Source code: Lib/string.py
See also
Text Sequence Type — str
String Methods
The constants defined in this module are:
The concatenation of the ascii_lowercase and ascii_uppercase constants described below. This value is not locale-dependent.
The lowercase letters 'abcdefghijklmnopqrstuvwxyz'. This value is not locale-dependent and will not change.
The uppercase letters 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'. This value is not locale-dependent and will not change.
The string '0123456789'.
The string '0123456789abcdefABCDEF'.
The string '01234567'.
String of ASCII characters which are considered punctuation characters in the C locale: !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~.
String of ASCII characters which are considered printable. This is a combination of digits, ascii_letters, punctuation, and whitespace.
A string containing all ASCII characters that are considered whitespace. This includes the characters space, tab, linefeed, return, formfeed, and vertical tab.
The built-in string class provides the ability to do complex variable substitutions and value formatting via the format() method described in PEP 3101. The Formatter class in the string module allows you to create and customize your own string formatting behaviors using the same implementation as the built-in format() method.
The Formatter class has the following public methods:
The primary API method. It takes a format string and an arbitrary set of positional and keyword arguments. It is just a wrapper that calls vformat().
Changed in version 3.7: A format string argument is now positional-only.
This function does the actual work of formatting. It is exposed as a separate function for cases where you want to pass in a predefined dictionary of arguments, rather than unpacking and repacking the dictionary as individual arguments using the *args and **kwargs syntax. vformat() does the work of breaking up the format string into character data and replacement fields. It calls the various methods described below.
In addition, the Formatter defines a number of methods that are intended to be replaced by subclasses:
Loop over the format_string and return an iterable of tuples (literal_text, field_name, format_spec, conversion). This is used by vformat() to break the string into either literal text, or replacement fields.
The values in the tuple conceptually represent a span of literal text followed by a single replacement field. If there is no literal text (which can happen if two replacement fields occur consecutively), then literal_text will be a zero-length string. If there is no replacement field, then the values of field_name, format_spec and conversion will be None.
Given field_name as returned by parse() (see above), convert it to an object to be formatted. Returns a tuple (obj, used_key). The default version takes strings of the form defined in PEP 3101, such as “0[name]” or “label.title”. args and kwargs are as passed in to vformat(). The return value used_key has the same meaning as the key parameter to get_value().
Retrieve a given field value. The key argument will be either an integer or a string. If it is an integer, it represents the index of the positional argument in args; if it is a string, then it represents a named argument in kwargs.
The args parameter is set to the list of positional arguments to vformat(), and the kwargs parameter is set to the dictionary of keyword arguments.
For compound field names, these functions are only called for the first component of the field name; subsequent components are handled through normal attribute and indexing operations.
So for example, the field expression ‘0.name’ would cause get_value() to be called with a key argument of 0. The name attribute will be looked up after get_value() returns by calling the built-in getattr() function.
If the index or keyword refers to an item that does not exist, then an IndexError or KeyError should be raised.
Implement checking for unused arguments if desired. The arguments to this function is the set of all argument keys that were actually referred to in the format string (integers for positional arguments, and strings for named arguments), and a reference to the args and kwargs that was passed to vformat. The set of unused args can be calculated from these parameters. check_unused_args() is assumed to raise an exception if the check fails.
format_field() simply calls the global format() built-in. The method is provided so that subclasses can override it.
Converts the value (returned by get_field()) given a conversion type (as in the tuple returned by the parse() method). The default version understands ‘s’ (str), ‘r’ (repr) and ‘a’ (ascii) conversion types.
The str.format() method and the Formatter class share the same syntax for format strings (although in the case of Formatter, subclasses can define their own format string syntax). The syntax is related to that of formatted string literals, but it is less sophisticated and, in particular, does not support arbitrary expressions.
Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.
The grammar for a replacement field is as follows:
