be assigned to it.
If __setattr__ wants to assign to an instance attribute, it should not simply executeself.name = value — this would cause a recursive call. Instead, it should insert thevalue in the dictionary of instance attributes, e.g. self.__dict__[name] = value.
__delattr__(self, name) Like __setattr__ but for attribute deletion instead of assignment.

3.3.3

Special methods for sequence and mapping types__len__(self) Called to implement the built-in function len(). Should return the length ofthe object, an integer >= 0. Also, an object whose __len__() method returns 0 is considered
to be false in a Boolean context.
__getitem__(self, key) Called to implement evaluation of self[key]. Note that the
special interpretation of negative keys (if the class wishes to emulate a sequence type) is up to
the __getitem__ method.
__setitem__(self, key, value) Called to implement assignment to self[key]. Same
note as for __getitem__.
__delitem__(self, key) Called to implement deletion of self[key]. Same note as for__getitem__.

3.3.4

Special methods for sequence types__getslice__(self, i, j) Called to implement evaluation of self[i:j]. Note thatmissing i or j are replaced by 0 or len(self), respectively, and len(self) has been
added (once) to originally negative i or j by the time this function is called (unlike for__getitem__).
__setslice__(self, i, j, sequence) Called
self[i:j]. Same notes as for __getslice__.

toimplement

assignment

to__delslice__(self, i, j) Called to implement deletion of self[i:j]. Same notes asfor __getslice__.

3.3.5

Special methods for numeric types__add__(self, other)__sub__(self, other)__mul__(self, other)__div__(self, other)__mod__(self, other)__divmod__(self, other)__pow__(self, other)16

__lshift__(self, other)__rshift__(self, other)__and__(self, other)__xor__(self, other)__or__(self, other)Called to implement the binary arithmetic operations (+, -, *, /, %, divmod(), pow(), <<,>>, &, ˆ, |).
__neg__(self)__pos__(self)__abs__(self)__invert__(self)
Called to implement the unary arithmetic operations (-, +, abs() and ˜).
__nonzero__(self) Called to implement boolean testing; should return 0 or 1. An alternative
name for this method is __len__.
__coerce__(self, other) Called to implement “mixed-mode” numeric arithmetic. Should
either return a tuple containing self and other converted to a common numeric type, or None
if no way of conversion is known. When the common type would be the type of other, it issufficient to return None, since the interpreter will also ask the other object to attempt a coercion(but sometimes, if the implementation of the other type cannot be changed, it is useful to do theconversion to the other type here).
Note that this method is not called to coerce the arguments to + and *, because these are also used
to implement sequence concatenation and repetition, respectively. Also note that, for the same
reason, in n*x, where n is a built-in number and x is an instance, a call to x.__mul__(n) is
made.1__int__(self)__long__(self)__float__(self)
Called to implement the built-in functions int(), long() and float(). Should return avalue of the appropriate type.
__oct__(self)__hex__(self)
Called to implement the built-in functions oct() and hex(). Should return a string value.

1

The interpreter should really distinguish between user-defined classes implementing sequences, mappings or numbers,
but currently it doesn’t — hence this strange exception.

17

Chapter 4

Execution model
4.1 Code blocks, execution frames, and name spaces
A code block is a piece of Python program text that can be executed as a unit, such as a module, aclass definition or a function body. Some code blocks (like modules) are executed only once, others(like function bodies) may be executed many times. Code blocks may textually contain other code
blocks. Code blocks may invoke other code blocks (that may or may not be textually contained inthem) as part of their execution, e.g. by invoking (calling) a function.
The following are code blocks: A module is a code block. A function body is a code block. A classdefinition is a code block. Each command typed interactively is a separate code block; a script file isa code block. The string argument passed to the built-in function eval and to the exec statement
are code blocks. And finally, the expression read and evaluated by the built-in function input is acode block.
A code block is executed in an execution frame. An execution frame contains some administrative
information (used for debugging), determines where and how execution continues after the code
block’s execution has completed, and (perhaps most importantly) defines two name spaces, the local
and the global name space, that affect execution of the code block.
A name space is a mapping from names (identifiers) to objects. A particular name space may bereferenced by more than one execution frame, and from other places as well. Adding a name to a namespace is called binding a name (to an object); changing the mapping of a name is called rebinding;removing a name is unbinding. Name spaces are functionally equivalent to dictionaries.
The local name space of an execution frame determines the default place where names are definedand searched. The global name space determines the place where names listed in global statements
are defined and searched, and where names that are not explicitly bound in the current code block aresearched.
Whether a name is local or global in a code block is determined by static inspection of the source
text for the code block: in the absence of global statements, a name that is bound anywhere in thecode block is local in the entire code block; all other names are considered global. The global
statement forces global interpretation of selected names throughout the code block. The following
constructs bind names: formal parameters, import statements, class and function definitions (these18bind the class or function name), and targets that are identifiers if occurring in an assignment, forloop header, or except clause header.
A target occurring in a del statement is also considered bound for this purpose (though the actual
semantics are to “unbind” the name).
When a global name is not found in the global name space, it is searched in the list of “built-in” names(which is actually the global name space of the module __builtin__). When a name is not foundat all, the NameError exception is raised.1
The following table lists the meaning of the local and global name space for various types of code
blocks. The name space for a particular module is automatically created when the module is firstreferenced. Note that in almost all cases, the global name space is the name space of the containing
module — scopes in Python do not nest!

Code block type
ModuleScriptInteractive command
Class definition
Function body
String passed to exec statement
String passed to eval()
File read by execfile()
Expression read by input

Global name spacen.s. for this modulen.s. for __main__n.s. for __main__global n.s. of containing blockglobal n.s. of containing blockglobal n.s. of cobtaining blockglobal n.s. of caller
global n.s. of caller
global n.s. of caller

Local name spacesame as global
same as global
same as global
new n.s.
new n.s.
local n.s. of containing block
local n.s. of caller
local n.s. of caller
local n.s. of caller

Notes:
n.s. means name space(1) The global and local name space for these can be overridden with optional extra arguments.
The built-in function vars() returns a dictionary representing the current local name space. The
effect of modifications to this dictionary on the name space are undefined.2

4.2 Exceptions
Exceptions are a means of breaking out of the normal flow of control of a code block in order tohandle errors or other exceptional conditions. An exception is raised at the point where the error
is detected; it may be handled by the surrounding code block or by any code block that directly or
indirectly invoked the code block where the error occurred.
If the code block contains exec statements or the construct from ^_^`^ import *, the semantics of names not explicitly
mentioned in a global statement change subtly: name lookup first searches the local name space, then the global one,then the built-in one.
2
The current implementation returns the dictionary actually used to implement the name space, except for functions,where the optimizer may cause the local name space to be implemented differently.
1

19

Notes

(1)
(1)
(1)

The Python interpreter raises an exception when it detects an run-time error (such as division byzero). A Python program can also explicitly raise an exception with the raise statement. Exception
handlers are specified with the try...except statement.
Python uses the “termination” model of error handling: an exception handler can find out what
happened and continue execution at an outer level, but it cannot repair the cause of the error and retry
the failing operation (except by re-entering the the offending piece of code from the top).
When an exception is not handled at all, the interpreter terminates execution of the program, or returns
to its interactive main loop.
Exceptions are identified by string objects. Two different string objects with the same value identify
different exceptions.
When an exception is raised, an object (maybe None) is passed as the exception’s “parameter”; thisobject does not affect the selection of an exception handler, but is passed to the selected exception
handler as additional information.
See also the description of the try and raise statements.