In less formal terms, the replacement field can start with a field_name that specifies the object whose value is to be formatted and inserted into the output instead of the replacement field. The field_name is optionally followed by a conversion field, which is preceded by an exclamation point '!', and a format_spec, which is preceded by a colon ':'. These specify a non-default format for the replacement value.
See also the Format Specification Mini-Language section.
The field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument. If the numerical arg_names in a format string are 0, 1, 2, … in sequence, they can all be omitted (not just some) and the numbers 0, 1, 2, … will be automatically inserted in that order. Because arg_name is not quote-delimited, it is not possible to specify arbitrary dictionary keys (e.g., the strings '10' or ':-]') within a format string. The arg_name can be followed by any number of index or attribute expressions. An expression of the form '.name' selects the named attribute using getattr(), while an expression of the form '[index]' does an index lookup using __getitem__().
Changed in version 3.1: The positional argument specifiers can be omitted for str.format(), so '{} {}'.format(a, b) is equivalent to '{0} {1}'.format(a, b).
Changed in version 3.4: The positional argument specifiers can be omitted for Formatter.
Some simple format string examples:
The conversion field causes a type coercion before formatting. Normally, the job of formatting a value is done by the __format__() method of the value itself. However, in some cases it is desirable to force a type to be formatted as a string, overriding its own definition of formatting. By converting the value to a string before calling __format__(), the normal formatting logic is bypassed.
Three conversion flags are currently supported: '!s' which calls str() on the value, '!r' which calls repr() and '!a' which calls ascii().
Some examples:
The format_spec field contains a specification of how the value should be presented, including such details as field width, alignment, padding, decimal precision and so on. Each value type can define its own “formatting mini-language” or interpretation of the format_spec.
Most built-in types support a common formatting mini-language, which is described in the next section.
A format_spec field can also include nested replacement fields within it. These nested replacement fields may contain a field name, conversion flag and format specification, but deeper nesting is not allowed. The replacement fields within the format_spec are substituted before the format_spec string is interpreted. This allows the formatting of a value to be dynamically specified.
See the Format examples section for some examples.
“Format specifications” are used within replacement fields contained within a format string to define how individual values are presented (see Format String Syntax and Formatted string literals). They can also be passed directly to the built-in format() function. Each formattable type may define how the format specification is to be interpreted.
Most built-in types implement the following options for format specifications, although some of the formatting options are only supported by the numeric types.
A general convention is that an empty format specification produces the same result as if you had called str() on the value. A non-empty format specification typically modifies the result.
The general form of a standard format specifier is:
If a valid align value is specified, it can be preceded by a fill character that can be any character and defaults to a space if omitted. It is not possible to use a literal curly brace (“{” or “}”) as the fill character in a formatted string literal or when using the str.format() method. However, it is possible to insert a curly brace with a nested replacement field. This limitation doesn’t affect the format() function.
The meaning of the various alignment options is as follows:
Option
Meaning
'<'
Forces the field to be left-aligned within the available space (this is the default for most objects).
'>'
Forces the field to be right-aligned within the available space (this is the default for numbers).
'='
Forces the padding to be placed after the sign (if any) but before the digits. This is used for printing fields in the form ‘+000000120’. This alignment option is only valid for numeric types. It becomes the default when ‘0’ immediately precedes the field width.
'^'
Forces the field to be centered within the available space.
Note that unless a minimum field width is defined, the field width will always be the same size as the data to fill it, so that the alignment option has no meaning in this case.
The sign option is only valid for number types, and can be one of the following:
Option
Meaning
'+'
indicates that a sign should be used for both positive as well as negative numbers.
'-'
indicates that a sign should be used only for negative numbers (this is the default behavior).
space
indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers.
The '#' option causes the “alternate form” to be used for the conversion. The alternate form is defined differently for different types. This option is only valid for integer, float and complex types. For integers, when binary, octal, or hexadecimal output is used, this option adds the prefix respective '0b', '0o', or '0x' to the output value. For float and complex the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for 'g' and 'G' conversions, trailing zeros are not removed from the result.
The ',' option signals the use of a comma for a thousands separator. For a locale aware separator, use the 'n' integer presentation type instead.
Changed in version 3.1: Added the ',' option (see also PEP 378).
The '_' option signals the use of an underscore for a thousands separator for floating point presentation types and for integer presentation type 'd'. For integer presentation types 'b', 'o', 'x', and 'X', underscores will be inserted every 4 digits. For other presentation types, specifying this option is an error.
Changed in version 3.6: Added the '_' option (see also PEP 515).
width is a decimal integer defining the minimum total field width, including any prefixes, separators, and other formatting characters. If not specified, then the field width will be determined by the content.
When no explicit alignment is given, preceding the width field by a zero ('0') character enables sign-aware zero-padding for numeric types. This is equivalent to a fill character of '0' with an alignment type of '='.
The precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating point value formatted with 'f' and 'F', or before and after the decimal point for a floating point value formatted with 'g' or 'G'. For non-number types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is not allowed for integer values.
Finally, the type determines how the data should be presented.
The available string presentation types are:
Type
Meaning
's'
String format. This is the default type for strings and may be omitted.
None
The same as 's'.
The available integer presentation types are:
Type
Meaning
'b'
Binary format. Outputs the number in base 2.
'c'
Character. Converts the integer to the corresponding unicode character before printing.
'd'
Decimal Integer. Outputs the number in base 10.
'o'
Octal format. Outputs the number in base 8.
'x'
Hex format. Outputs the number in base 16, using lower-case letters for the digits above 9.
'X'
Hex format. Outputs the number in base 16, using upper-case letters for the digits above 9.
'n'
Number. This is the same as 'd', except that it uses the current locale setting to insert the appropriate number separator characters.
None
The same as 'd'.
In addition to the above presentation types, integers can be formatted with the floating point presentation types listed below (except 'n' and None). When doing so, float() is used to convert the integer to a floating point number before formatting.
The available presentation types for float and Decimal values are:
Type
Meaning
'e'
Scientific notation. For a given precision p, formats the number in scientific notation with the letter ‘e’ separating the coefficient from the exponent. The coefficient has one digit before and p digits after the decimal point, for a total of p + 1 significant digits. With no precision given, uses a precision of 6 digits after the decimal point for float, and shows all coefficient digits for Decimal. If no digits follow the decimal point, the decimal point is also removed unless the # option is used.
'E'
Scientific notation. Same as 'e' except it uses an upper case ‘E’ as the separator character.
'f'
Fixed-point notation. For a given precision p, formats the number as a decimal number with exactly p digits following the decimal point. With no precision given, uses a precision of 6 digits after the decimal point for float, and uses a precision large enough to show all coefficient digits for Decimal. If no digits follow the decimal point, the decimal point is also removed unless the # option is used.
'F'
Fixed-point notation. Same as 'f', but converts nan to NAN and inf to INF.
'g'
General format. For a given precision p >= 1, this rounds the number to p significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. A precision of 0 is treated as equivalent to a precision of 1.
The precise rules are as follows: suppose that the result formatted with presentation type 'e' and precision p-1 would have exponent exp. Then, if m <= exp < p, where m is -4 for floats and -6 for Decimals, the number is formatted with presentation type 'f' and precision p-1-exp. Otherwise, the number is formatted with presentation type 'e' and precision p-1. In both cases insignificant trailing zeros are removed from the significand, and the decimal point is also removed if there are no remaining digits following it, unless the '#' option is used.
With no precision given, uses a precision of 6 significant digits for float. For Decimal, the coefficient of the result is formed from the coefficient digits of the value; scientific notation is used for values smaller than 1e-6 in absolute value and values where the place value of the least significant digit is larger than 1, and fixed-point notation is used otherwise.
Positive and negative infinity, positive and negative zero, and nans, are formatted as inf, -inf, 0, -0 and nan respectively, regardless of the precision.
'G'
General format. Same as 'g' except switches to 'E' if the number gets too large. The representations of infinity and NaN are uppercased, too.
'n'
Number. This is the same as 'g', except that it uses the current locale setting to insert the appropriate number separator characters.
'%'
Percentage. Multiplies the number by 100 and displays in fixed ('f') format, followed by a percent sign.