20

Chapter 5

Expressions and conditions
Note: In this and the following chapters, extended BNF notation will be used to describe syntax, notlexical analysis.
This chapter explains the meaning of the elements of expressions and conditions. Conditions are asuperset of expressions, and a condition may be used wherever an expression is required by enclosing
it in parentheses. The only places where expressions are used in the syntax instead of conditions is inexpression statements and on the right-hand side of assignment statements; this catches some nasty
bugs like accidentally writing x == 1 instead of x = 1.
The comma plays several roles in Python’s syntax. It is usually an operator with a lower precedence
than all others, but occasionally serves other purposes as well; e.g. it separates function arguments, isused in list and dictionary constructors, and has special semantics in print statements.
When (one alternative of) a syntax rule has the form
name:othernameand no semantics are given, the semantics of this form of name are the same as for othername.

5.1 Arithmetic conversions
When a description of an arithmetic operator below uses the phrase “the numeric arguments areconverted to a common type”, this both means that if either argument is not a number, a TypeError
exception is raised, and that otherwise the following conversions are applied:a first, if either argument is a floating point number, the other is converted to floating point;a else, if either argument is a long integer, the other is converted to long integer;a otherwise, both must be plain integers and no conversion is necessary.

21

5.2 Atoms
Atoms are the most basic elements of expressions. Forms enclosed in reverse quotes or in parentheses,
brackets or braces are also categorized syntactically as atoms. The syntax for atoms is:
atom:
identifier | literal | enclosure
enclosure: parenth_form|list_display|dict_display|string_conversion

5.2.1

Identifiers (Names)

An identifier occurring as an atom is a reference to a local, global or built-in name binding. If aname is assigned to anywhere in a code block (even in unreachable code), and is not mentioned ina global statement in that code block, then it refers to a local name throughout that code block.
When it is not assigned to anywhere in the block, or when it is assigned to but also explicitly listed ina global statement, it refers to a global name if one exists, else to a built-in name (and this binding
may dynamically change).
When the name is bound to an object, evaluation of the atom yields that object. When a name is notbound, an attempt to evaluate it raises a NameError exception.

5.2.2

Literals

Python knows string and numeric literals:
literal:stringliteral | integer | longinteger | floatnumber

Evaluation of a literal yields an object of the given type (string, integer, long integer, floating point
number) with the given value. The value may be approximated in the case of floating point literals.
See section 2.4 for details.
All literals correspond to immutable data types, and hence the object’s identity is less important thanits value. Multiple evaluations of literals with the same value (either the same occurrence in the
program text or a different occurrence) may obtain the same object or a different object with the samevalue.
(In the original implementation, all literals in the same code block with the same type and value yield
the same object.)

5.2.3

Parenthesized forms

A parenthesized form is an optional condition list enclosed in parentheses:parenth_form:"(" [condition_list] ")"A parenthesized condition list yields whatever that condition list yields.
22

An empty pair of parentheses yields an empty tuple object. Since tuples are immutable, the rules forliterals apply here.
(Note that tuples are not formed by the parentheses, but rather by use of the comma operator.
The exception is the empty tuple, for which parentheses are required — allowing unparenthesized“nothing” in expressions would cause ambiguities and allow common typos to pass uncaught.)

5.2.4

List displays

A list display is a possibly empty series of conditions enclosed in square brackets:
list_display:"[" [condition_list] "]"A list display yields a new list object.
If it has no condition list, the list object has no items. Otherwise, the elements of the condition list
are evaluated from left to right and inserted in the list object in that order.

5.2.5

Dictionary displays

A dictionary display is a possibly empty series of key/datum pairs enclosed in curly braces:
dict_display:"{" [key_datum_list] "}"key_datum_list: key_datum ("," key_datum)* [","]
key_datum:
condition ":" condition
A dictionary display yields a new dictionary object.
The key/datum pairs are evaluated from left to right to define the entries of the dictionary: each key
object is used as a key into the dictionary to store the corresponding datum.
Restrictions on the types of the key values are listed earlier in section 3.2. Clashes between duplicate
keys are not detected; the last datum (textually rightmost in the display) stored for a given key value
prevails.

5.2.6

String conversions

A string conversion is a condition list enclosed in reverse (or backward) quotes:
string_conversion: "‘" condition_list "‘"A string conversion evaluates the contained condition list and converts the resulting object into a string
according to rules specific to its type.
If the object is a string, a number, None, or a tuple, list or dictionary containing only objects whosetype is one of these, the resulting string is a valid Python expression which can be passed to the built-in
23

function eval() to yield an expression with the same value (or an approximation, if floating point
numbers are involved).
(In particular, converting a string adds quotes around it and converts “funny” characters to escapesequences that are safe to print.)
It is illegal to attempt to convert recursive objects (e.g. lists or dictionaries that contain a reference tothemselves, directly or indirectly.)
The built-in function repr() performs exactly the same conversion in its argument as enclosing
it it reverse quotes does. The built-in function str() performs a similar but more user-friendly
conversion.

5.3 Primaries
Primaries represent the most tightly bound operations of the language. Their syntax is:
primary:

5.3.1

atom | attributeref | subscription | slicing | call

Attribute references

An attribute reference is a primary followed by a period and a name:attributeref:

primary "." identifier

The primary must evaluate to an object of a type that supports attribute references, e.g. a module ora list. This object is then asked to produce the attribute whose name is the identifier. If this attributeis not available, the exception AttributeError is raised. Otherwise, the type and value of theobject produced is determined by the object. Multiple evaluations of the same attribute reference may
yield different objects.

5.3.2

Subscriptions

A subscription selects an item of a sequence (string, tuple or list) or mapping (dictionary) object:subscription:

primary "[" condition "]"The primary must evaluate to an object of a sequence or mapping type.
If it is a mapping, the condition must evaluate to an object whose value is one of the keys of the
mapping, and the subscription selects the value in the mapping that corresponds to that key.
If it is a sequence, the condition must evaluate to a plain integer. If this value is negative, the lengthof the sequence is added to it (so that, e.g. x[-1] selects the last item of x.) The resulting value

24

must be a nonnegative integer smaller than the number of items in the sequence, and the subscription
selects the item whose index is that value (counting from zero).
A string’s items are characters. A character is not a separate data type but a string of exactly one
character.

5.3.3

Slicings

A slicing (or slice) selects a range of items in a sequence (string, tuple or list) object:slicing:

primary "[" [condition] ":" [condition] "]"The primary must evaluate to a sequence object. The lower and upper bound expressions, if present,
must evaluate to plain integers; defaults are zero and the sequence’s length, respectively. If either
bound is negative, the sequence’s length is added to it. The slicing now selects all items with index ksuch that i <= k < j where i and j are the specified lower and upper bounds. This may be an empty
sequence. It is not an error if i or j lie outside the range of valid indexes (such items don’t exist so
they aren’t selected).

5.3.4

Calls

A call calls a callable object (e.g. a function) with a possibly empty series of arguments:
call:

primary "(" [condition_list] ")"The primary must evaluate to a callable object (user-defined functions, built-in functions, methods ofbuilt-in objects, class objects, and methods of class instances are callable). If it is a class, the argument
list must be empty; otherwise, the arguments are evaluated.
A call always returns some value, possibly None, unless it raises an exception. How this value iscomputed depends on the type of the callable object. If it is:a user-defined function: the code block for the function is executed, passing it the argument list.
The first thing the code block will do is bind the formal parameters to the arguments; this is
described in section 7.5. When the code block executes a return statement, this specifies thereturn value of the function call.
a built-in function or method: the result is up to the interpreter; see the library reference manual
for the descriptions of built-in functions and methods.
a class object: a new instance of that class is returned.
a class instance method: the corresponding user-defined function is called, with an argument listthat is one longer than the argument list of the call: the instance becomes the first argument.