None
For float this is the same as 'g', except that when fixed-point notation is used to format the result, it always includes at least one digit past the decimal point. The precision used is as large as needed to represent the given value faithfully.
For Decimal, this is the same as either 'g' or 'G' depending on the value of context.capitals for the current decimal context.
The overall effect is to match the output of str() as altered by the other format modifiers.
This section contains examples of the str.format() syntax and comparison with the old %-formatting.
In most of the cases the syntax is similar to the old %-formatting, with the addition of the {} and with : used instead of %. For example, '%03.2f' can be translated to '{:03.2f}'.
The new format syntax also supports new and different options, shown in the following examples.
Accessing arguments by position:
Accessing arguments by name:
Accessing arguments’ attributes:
Accessing arguments’ items:
Replacing %s and %r:
Aligning the text and specifying a width:
Replacing %+f, %-f, and % f and specifying a sign:
Replacing %x and %o and converting the value to different bases:
Using the comma as a thousands separator:
Expressing a percentage:
Using type-specific formatting:
Nesting arguments and more complex examples:
Template strings provide simpler string substitutions as described in PEP 292. A primary use case for template strings is for internationalization (i18n) since in that context, the simpler syntax and functionality makes it easier to translate than other built-in string formatting facilities in Python. As an example of a library built on template strings for i18n, see the flufl.i18n package.
Template strings support $-based substitutions, using the following rules:
Any other appearance of $ in the string will result in a ValueError being raised.
The string module provides a Template class that implements these rules. The methods of Template are:
The constructor takes a single argument which is the template string.
Performs the template substitution, returning a new string. mapping is any dictionary-like object with keys that match the placeholders in the template. Alternatively, you can provide keyword arguments, where the keywords are the placeholders. When both mapping and kwds are given and there are duplicates, the placeholders from kwds take precedence.
Like substitute(), except that if placeholders are missing from mapping and kwds, instead of raising a KeyError exception, the original placeholder will appear in the resulting string intact. Also, unlike with substitute(), any other appearances of the $ will simply return $ instead of raising ValueError.
While other exceptions may still occur, this method is called “safe” because it always tries to return a usable string instead of raising an exception. In another sense, safe_substitute() may be anything other than safe, since it will silently ignore malformed templates containing dangling delimiters, unmatched braces, or placeholders that are not valid Python identifiers.
Template instances also provide one public data attribute:
This is the object passed to the constructor’s template argument. In general, you shouldn’t change it, but read-only access is not enforced.
Here is an example of how to use a Template:
Advanced usage: you can derive subclasses of Template to customize the placeholder syntax, delimiter character, or the entire regular expression used to parse template strings. To do this, you can override these class attributes:
idpattern – This is the regular expression describing the pattern for non-braced placeholders. The default value is the regular expression (?a:[_a-z][_a-z0-9]*). If this is given and braceidpattern is None this pattern will also apply to braced placeholders.
Note
Since default flags is re.IGNORECASE, pattern [a-z] can match with some non-ASCII characters. That’s why we use the local a flag here.
Changed in version 3.7: braceidpattern can be used to define separate patterns used inside and outside the braces.
braceidpattern – This is like idpattern but describes the pattern for braced placeholders. Defaults to None which means to fall back to idpattern (i.e. the same pattern is used both inside and outside braces). If given, this allows you to define different patterns for braced and unbraced placeholders.
New in version 3.7.
flags – The regular expression flags that will be applied when compiling the regular expression used for recognizing substitutions. The default value is re.IGNORECASE. Note that re.VERBOSE will always be added to the flags, so custom idpatterns must follow conventions for verbose regular expressions.
New in version 3.2.
Alternatively, you can provide the entire regular expression pattern by overriding the class attribute pattern. If you do this, the value must be a regular expression object with four named capturing groups. The capturing groups correspond to the rules given above, along with the invalid placeholder rule:
Split the argument into words using str.split(), capitalize each word using str.capitalize(), and join the capitalized words using str.join(). If the optional second argument sep is absent or None, runs of whitespace characters are replaced by a single space and leading and trailing whitespace are removed, otherwise sep is used to split and join the words.


The concatenation of the ascii_lowercase and ascii_uppercase constants described below. This value is not locale-dependent.


The lowercase letters 'abcdefghijklmnopqrstuvwxyz'. This value is not locale-dependent and will not change.


The uppercase letters 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'. This value is not locale-dependent and will not change.


Split the argument into words using str.split(), capitalize each word using str.capitalize(), and join the capitalized words using str.join(). If the optional second argument sep is absent or None, runs of whitespace characters are replaced by a single space and leading and trailing whitespace are removed, otherwise sep is used to split and join the words.


The Formatter class has the following public methods:
The primary API method. It takes a format string and an arbitrary set of positional and keyword arguments. It is just a wrapper that calls vformat().
Changed in version 3.7: A format string argument is now positional-only.
This function does the actual work of formatting. It is exposed as a separate function for cases where you want to pass in a predefined dictionary of arguments, rather than unpacking and repacking the dictionary as individual arguments using the *args and **kwargs syntax. vformat() does the work of breaking up the format string into character data and replacement fields. It calls the various methods described below.
In addition, the Formatter defines a number of methods that are intended to be replaced by subclasses:
Loop over the format_string and return an iterable of tuples (literal_text, field_name, format_spec, conversion). This is used by vformat() to break the string into either literal text, or replacement fields.
The values in the tuple conceptually represent a span of literal text followed by a single replacement field. If there is no literal text (which can happen if two replacement fields occur consecutively), then literal_text will be a zero-length string. If there is no replacement field, then the values of field_name, format_spec and conversion will be None.
Given field_name as returned by parse() (see above), convert it to an object to be formatted. Returns a tuple (obj, used_key). The default version takes strings of the form defined in PEP 3101, such as “0[name]” or “label.title”. args and kwargs are as passed in to vformat(). The return value used_key has the same meaning as the key parameter to get_value().
Retrieve a given field value. The key argument will be either an integer or a string. If it is an integer, it represents the index of the positional argument in args; if it is a string, then it represents a named argument in kwargs.
The args parameter is set to the list of positional arguments to vformat(), and the kwargs parameter is set to the dictionary of keyword arguments.
For compound field names, these functions are only called for the first component of the field name; subsequent components are handled through normal attribute and indexing operations.
So for example, the field expression ‘0.name’ would cause get_value() to be called with a key argument of 0. The name attribute will be looked up after get_value() returns by calling the built-in getattr() function.
If the index or keyword refers to an item that does not exist, then an IndexError or KeyError should be raised.
Implement checking for unused arguments if desired. The arguments to this function is the set of all argument keys that were actually referred to in the format string (integers for positional arguments, and strings for named arguments), and a reference to the args and kwargs that was passed to vformat. The set of unused args can be calculated from these parameters. check_unused_args() is assumed to raise an exception if the check fails.
format_field() simply calls the global format() built-in. The method is provided so that subclasses can override it.
Converts the value (returned by get_field()) given a conversion type (as in the tuple returned by the parse() method). The default version understands ‘s’ (str), ‘r’ (repr) and ‘a’ (ascii) conversion types.


Implement checking for unused arguments if desired. The arguments to this function is the set of all argument keys that were actually referred to in the format string (integers for positional arguments, and strings for named arguments), and a reference to the args and kwargs that was passed to vformat. The set of unused args can be calculated from these parameters. check_unused_args() is assumed to raise an exception if the check fails.


Converts the value (returned by get_field()) given a conversion type (as in the tuple returned by the parse() method). The default version understands ‘s’ (str), ‘r’ (repr) and ‘a’ (ascii) conversion types.


The primary API method. It takes a format string and an arbitrary set of positional and keyword arguments. It is just a wrapper that calls vformat().
Changed in version 3.7: A format string argument is now positional-only.


format_field() simply calls the global format() built-in. The method is provided so that subclasses can override it.


Given field_name as returned by parse() (see above), convert it to an object to be formatted. Returns a tuple (obj, used_key). The default version takes strings of the form defined in PEP 3101, such as “0[name]” or “label.title”. args and kwargs are as passed in to vformat(). The return value used_key has the same meaning as the key parameter to get_value().


Retrieve a given field value. The key argument will be either an integer or a string. If it is an integer, it represents the index of the positional argument in args; if it is a string, then it represents a named argument in kwargs.
The args parameter is set to the list of positional arguments to vformat(), and the kwargs parameter is set to the dictionary of keyword arguments.
For compound field names, these functions are only called for the first component of the field name; subsequent components are handled through normal attribute and indexing operations.
So for example, the field expression ‘0.name’ would cause get_value() to be called with a key argument of 0. The name attribute will be looked up after get_value() returns by calling the built-in getattr() function.
If the index or keyword refers to an item that does not exist, then an IndexError or KeyError should be raised.