25

5.4 Unary arithmetic operations
All unary arithmetic (and bit-wise) operations have the same priority:
u_expr:primary | "-" u_expr | "+" u_expr | "˜" u_expr

The unary "-" (minus) operator yields the negation of its numeric argument.
The unary "+" (plus) operator yields its numeric argument unchanged.
The unary "˜" (invert) operator yields the bit-wise inversion of its plain or long integer argument.
The bit-wise inversion of x is defined as -(x+1).
In all three cases, if the argument does not have the proper type, a TypeError exception is raised.

5.5 Binary arithmetic operations
The binary arithmetic operations have the conventional priority levels. Note that some of these
operations also apply to certain non-numeric types. There is no “power” operator, so there are only
two levels, one for multiplicative operators and one for additive operators:
m_expr:a_expr:u_expr | m_expr "*" u_expr| m_expr "/" u_expr | m_expr "%" u_expr
m_expr | aexpr "+" m_expr | aexpr "-" m_expr

The "*" (multiplication) operator yields the product of its arguments. The arguments must either
both be numbers, or one argument must be a plain integer and the other must be a sequence. In theformer case, the numbers are converted to a common type and then multiplied together. In the latter
case, sequence repetition is performed; a negative repetition factor yields an empty sequence.
The "/" (division) operator yields the quotient of its arguments. The numeric arguments are first
converted to a common type. Plain or long integer division yields an integer of the same type; theresult is that of mathematical division with the ‘floor’ function applied to the result. Division by zeroraises the ZeroDivisionError exception.
The "%" (modulo) operator yields the remainder from the division of the first argument by thesecond. The numeric arguments are first converted to a common type. A zero right argument
raises the ZeroDivisionError exception. The arguments may be floating point numbers, e.g.
3.14 % 0.7 equals 0.34. The modulo operator always yields a result with the same sign as its
second operand (or zero); the absolute value of the result is strictly smaller than the second operand.
The integer division and modulo operators are connected by the following identity:x == (x/y)*y + (x%y). Integer division and modulo are also connected with the built-infunction divmod(): divmod(x, y) == (x/y, x%y). These identities don’t hold for floating point numbers; there a similar identity holds where x/y is replaced by floor(x/y)).
The "+" (addition) operator yields the sum of its arguments. The arguments must either both benumbers, or both sequences of the same type. In the former case, the numbers are converted to acommon type and then added together. In the latter case, the sequences are concatenated.
26

The "-" (subtraction) operator yields the difference of its arguments. The numeric arguments are
first converted to a common type.

5.6 Shifting operations
The shifting operations have lower priority than the arithmetic operations:shift_expr:

a_expr | shift_expr ( "<<" | ">>" ) a_expr

These operators accept plain or long integers as arguments. The arguments are converted to acommon type. They shift the first argument to the left or right by the number of bits given by thesecond argument.
A right shift by n bits is defined as division by pow(2,n). A left shift by n bits is defined as
multiplication with pow(2,n); for plain integers there is no overflow check so this drops bits andflips the sign if the result is not less than pow(2,31) in absolute value.
Negative shift counts raise a ValueError exception.

5.7 Binary bit-wise operations
Each of the three bitwise operations has a different priority level:
and_expr:
xor_expr:
or_expr:shift_expr | and_expr "&" shift_expr
and_expr | xor_expr "ˆ" and_expr
xor_expr | or_expr "|" xor_expr

The "&" operator yields the bitwise AND of its arguments, which must be plain or long integers. The
arguments are converted to a common type.
The "ˆ" operator yields the bitwise XOR (exclusive OR) of its arguments, which must be plain orlong integers. The arguments are converted to a common type.
The "|" operator yields the bitwise (inclusive) OR of its arguments, which must be plain or longintegers. The arguments are converted to a common type.

5.8 ComparisonsContrary to C, all comparison operations in Python have the same priority, which is lower than that of
any arithmetic, shifting or bitwise operation. Also contrary to C, expressions like a < b < c havethe interpretation that is conventional in mathematics:
comparison:
comp_operator:

or_expr (comp_operator or_expr)*"<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"27Comparisons yield integer values: 1 for true, 0 for false.
Comparisons can be chained arbitrarily, e.g. x < y <= z is equivalent to x < y and y <= z,
except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is foundto be false).
Formally, if a, b, c, \)\\ , y, z are expressions and opa, opb, \)\)\ , opy are comparison operators, thena opa b opb c \\)\ y opy z is equivalent to a opa b and b opb c and \)\)\ and y opy z, except that each
expression is evaluated at most once.
Note that a opa b opb c doesn’t imply any kind of comparison between a and c, so that e.g.
x < y > z is perfectly legal (though perhaps not pretty).
The forms <> and != are equivalent; for consistency with C, != is preferred; where != is mentioned
below <> is also implied.
The operators "<", ">", "==", ">=", "<=", and "!=" compare the values of two objects.
The objects needn’t have the same type. If both are numbers, they are coverted to a common
type. Otherwise, objects of different types always compare unequal, and are ordered consistently but
arbitrarily.
(This unusual definition of comparison is done to simplify the definition of operations like sorting andthe in and not in operators.)Comparison of objects of the same type depends on the type:a Numbers are compared arithmetically.
a Strings are compared lexicographically using the numeric equivalents (the result of the built-infunction ord) of their characters.
a Tuples and lists are compared lexicographically using comparison of corresponding items.
a Mappings (dictionaries) are compared through lexicographic comparison of their sorted (key,value) lists.1
a Most other types compare unequal unless they are the same object; the choice whether one
object is considered smaller or larger than another one is made arbitrarily but consistently
within one execution of a program.
The operators in and not in test for sequence membership: if y is a sequence, x in y is true if
and only if there exists an index i such that x = y[i]. x not in y yields the inverse truth value.
The exception TypeError is raised when y is not a sequence, or when y is a string and x is not astring of length one.2
The operators is and is not test for object identity: x is y is true if and only if x and y are thesame object. x is not y yields the inverse truth value.
1

This is expensive since it requires sorting the keys first, but about the only sensible definition. An earlier versionof Python compared dictionaries by identity only, but this caused surprises because people expected to be able to test a
dictionary for emptiness by comparing it to bdc .
2
The latter restriction is sometimes a nuisance.

28

5.9 Boolean operations
Boolean operations have the lowest priority of all Python operations:condition:
or_test | lambda_form
or_test:and_test | or_test "or" and_test
and_test:
not_test | and_test "and" not_test
not_test:
comparison | "not" not_test
lambda_form: "lambda" [parameter_list]: condition
In the context of Boolean operations, and also when conditions are used by control flow statements, thefollowing values are interpreted as false: None, numeric zero of all types, empty sequences (strings,
tuples and lists), and empty mappings (dictionaries). All other values are interpreted as true.
The operator not yields 1 if its argument is false, 0 otherwise.
The condition x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated andthe resulting value is returned.
The condition x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated andthe resulting value is returned.
(Note that and and or do not restrict the value and type they return to 0 and 1, but rather return thelast evaluated argument. This is sometimes useful, e.g. if s is a string that should be replaced by adefault value if it is empty, the expression s or ’foo’ yields the desired value. Because not has
to invent a value anyway, it does not bother to return a value of the same type as its argument, so e.g.
not ’foo’ yields 0, not ’’.)
Lambda forms (lambda expressions) have the same syntactic position as conditions. They are a
shorthand to create anonymous functions; the expression lambda arguments: condition yields
a function object that behaves virtually identical to one defined with def name (arguments):
return condition. See section 7.5 for the syntax of parameter lists. Note that functions created withlambda forms cannot contain statements.

5.10 Expression lists and condition lists
expr_list:
cond_list:

or_expr ("," or_expr)* [","]
condition ("," condition)* [","]The only difference between expression lists and condition lists is the lowest priority of operatorsthat can be used in them without being enclosed in parentheses; condition lists allow all operators,
while expression lists don’t allow comparisons and Boolean operators (they do allow bitwise and shiftoperators though).
Expression lists are used in expression statements and assignments; condition lists are used everywhere
else where a list of comma-separated values is required.
An expression (condition) list containing at least one comma yields a tuple. The length of the tuple
is the number of expressions (conditions) in the list. The expressions (conditions) are evaluated from29

left to right. (Condition lists are used syntactically is a few places where no tuple is constructed but alist of values is needed nevertheless.)
The trailing comma is required only to create a single tuple (a.k.a. a singleton); it is optional in allother cases. A single expression (condition) without a trailing comma doesn’t create a tuple, but rather
yields the value of that expression (condition).
(To create an empty tuple, use an empty pair of parentheses: ().)

5.11 Summary
The following table summarizes the operator precedences in Python, from lowest precedence (leastbinding) to highest precedence (most binding). Operators in the same box have the same precedence.
Unless the syntax is explicitly given, operators are binary. Operators in the same box group left toright (except for comparisons, which chain from left to right — see above).
or
and
not x
in, not in
is, is not<, <=, >, >=, <>, !=, =|ˆ&<<, >>+, *, /, %+x, -x˜x
x.attributex[index]x[index:index]f (arguments...)(expressions\)\\ )[expressions\)\\ ]{key:datum\)\\ }‘expression\\)\ ‘

Boolean OR
Boolean AND
Boolean NOTMembership tests
Identity tests
Comparisons
Bitwise OR
Bitwise XOR
Bitwise AND
ShiftsAddition and subtraction
Multiplication, division, remainder
Positive, negative
Bitwise not
Attribute reference
Subscription
SlicingFunction call
Binding or tuple display
List display
Dictionary display
String conversion

30

Chapter 6

Simple statements
Simple statements are comprised within a single logical line. Several simple statements may occur
on a single line separated by semicolons. The syntax for simple statements is:
simple_stmt:

expression_stmt| assignment_stmt| pass_stmt| del_stmt| print_stmt| return_stmt| raise_stmt| break_stmt| continue_stmt| import_stmt| global_stmt| access_stmt| exec_stmt

6.1 Expression statements
Expression statements are used (mostly interactively) to compute and write a value, or (usually) tocall a procedure (a function that returns no meaningful result; in Python, procedures return the valueNone):
expression_stmt: expression_list
An expression statement evaluates the expression list (which may be a single expression). If the value
is not None, it is converted to a string using the rules for string conversions (expressions in reverse
quotes), and the resulting string is written to standard output (see section 6.5) on a line by itself.
(The exception for None is made so that procedure calls, which are syntactically equivalent toexpressions, do not cause any output. A tuple with only None items is written normally.)31

6.2 Assignment statements
Assignment statements are used to (re)bind names to values and to modify attributes or items ofmutable objects:
assignment_stmt: (target_list "=")+ expression_list
target_list:
target ("," target)* [","]
target:identifier | "(" target_list ")" | "[" target_list "]"| attributeref | subscription | slicing(See section 5.3 for the syntax definitions for the last three symbols.)
An assignment statement evaluates the expression list (remember that this can be a single expression
or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to eachof the target lists, from left to right.
Assignment is defined recursively depending on the form of the target (list). When a target is part ofa mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately
perform the assignment and decide about its validity, and may raise an exception if the assignmentis unacceptable. The rules observed by various types and the exceptions raised are given with thedefinition of the object types (see section 3.2).
Assignment of an object to a target list is recursively defined as follows.
a If the target list is a single target: the object is assigned to that target.
a If the target list is a comma-separated list of targets: the object must be a tuple with the same
number of items as the list contains targets, and the items are assigned, from left to right, to thecorresponding targets.
Assignment of an object to a single target is recursively defined as follows.
a If the target is an identifier (name):– If the name does not occur in a global statement in the current code block: the name isbound to the object in the current local name space.
– Otherwise: the name is bound to the object in the current global name space.
The name is rebound if it was already bound.
a If the target is a target list enclosed in parentheses: the object is assigned to that target list as
described above.
a If the target is a target list enclosed in square brackets: the object must be a list with the same
number of items as the target list contains targets, and its items are assigned, from left to right,
to the corresponding targets.

32

a If the target is an attribute reference: The primary expression in the reference is evaluated. It
should yield an object with assignable attributes; if this is not the case, TypeError is raised.
That object is then asked to assign the assigned object to the given attribute; if it cannot performthe assignment, it raises an exception (usually but not necessarily AttributeError).
a If the target is a subscription: The primary expression in the reference is evaluated. It should
yield either a mutable sequence (list) object or a mapping (dictionary) object. Next, the subscript
expression is evaluated.
If the primary is a mutable sequence object (a list), the subscript must yield a plain integer. If
it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative
integer less than the sequence’s length, and the sequence is asked to assign the assigned object
to its item with that index. If the index is out of range, IndexError is raised (assignment toa subscripted sequence cannot add new items to a list).
If the primary is a mapping (dictionary) object, the subscript must have a type compatible withthe mapping’s key type, and the mapping is then asked to create a key/datum pair which maps
the subscript to the assigned object. This can either replace an existing key/value pair with thesame key value, or insert a new key/value pair (if no key with the same value existed).
a If the target is a slicing: The primary expression in the reference is evaluated. It should yielda mutable sequence object (e.g. a list). The assigned object should be a sequence object of
the same type. Next, the lower and upper bound expressions are evaluated, insofar they arepresent; defaults are zero and the sequence’s length. The bounds should evaluate to (small)
integers. If either bound is negative, the sequence’s length is added to it. The resulting bounds
are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequenceobject is asked to replace the slice with the items of the assigned sequence. The length of theslice may be different from the length of the assigned sequence, thus changing the length of thetarget sequence, if the object allows it.
(In the current implementation, the syntax for targets is taken to be the same as for expressions, andinvalid syntax is rejected during the code generation phase, causing less detailed error messages.)
WARNING: Although the definition of assignment implies that overlaps between the left-hand sideand the right-hand side are ‘safe’ (e.g. a, b = b, a swaps two variables), overlaps within thecollection of assigned-to variables are not safe! For instance, the following program prints @[0, 2]@:x = [0, 1]i = 0i, x[i] = 1, 2
print x6.3 The pass statement
pass_stmt:"pass"

pass is a null operation — when it is executed, nothing happens. It is useful as a placeholder whena statement is required syntactically, but no code needs to be executed, for example:
33

def f(arg): pass

# a function that does nothing (yet)class C: pass# an class with no methods (yet)

6.4 The del statement
del_stmt:"del" target_list

Deletion is recursively defined very similar to the way assignment is defined. Rather that spelling it
out in full details, here are some hints.
Deletion of a target list recursively deletes each target, from left to right.
Deletion of a name removes the binding of that name (which must exist) from the local or global namespace, depending on whether the name occurs in a global statement in the same code block.
Deletion of attribute references, subscriptions and slicings is passed to the primary object involved;
deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but eventhis is determined by the sliced object).