Loop over the format_string and return an iterable of tuples (literal_text, field_name, format_spec, conversion). This is used by vformat() to break the string into either literal text, or replacement fields.
The values in the tuple conceptually represent a span of literal text followed by a single replacement field. If there is no literal text (which can happen if two replacement fields occur consecutively), then literal_text will be a zero-length string. If there is no replacement field, then the values of field_name, format_spec and conversion will be None.


This function does the actual work of formatting. It is exposed as a separate function for cases where you want to pass in a predefined dictionary of arguments, rather than unpacking and repacking the dictionary as individual arguments using the *args and **kwargs syntax. vformat() does the work of breaking up the format string into character data and replacement fields. It calls the various methods described below.


String of ASCII characters which are considered printable. This is a combination of digits, ascii_letters, punctuation, and whitespace.


String of ASCII characters which are considered punctuation characters in the C locale: !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~.


The constructor takes a single argument which is the template string.
Performs the template substitution, returning a new string. mapping is any dictionary-like object with keys that match the placeholders in the template. Alternatively, you can provide keyword arguments, where the keywords are the placeholders. When both mapping and kwds are given and there are duplicates, the placeholders from kwds take precedence.
Like substitute(), except that if placeholders are missing from mapping and kwds, instead of raising a KeyError exception, the original placeholder will appear in the resulting string intact. Also, unlike with substitute(), any other appearances of the $ will simply return $ instead of raising ValueError.
While other exceptions may still occur, this method is called “safe” because it always tries to return a usable string instead of raising an exception. In another sense, safe_substitute() may be anything other than safe, since it will silently ignore malformed templates containing dangling delimiters, unmatched braces, or placeholders that are not valid Python identifiers.
Template instances also provide one public data attribute:
This is the object passed to the constructor’s template argument. In general, you shouldn’t change it, but read-only access is not enforced.


Like substitute(), except that if placeholders are missing from mapping and kwds, instead of raising a KeyError exception, the original placeholder will appear in the resulting string intact. Also, unlike with substitute(), any other appearances of the $ will simply return $ instead of raising ValueError.
While other exceptions may still occur, this method is called “safe” because it always tries to return a usable string instead of raising an exception. In another sense, safe_substitute() may be anything other than safe, since it will silently ignore malformed templates containing dangling delimiters, unmatched braces, or placeholders that are not valid Python identifiers.


Performs the template substitution, returning a new string. mapping is any dictionary-like object with keys that match the placeholders in the template. Alternatively, you can provide keyword arguments, where the keywords are the placeholders. When both mapping and kwds are given and there are duplicates, the placeholders from kwds take precedence.


This is the object passed to the constructor’s template argument. In general, you shouldn’t change it, but read-only access is not enforced.


A string containing all ASCII characters that are considered whitespace. This includes the characters space, tab, linefeed, return, formfeed, and vertical tab.