6.5 The print statement
print_stmt:"print" [ condition ("," condition)* [","] ]print evaluates each condition in turn and writes the resulting object to standard output (see below).
If an object is not a string, it is first converted to a string using the rules for string conversions. The(resulting or original) string is then written. A space is written before each object is (converted and)written, unless the output system believes it is positioned at the beginning of a line. This is the case:(1) when no characters have yet been written to standard output; or (2) when the last character written
to standard output is \n; or (3) when the last write operation on standard output was not a printstatement. (In some cases it may be functional to write an empty string to standard output for thisreason.)A "\n" character is written at the end, unless the print statement ends with a comma. This is the
only action if the statement contains just the keyword print.
Standard output is defined as the file object named stdout in the built-in module sys. If no suchobject exists, or if it is not a writable file, a RuntimeError exception is raised. (The original
implementation attempts to write to the system’s original standard output instead, but this is not safe,
and should be fixed.)

6.6 The return statement
return_stmt:"return" [condition_list]

34

return may only occur syntactically nested in a function definition, not within a nested classdefinition.
If a condition list is present, it is evaluated, else None is substituted.
return leaves the current function call with the condition list (or None) as return value.
When return passes control out of a try statement with a finally clause, that finally clause is
executed before really leaving the function.

6.7 The raise statement
raise_stmt:"raise" condition ["," condition]raise evaluates its first condition, which must yield a string, class, or instance object. If there is asecond condition, this is evaluated, else None is substituted. If the first condition is a class object, thenthe second condition must be an instance of that class or one of its derivatives. If the first condition isan instance object, the second condition must be None.
If the first object is a class or string, it then raises the exception identified by the first object, withthe second one (or None) as its parameter. If the first object is an instance, it raises the exception
identified by the class of the object, with the instance as its parameter.

6.8 The break statement
break_stmt:"break"break may only occur syntactically nested in a for or while loop, but not nested in a function orclass definition within that loop.
It terminates the nearest enclosing loop, skipping the optional else clause if the loop has one.
If a for loop is terminated by break, the loop control target keeps its current value.
When break passes control out of a try statement with a finally clause, that finally clause is
executed before really leaving the loop.

6.9 The continue statement
continue_stmt:"continue"

continue may only occur syntactically nested in a for or while loop, but not nested in a function
or class definition or try statement within that loop.1
It continues with the next cycle of the nearest enclosing loop.
1

Except that it may currently occur within an except clause.

35

6.10 The import statement
import_stmt:"import" identifier ("," identifier)*| "from" identifier "import" identifier ("," identifier)*| "from" identifier "import" "*"Import statements are executed in two steps: (1) find a module, and initialize it if necessary; (2) define
a name or names in the local name space (of the scope where the import statement occurs). The
first form (without from) repeats these steps for each identifier in the list, the from form performs
them once, with the first identifier specifying the module name.
The system maintains a table of modules that have been initialized, indexed by module name. (Thecurrent implementation makes this table accessible as sys.modules.) When a module name isfound in this table, step (1) is finished. If not, a search for a module definition is started. This firstlooks for a built-in module definition, and if no built-in module if the given name is found, it searches
a user-specified list of directories for a file whose name is the module name with extension ".py".
(The current implementation uses the list of strings sys.path as the search path; it is initialized
from the shell environment variable $PYTHONPATH, with an installation-dependent default.)
If a built-in module is found, its built-in initialization code is executed and step (1) is finished. If nomatching file is found, ImportError is raised. If a file is found, it is parsed, yielding an executablecode block. If a syntax error occurs, SyntaxError is raised. Otherwise, an empty module of thegiven name is created and inserted in the module table, and then the code block is executed in thecontext of this module. Exceptions during this execution terminate step (1).
When step (1) finishes without raising an exception, step (2) can begin.
The first form of import statement binds the module name in the local name space to the module
object, and then goes on to import the next identifier, if any. The from from does not bind the module
name: it goes through the list of identifiers, looks each one of them up in the module found in step(1), and binds the name in the local name space to the object thus found. If a name is not found,
ImportError is raised. If the list of identifiers is replaced by a star (*), all names defined in themodule are bound, except those beginning with an underscore(_).
Names bound by import statements may not occur in global statements in the same scope.
The from form with * may only occur in a module scope.
(The current implementation does not enforce the latter two restrictions, but programs should notabuse this freedom, as future implementations may enforce them or silently change the meaning of
the program.)

6.11 The global statement
global_stmt:"global" identifier ("," identifier)*

The global statement is a declaration which holds for the entire current scope. It means that thelisted identifiers are to be interpreted as globals. While using global names is automatic if they are
not defined in the local scope, assigning to global names would be impossible without global.
36

Names listed in a global statement must not be used in the same scope before that global
statement is executed.
Names listed in a global statement must not be defined as formal parameters or in a for loop
control target, class definition, function definition, or import statement.
(The current implementation does not enforce the latter two restrictions, but programs should notabuse this freedom, as future implementations may enforce them or silently change the meaning of
the program.)

6.12 The access statement
access_stmt:"access" ...

This statement will be used in the future to control access to instance and class variables. Currently
its syntax and effects are undefined; however the keyword access is a reserved word for the parser.

6.13 The exec statement
exec_stmt:"exec" expression ["in" expression ["," expression]]

This statement supports dynamic execution of Python code. The first expression should evaluate toeither a string, an open file object, or a code object. If it is a string, the string is parsed as a suite of
Python statements which is then executed (unless a syntax error occurs). If it is an open file, the fileis parsed until EOF and executed. If it is a code object, it is simply executed.
In all cases, if the optional parts are omitted, the code is executed in the current scope. If only the firstexpression after in is specified, it should be a dictionary, which will be used for both the global andthe local variables. If two expressions are given, both must be dictionaries and they are used for the
global and local variables, respectively.
Hints: dynamic evaluation of expressions is supported by the built-in function eval(). The built-infunction vars() returns the current local dictionary, which may be useful to pass around for use by
exec.

37

Chapter 7

Compound statements
Compound statements contain (groups of) other statements; they affect or control the execution of
those other statements in some way. In general, compound statements span multiple lines, althoughin simple incarnations a whole compound statement may be contained in one line.
The if, while and for statements implement traditional control flow constructs. try specifies
exception handlers and/or cleanup code for a group of statements. Function and class definitions arealso syntactically compound statements.
Compound statements consist of one or more ‘clauses’. A clause consists of a header and a ‘suite’.
The clause headers of a particular compound statement are all at the same indentation level. Eachclause header begins with a uniquely identifying keyword and ends with a colon. A suite is agroup of statements controlled by a clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header’s colon, or it can be one or moreindented statements on subsequent lines. Only the latter form of suite can contain nested compoundstatements; the following is illegal, mostly because it wouldn’t be clear to which if clause a following
else clause would belong:
if test1: if test2: print x
Also note that the semicolon binds tighter than the colon in this context, so that in the following
example, either all or none of the print statements are executed:
if x < y < z: print x; print y; print z
Summarizing:
compound_stmt:
suite:
statement:
stmt_list:

if_stmt | while_stmt | for_stmt| try_stmt | funcdef | classdef
stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
stmt_list NEWLINE | compound_stmt
simple_stmt (";" simple_stmt)* [";"]

Note that statements always end in a NEWLINE possibly followed by a DEDENT.
38

Also note that optional continuation clauses always begin with a keyword that cannot start a statement,
thus there are no ambiguities (the ‘dangling else’ problem is solved in Python by requiring nested
if statements to be indented).
The formatting of the grammar rules in the following sections places each clause on a separate line
for clarity.

7.1 The if statement
The if statement is used for conditional execution:
if_stmt:"if" condition ":" suite("elif" condition ":" suite)*["else" ":" suite]

It selects exactly one of the suites by evaluating the conditions one by one until one is found to be true(see section 5.9 for the definition of true and false); then that suite is executed (and no other part ofthe if statement is executed or evaluated). If all conditions are false, the suite of the else clause, if
present, is executed.

7.2 The while statement
The while statement is used for repeated execution as long as a condition is true:
while_stmt:"while" condition ":" suite["else" ":" suite]

This repeatedly tests the condition and, if it is true, executes the first suite; if the condition is false(which may be the first time it is tested) the suite of the else clause, if present, is executed and theloop terminates.
A break statement executed in the first suite terminates the loop without executing the else clause’s
suite. A continue statement executed in the first suite skips the rest of the suite and goes back totesting the condition.

7.3 The for statement
The for statement is used to iterate over the elements of a sequence (string, tuple or list):
for_stmt:"for" target_list "in" condition_list ":" suite["else" ":" suite]39

The condition list is evaluated once; it should yield a sequence. The suite is then executed once foreach item in the sequence, in the order of ascending indices. Each item in turn is assigned to thetarget list using the standard rules for assignments, and then the suite is executed. When the items
are exhausted (which is immediately when the sequence is empty), the suite in the else clause, if
present, is executed, and the loop terminates.
A break statement executed in the first suite terminates the loop without executing the else clause’s
suite. A continue statement executed in the first suite skips the rest of the suite and continues with
the next item, or with the else clause if there was no next item.
The suite may assign to the variable(s) in the target list; this does not affect the next item assigned toit.
The target list is not deleted when the loop is finished, but if the sequence is empty, it will not havebeen assigned to at all by the loop.
Hint: the built-in function range() returns a sequence of integers suitable to emulate the effect of
Pascal’s for i := a to b do; e.g. range(3) returns the list [0, 1, 2].
Warning: There is a subtlety when the sequence is being modified by the loop (this can only occur
for mutable sequences, i.e. lists). An internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has reached the length of the sequence
the loop terminates. This means that if the suite deletes the current (or a previous) item from thesequence, the next item will be skipped (since it gets the index of the current item which has alreadybeen treated). Likewise, if the suite inserts an item in the sequence before the current item, the current
item will be treated again the next time through the loop. This can lead to nasty bugs that can be
avoided by making a temporary copy using a slice of the whole sequence, e.g.
for x in a[:]:if x < 0: a.remove(x)7.4 The try statement
The try statement specifies exception handlers and/or cleanup code for a group of statements:
try_stmt:
try_exc_stmt:

try_fin_stmt:

try_exc_stmt | try_fin_stmt"try" ":" suite("except" [condition ["," target]] ":" suite)+["else" ":" suite]"try" ":" suite"finally" ":" suite

There are two forms of try statement: try...except and try...finally. These forms
cannot be mixed.
The try...except form specifies one or more exception handlers (the except clauses). Whenno exception occurs in the try clause, no exception handler is executed. When an exception occurs
in the try suite, a search for an exception handler is started. This inspects the except clauses in turn40

until one is found that matches the exception. A condition-less except clause, if present, must be last;
it matches any exception. For an except clause with a condition, that condition is evaluated, and theclause matches the exception if the resulting object is “compatible” with the exception. An object iscompatible with an exception if it is either the object that identifies the exception, or (for exceptions
that are classes) it is a base class of the exception, or it is a tuple containing an item that is compatible
with the exception. Note that the object identities must match, i.e. it must be the same object, not justan object with the same value.
If no except clause matches the exception, the search for an exception handler continues in thesurrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause raises an exception, the original
search for a handler is cancelled and a search starts for the new exception in the surrounding code andon the call stack (it is treated as if the entire try statement raised the exception).
When a matching except clause is found, the exception’s parameter is assigned to the target specified
in that except clause, if present, and the except clause’s suite is executed. When the end of this suite
is reached, execution continues normally after the entire try statement. (This means that if two nested
handlers exist for the same exception, and the exception occurs in the try clause of the inner handler,
the outer handler will not handle the exception.)
Before an except clause’s suite is executed, details about the exception are assigned to three variables inthe sys module: sys.exc_type receives the object identifying the exception; sys.exc_value
receives the exception’s parameter; sys.exc_traceback receives a traceback object (see section
3.2) identifying the point in the program where the exception occurred.
The optional else clause is executed when no exception occurs in the try clause. Exceptions in theelse clause are not handled by the preceding except clauses.
The try...finally form specifies a ‘cleanup’ handler. The try clause is executed. When no
exception occurs, the finally clause is executed. When an exception occurs in the try clause,the exception is temporarily saved, the finally clause is executed, and then the saved exception
is re-raised. If the finally clause raises another exception or executes a return, break orcontinue statement, the saved exception is lost.
When a return or break statement is executed in the try suite of a try...finally statement,
the finally clause is also executed ‘on the way out’. A continue statement is illegal in the try
clause. (The reason is a problem with the current implementation — this restriction may be lifted in
the future).

7.5 Function definitions
A function definition defines a user-defined function object (see section 3.2):
funcdef:"def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (defparameter ",")* ("*" identifier | defparameter [","])
defparameter:parameter ["=" condition]
sublist:parameter ("," parameter)* [","]
parameter:
identifier | "(" sublist ")"funcname:
identifier41

A function definition is an executable statement. Its execution binds the function name in the current
local name space to a function object (a wrapper around the executable code for the function). Thisfunction object contains a reference to the current global name space as the global name space to be
used when the function is called.
The function definition does not execute the function body; this gets executed only when the function
is called.
When one or more top-level parameters have the form parameter = condition, the function is said tohave “default parameter values”. Default parameter values are evaluated when the function definitionis executed. For a parameter with a default value, the correponding argument may be omitted froma call, in which case the parameter’s default value is substituted. If a parameter has a default value,
all following parameters must also have a default value — this is a syntactic restriction that is not
expressed by the grammar.1
Function call semantics are described in section 5.3.4. When a user-defined function is called, firstmissing arguments for which a default value exists are supplied; then the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:a If there are no formal parameters, there must be no arguments.
a If the formal parameter list does not end in a star followed by an identifier, there must be exactly
as many arguments as there are parameters in the formal parameter list (at the top level); thearguments are assigned to the formal parameters one by one. Note that the presence or absenceof a trailing comma at the top level in either the formal or the actual parameter list makes nodifference. The assignment to a formal parameter is performed as if the parameter occurs on theleft hand side of an assignment statement whose right hand side’s value is that of the argument.
a If the formal parameter list ends in a star followed by an identifier, preceded by zero or morecomma-followed parameters, there must be at least as many arguments as there are parameters
preceding the star. Call this number N. The first N arguments are assigned to the corresponding
formal parameters in the way descibed above. A tuple containing the remaining arguments, ifany, is then assigned to the identifier following the star. This variable will always be a tuple: if
there are no extra arguments, its value is (), if there is just one extra argument, it is a singleton
tuple.
Note that the ‘variable length parameter list’ feature only works at the top level of the parameter list;
individual parameters use a model corresponding more closely to that of ordinary assignment. Whilethe latter model is generally preferable, because of the greater type safety it offers (wrong-sized tuples
aren’t silently mistreated), variable length parameter lists are a sufficiently accepted practice in mostprogramming languages that a compromise has been worked out. (And anyway, assignment has noequivalent for empty argument lists.)
It is also possible to create anonymous functions (functions not bound to a name), for immediate use
in expressions. This uses lambda forms, described in section 5.9.
1

Currently this is not checked; instead, def f(a=1,b) is interpreted as def f(a=1,b=None).

42

7.6 Class definitions
A class definition defines a class object (see section 3.2):
classdef:
inheritance:classname:"class" classname [inheritance] ":" suite"(" [condition_list] ")"identifier

A class definition is an executable statement. It first evaluates the inheritance list, if present. Each
item in the inheritance list should evaluate to a class object. The class’s suite is then executed in a
new execution frame (see section 4.1), using a newly created local name space and the original global
name space. (Usually, the suite contains only function definitions.) When the class’s suite finishes
execution, its execution frame is discarded but its local name space is saved. A class object is thencreated using the inheritance list for the base classes and the saved local name space for the attributedictionary. The class name is bound to this class object in the original local name space.