Source code: Lib/stringprep.py
When identifying things (such as host names) in the internet, it is often necessary to compare such identifications for “equality”. Exactly how this comparison is executed may depend on the application domain, e.g. whether it should be case-insensitive or not. It may be also necessary to restrict the possible identifications, to allow only identifications consisting of “printable” characters.
RFC 3454 defines a procedure for “preparing” Unicode strings in internet protocols. Before passing strings onto the wire, they are processed with the preparation procedure, after which they have a certain normalized form. The RFC defines a set of tables, which can be combined into profiles. Each profile must define which tables it uses, and what other optional parts of the stringprep procedure are part of the profile. One example of a stringprep profile is nameprep, which is used for internationalized domain names.
The module stringprep only exposes the tables from RFC 3454. As these tables would be very large to represent them as dictionaries or lists, the module uses the Unicode character database internally. The module source code itself was generated using the mkstringprep.py utility.
As a result, these tables are exposed as functions, not as data structures. There are two kinds of tables in the RFC: sets and mappings. For a set, stringprep provides the “characteristic function”, i.e. a function that returns True if the parameter is part of the set. For mappings, it provides the mapping function: given the key, it returns the associated value. Below is a list of all functions available in the module.
Determine whether code is in tableA.1 (Unassigned code points in Unicode 3.2).
Determine whether code is in tableB.1 (Commonly mapped to nothing).
Return the mapped value for code according to tableB.2 (Mapping for case-folding used with NFKC).
Return the mapped value for code according to tableB.3 (Mapping for case-folding used with no normalization).
Determine whether code is in tableC.1.1 (ASCII space characters).
Determine whether code is in tableC.1.2 (Non-ASCII space characters).
Determine whether code is in tableC.1 (Space characters, union of C.1.1 and C.1.2).
Determine whether code is in tableC.2.1 (ASCII control characters).
Determine whether code is in tableC.2.2 (Non-ASCII control characters).
Determine whether code is in tableC.2 (Control characters, union of C.2.1 and C.2.2).
Determine whether code is in tableC.3 (Private use).
Determine whether code is in tableC.4 (Non-character code points).
Determine whether code is in tableC.5 (Surrogate codes).
Determine whether code is in tableC.6 (Inappropriate for plain text).
Determine whether code is in tableC.7 (Inappropriate for canonical representation).
Determine whether code is in tableC.8 (Change display properties or are deprecated).
Determine whether code is in tableC.9 (Tagging characters).
Determine whether code is in tableD.1 (Characters with bidirectional property “R” or “AL”).
Determine whether code is in tableD.2 (Characters with bidirectional property “L”).


Determine whether code is in tableA.1 (Unassigned code points in Unicode 3.2).


Determine whether code is in tableB.1 (Commonly mapped to nothing).


Determine whether code is in tableC.1.1 (ASCII space characters).


Determine whether code is in tableC.1 (Space characters, union of C.1.1 and C.1.2).


Determine whether code is in tableC.1.2 (Non-ASCII space characters).


Determine whether code is in tableC.2.1 (ASCII control characters).


Determine whether code is in tableC.2 (Control characters, union of C.2.1 and C.2.2).


Determine whether code is in tableC.2.2 (Non-ASCII control characters).


Determine whether code is in tableC.3 (Private use).


Determine whether code is in tableC.4 (Non-character code points).


Determine whether code is in tableC.5 (Surrogate codes).


Determine whether code is in tableC.6 (Inappropriate for plain text).


Determine whether code is in tableC.7 (Inappropriate for canonical representation).


Determine whether code is in tableC.8 (Change display properties or are deprecated).


Determine whether code is in tableC.9 (Tagging characters).


Determine whether code is in tableD.1 (Characters with bidirectional property “R” or “AL”).


Determine whether code is in tableD.2 (Characters with bidirectional property “L”).


Return the mapped value for code according to tableB.2 (Mapping for case-folding used with NFKC).


Return the mapped value for code according to tableB.3 (Mapping for case-folding used with no normalization).