43

Chapter 8Top-level components
The Python interpreter can get its input from a number of sources: from a script passed to it as
standard input or as program argument, typed in interactively, from a module source file, etc. This
chapter gives the syntax used in these cases.

8.1 Complete Python programs
While a language specification need not prescribe how the language interpreter is invoked, it is useful
to have a notion of a complete Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are available, but none have been initialized,
except for sys (various system services), __builtin__ (built-in functions, exceptions and None)and __main__. The latter is used to provide the local and global name space for execution of thecomplete program.
The syntax for a complete Python program is that for file input, described in the next section.
The interpreter may also be invoked in interactive mode; in this case, it does not read and execute acomplete program but reads and executes one statement (possibly compound) at a time. The initialenvironment is identical to that of a complete program; each statement is executed in the name spaceof __main__.
Under UNIX, a complete program can be passed to the interpreter in three forms: with the -c string
command line option, as a file passed as the first command line argument, or as standard input. If thefile or standard input is a tty device, the interpreter enters interactive mode; otherwise, it executes thefile as a complete program.

8.2 File input
All input read from non-interactive files has the same form:
file_input:(NEWLINE | statement)*

This syntax is used in the following situations:44

a when parsing a complete Python program (from a file or from a string);
a when parsing a module;
a when parsing a string passed to the exec statement;

8.3 Interactive input
Input in interactive mode is parsed using the following grammar:
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
Note that a (top-level) compound statement must be followed by a blank line in interactive mode; thisis needed to help the parser detect the end of the input.

8.4 Expression input
There are two forms of expression input. Both ignore leading whitespace.
The string argument to eval() must have the following form:
eval_input:

condition_list NEWLINE*

The input line read by input() must have the following form:
input_input:

condition_list NEWLINE

Note: to read ‘raw’ input line without interpretation, you can use the built-in function raw_input()or the readline() method of file objects.

45Index__abs__ , 17__add__ , 17__and__ , 17__bases__ , 13__builtin__ (built-in module), 19, 44__call__ , 15__class__ , 13__cmp__ , 14, 15__coerce__ , 17__del__ , 15__delattr__ , 16__delitem__ , 16__delslice__ , 16__dict__ , 13, 16__div__ , 17__divmod__ , 17__float__ , 17__getattr__ , 15__getitem__ , 14, 16__getslice__ , 16__hash__ , 15__hex__ , 17__init__ , 12, 14__int__ , 17__invert__ , 17__len__ , 16__long__ , 17__lshift__ , 17__main__ (built-in module), 19, 44__members__ , 10__methods__ , 10__mod__ , 17__mul__ , 17__name__ , 13__neg__ , 17__nonzero__ , 17__oct__ , 17__or__ , 17__pos__ , 17__pow__ , 17__repr__ , 14, 15__rshift__ , 17__setattr__ , 15, 16__setitem__ , 16__setslice__ , 16__str__ , 14, 15__sub__ , 17__xor__ , 17accessstatement, 37
actualparameter, 42
addition, 26
andbit-wise, 27
andoperator, 29
anonmymous
function, 29
argument, 42function, 12arithmetic
conversion, 21
operation, binary, 26
operation, unary, 26
ASCII, 2, 6–8, 11
assignment
attribute, 32, 33
class attribute, 13class instance attribute, 13
slicing, 33
statement, 11, 21, 32
subscription, 33
target list, 32
atom, 22
attribute, 10
assignment, 32, 33
46

assignment, class, 13
assignment, class instance, 13class, 13class instance, 13deletion, 34
generic special, 10
reference, 24
special, 10

built-in function, 25
built-in method, 25class instance, 25class object, 12, 13, 25
function, 12, 25, 42
instance, 15, 25
method, 25
procedure, 31
user-defined function, 25, 42
callable
object, 12, 25
chaining
comparisons, 28
character, 11, 25
character set, 11chr (built-in function), 11
class
attribute, 13
attribute assignment, 13
constructor, 14
definition, 35, 43
instance, 13
name, 43
object, 12, 13, 25, 43
class instanceattribute, 13
attribute assignment, 13call, 25
object, 12, 13, 25
class object
call, 12, 13, 25
clause, 38
cmp (built-in function), 15
co_code , 14
co_consts , 14
co_filename , 14
co_names , 14code
block, 18
object, 13
code block, 18, 22, 36
comma, 21, 23
trailing, 30, 34
command line, 44comment, 3comparison, 27
string, 11

back-quotes, 15, 23
backslash character, 3
backward
quotes, 15, 23
binaryarithmetic operation, 26
bit-wise operation, 27
binding
global name, 36
name, 18, 22, 32, 36, 42, 43
bit-wiseand, 27
operation, binary, 27
operation, unary, 26
or, 27
xor, 27
blank line, 4
block
code, 18
BNF, 1, 21
Booleanoperation, 29
breakstatement, 35, 39–41
built-inmethod, 12
module, 36
name, 22
built-in function
call, 25
object, 12, 25
built-in method
call, 25
object, 12, 25
byte, 11C, 6, 10–13, 27
call, 2547

comparisons, 15
chaining, 28
compoundstatement, 38
condition, 21
list, 29
constant, 6
constructor
class, 14
container, 9, 13
continuestatement, 35, 39–41
conversion
arithmetic, 21
string, 15, 23, 31

elif
keyword, 39
else
dangling, 39
else
keyword, 35, 39–41
empty
list, 23
tuple, 11, 23
error handling, 19
errors, 19
escape sequence, 6eval (built-in function), 45
exc_traceback , 14, 41
exc_type , 41
exc_value , 41
exceptkeyword, 41
exception, 19, 35
handler, 14
ImportError, 36
NameError, 22
raising, 35
RuntimeError, 34
SyntaxError, 36
TypeError, 21, 23, 26
ValueError, 27
ZeroDivisionError, 26
exception handler, 19
exclusiveor, 27
exec
statement, 19, 37
executionframe, 18, 43
stack, 14execution model, 18
expression, 21
lambda, 29
list, 29, 31, 32
statement, 31
extension
filename, 36
module, 10

dangling
else, 39
data, 9type, 10type, immutable, 22
datum, 23decimal literal, 7
DEDENT token, 4, 38
default
parameter value, 42
definition
class, 35, 43
function, 35, 41
delstatement, 11, 15, 34
delete, 11deletion
attribute, 34
target, 34
target list, 34
delimiters, 8
dictionary
display, 23
object, 12, 13, 15, 23, 24, 33
display
dictionary, 23
list, 23
tuple, 23division, 26

f_back , 14
f_code , 14

EBCDIC, 1148

statement, 18, 19, 22, 32, 34, 36
grammar, 1grouping, 4f_globals , 14f_lasti , 14f_lineno , 14
f_locals , 14fileobject, 13, 45
filename
extension, 36
finally
keyword, 35, 41
floating point
number, 11, 21
object, 11
floating point literal, 7
forstatement, 35, 39
form
lambda, 29, 42
formalparameter, 42
frame
execution, 18, 43
object, 14
fromkeyword, 36
statement, 19
from ... import * , 36
func_code , 12
func_globals , 12functionanonmymous, 29
argument, 12call, 12, 25, 42
call, user-defined, 25, 42
definition, 35, 41
name, 42
object, 12, 25, 41
user-defined, 12

handle an exception, 19
handler
exception, 14
hash (built-in function), 15
hash character, 3
hexadecimal literal, 7hierarchy
type, 10
identifier, 5, 22
identity
test, 28
identity of an object, 9
ifstatement, 39
im_func , 12
im_self , 12
immutable
data type, 22
object, 11
immutable object, 9
immutable sequenceobject, 11
importstatement, 13, 36
ImportError
exception, 36
importingmodule, 36
inkeyword, 40
operator, 28
inclusiveor, 27INDENT token, 4
indentation, 4index operation, 11
inheritance, 43
initialization
module, 36
input, 45
raw, 45
input (built-in function), 45

garbage collection, 9
generic
special attribute, 10
globalname, 22
name binding, 36
name space, 12, 18
global49

condition, 29
deletion target, 34
display, 23
empty, 23expression, 29, 31, 32
object, 11, 23–25, 33
target, 32, 40
literal, 6, 22
localname space, 18
logical line, 3
longinteger, 21
long integer
object, 10long integer literal, 7
loop
over mutable sequence, 40
statement, 35, 39
loop control
target, 35

instance
call, 15, 25class, 13
object, 12, 13, 25
integerlong, 21
object, 10
plain, 21representation, 11
integer literal, 7interactive mode, 44internal type, 13interpreter, 44
inversion, 26
invocation, 12
isoperator, 28
is notoperator, 28
itemsequence, 24
string, 25
item selection, 11

makefile (built-in function), 13
mapping
object, 12, 13, 24, 33
membership
test, 28
methodbuilt-in, 12call, 25
object, 12, 25
user-defined, 12
minus, 26
modulebuilt-in, 36
extension, 10
importing, 36
initialization, 36
name, 36
name space, 13
object, 13, 24
user-defined, 36
modules , 36
modulo, 26
multiplication, 26
mutableobject, 11, 12, 32, 33

key, 23
key/datum pair, 23
keyword, 6
elif, 39else, 35, 39–41
except, 41
finally, 35, 41
from, 36
in, 40lambdaexpression, 29
form, 29, 42
last_traceback , 14leading whitespace, 4len (built-in function), 11, 12
lexical analysis, 3
lexical definitions, 2line continuation, 3line joining, 3line structure, 3
list
assignment, target, 32
50

mutable object, 9mutable sequece
object, 11
mutable sequence
loop over, 40

callable, 12, 25
class, 12, 13, 25, 43
class instance, 12, 13, 25
code, 13
dictionary, 12, 13, 15, 23, 24, 33
file, 13, 45
floating point, 11
frame, 14function, 12, 25, 41
immutable, 11
immutable sequence, 11
instance, 12, 13, 25
integer, 10
list, 11, 23–25, 33
long integer, 10
mapping, 12, 13, 24, 33
method, 12, 25
module, 13, 24
mutable, 11, 12, 32, 33
mutable sequece, 11None, 10
number, 10, 13
numeric, 10
plain integer, 10
recursive, 24
seqence, 11sequence, 13, 24, 25, 28, 33, 39
string, 11, 24, 25
traceback, 14, 41
tuple, 11, 24, 25, 30
user-defined function, 12, 25, 41
user-defined method, 12
object closure, 12
octal literal, 7
open (built-in function), 13
operation
binary arithmetic, 26
binary bit-wise, 27
Boolean, 29
null, 33
shifting, 27
unary arithmetic, 26
unary bit-wise, 26
operatorand, 29in, 28is, 28

name, 5, 22
binding, 18, 22, 32, 36, 42, 43
binding, global, 36
built-in, 22class, 43
function, 42
global, 22
module, 36
rebinding, 18, 32
unbinding, 18, 34
name space, 18
global, 12, 18
local, 18
module, 13
NameError
exception, 22
NameError , 19
negation, 26
newlinesuppression, 34
NEWLINE token, 3, 38
Noneobject, 10
None , 10, 31
notoperator, 29
not inoperator, 28
notation, 1
null
operation, 33
number, 7
floating point, 11, 21
object, 10, 13
numeric
object, 10
numeric literal, 7object, 9
built-in function, 12, 25
built-in method, 12, 25
51

is not, 28
not, 29
not in, 28
or, 29operators, 8
orbit-wise, 27
exclusive, 27
inclusive, 27
oroperator, 29
ord (built-in function), 11
output, 31, 34
standard, 31, 34

raise an exception, 19
raising
exception, 35
range (built-in function), 40
raw input, 45
raw_index (built-in function), 45
readline , 45
rebinding
name, 18, 32
recursive
object, 24
reference
attribute, 24
reference counting, 9repr (built-in function), 15, 24
representation
integer, 11
reserved word, 6
returnstatement, 34, 41
reversequotes, 15, 23
RuntimeError
exception, 34

parameter, 42
actual, 42
formal, 42
value, default, 42
parameter list
variable length, 42parenthesized form, 22parser, 3
Pascal, 40
pass
statement, 33
path , 36
physical line, 3, 6
plain
integer, 21
plain integer
object, 10
plain integer literal, 7plus, 26
popen (built-in function), 13
primary, 24
printstatement, 15, 34
procedurecall, 31
program, 44

seqence
object, 11
sequence
item, 24
object, 13, 24, 25, 28, 33, 39
shifting
operation, 27
simplestatement, 31
singleton
tuple, 11slice, 25slicing, 11, 25
assignment, 33
space, 4specialattribute, 10
attribute, generic, 10
stack
execution, 14
trace, 14
standard

quotesbackward, 15, 23
reverse, 15, 23
raise
statement, 3552

sys.exc_traceback , 14
sys.last_traceback , 14
sys.modules , 36
sys.path , 36
sys.stderr , 13sys.stdin , 13sys.stdout , 13

output, 31, 34
Standard C, 6
standard input, 44
statement
access, 37
assignment, 11, 21, 32
break, 35, 39–41
compound, 38
continue, 35, 39–41
del, 11, 15, 34
exec, 19, 37
expression, 31
for, 35, 39
from, 19
global, 18, 19, 22, 32, 34, 36
if, 39import, 13, 36
loop, 35, 39
pass, 33
print, 15, 34
raise, 35
return, 34, 41
simple, 31
try, 14, 40
while, 35, 39
statement grouping, 4stderr , 13
stdin , 13
stdio, 13
stdout , 13, 34
str (built-in function), 15, 24
stringcomparison, 11
conversion, 15, 23, 31
item, 25
object, 11, 24, 25
string literal, 6
subscription, 11, 12, 24
assignment, 33
subtraction, 27
suite, 38
suppression
newline, 34
syntax, 1, 21
SyntaxError
exception, 36
sys (built-in module), 34, 36, 41, 44

tab, 4target, 32
deletion, 34
list, 32, 40
list assignment, 32
list, deletion, 34
loop control, 35
tb_frame , 14
tb_lasti , 14tb_lineno , 14tb_next , 14
test
identity, 28
membership, 28
token, 3
trace
stack, 14
tracebackobject, 14, 41
trailing
comma, 30, 34
try
statement, 14, 40
tuple
display, 23
empty, 11, 23
object, 11, 24, 25, 30
singleton, 11
type, 10
data, 10hierarchy, 10
immutable data, 22
type of an object, 9TypeError
exception, 21, 23, 26
unaryarithmetic operation, 26
bit-wise operation, 26
53

unbinding
name, 18, 34
UNIX, 44
unreachable object, 9
unrecognized escape sequence, 7
user-defined
function, 12function call, 25, 42
method, 12
module, 36
user-defined function
object, 12, 25, 41
user-defined method
object, 12
value
default parameter, 42
value of an object, 9
ValueError
exception, 27
valueswriting, 31, 34
variable lengthparameter list, 42
while
statement, 35, 39
whitespace, 4writingvalues, 31, 34
xor
bit-wise, 27
ZeroDivisionError
exception, 26

54