Source code: Lib/struct.py
This module performs conversions between Python values and C structs represented as Python bytes objects. This can be used in handling binary data stored in files or from network connections, among other sources. It uses Format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Python values.
Note
By default, the result of packing a given C struct includes pad bytes in order to maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. This behavior is chosen so that the bytes of a packed struct correspond exactly to the layout in memory of the corresponding C struct. To handle platform-independent data formats or omit implicit pad bytes, use standard size and alignment instead of native size and alignment: see Byte Order, Size, and Alignment for details.
Several struct functions (and methods of Struct) take a buffer argument. This refers to objects that implement the Buffer Protocol and provide either a readable or read-writable buffer. The most common types used for that purpose are bytes and bytearray, but many other types that can be viewed as an array of bytes implement the buffer protocol, so that they can be read/filled without additional copying from a bytes object.
The module defines the following exception and functions:
Exception raised on various occasions; argument is a string describing what is wrong.
Return a bytes object containing the values v1, v2, … packed according to the format string format. The arguments must match the values required by the format exactly.
Pack the values v1, v2, … according to the format string format and write the packed bytes into the writable buffer buffer starting at position offset. Note that offset is a required argument.
Unpack from the buffer buffer (presumably packed by pack(format, ...)) according to the format string format. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes must match the size required by the format, as reflected by calcsize().
Unpack from buffer starting at position offset, according to the format string format. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes, starting at position offset, must be at least the size required by the format, as reflected by calcsize().
Iteratively unpack from the buffer buffer according to the format string format. This function returns an iterator which will read equally-sized chunks from the buffer until all its contents have been consumed. The buffer’s size in bytes must be a multiple of the size required by the format, as reflected by calcsize().
Each iteration yields a tuple as specified by the format string.
New in version 3.4.
Return the size of the struct (and hence of the bytes object produced by pack(format, ...)) corresponding to the format string format.
Format strings are the mechanism used to specify the expected layout when packing and unpacking data. They are built up from Format Characters, which specify the type of data being packed/unpacked. In addition, there are special characters for controlling the Byte Order, Size, and Alignment.
By default, C types are represented in the machine’s native format and byte order, and properly aligned by skipping pad bytes if necessary (according to the rules used by the C compiler).
Alternatively, the first character of the format string can be used to indicate the byte order, size and alignment of the packed data, according to the following table:
Character
Byte order
Size
Alignment
@
native
native
native
=
native
standard
none
<
little-endian
standard
none
>
big-endian
standard
none
!
network (= big-endian)
standard
none
If the first character is not one of these, '@' is assumed.
Native byte order is big-endian or little-endian, depending on the host system. For example, Intel x86 and AMD64 (x86-64) are little-endian; Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature switchable endianness (bi-endian). Use sys.byteorder to check the endianness of your system.
Native size and alignment are determined using the C compiler’s sizeof expression. This is always combined with native byte order.
Standard size depends only on the format character; see the table in the Format Characters section.
Note the difference between '@' and '=': both use native byte order, but the size and alignment of the latter is standardized.
The form '!' represents the network byte order which is always big-endian as defined in IETF RFC 1700.
There is no way to indicate non-native byte order (force byte-swapping); use the appropriate choice of '<' or '>'.
Notes:
Format characters have the following meaning; the conversion between C and Python values should be obvious given their types. The ‘Standard size’ column refers to the size of the packed value in bytes when using standard size; that is, when the format string starts with one of '<', '>', '!' or '='. When using native size, the size of the packed value is platform-dependent.
Format
C Type
Python type
Standard size
Notes
x
pad byte
no value
c
char
bytes of length 1
1
b
signed char
integer
1
(1), (2)
B
unsigned char
integer
1
(2)
?
_Bool
bool
1
(1)
h
short
integer
2
(2)
H
unsigned short
integer
2
(2)
i
int
integer
4
(2)
I
unsigned int
integer
4
(2)
l
long
integer
4
(2)
L
unsigned long
integer
4
(2)
q
long long
integer
8
(2)
Q
unsigned long
long
integer
8
(2)
n
ssize_t
integer
(3)
N
size_t
integer
(3)
e
(6)
float
2
(4)
f
float
float
4
(4)
d
double
float
8
(4)
s
char[]
bytes
p
char[]
bytes
P
void *
integer
(5)
Changed in version 3.3: Added support for the 'n' and 'N' formats.
Changed in version 3.6: Added support for the 'e' format.
Notes:
When attempting to pack a non-integer using any of the integer conversion codes, if the non-integer has a __index__() method then that method is called to convert the argument to an integer before packing.
Changed in version 3.2: Added use of the __index__() method for non-integers.
A format character may be preceded by an integral repeat count. For example, the format string '4h' means exactly the same as 'hhhh'.
Whitespace characters between formats are ignored; a count and its format must not contain whitespace though.
For the 's' format character, the count is interpreted as the length of the bytes, not a repeat count like for the other format characters; for example, '10s' means a single 10-byte string, while '10c' means 10 characters. If a count is not given, it defaults to 1. For packing, the string is truncated or padded with null bytes as appropriate to make it fit. For unpacking, the resulting bytes object always has exactly the specified number of bytes. As a special case, '0s' means a single, empty string (while '0c' means 0 characters).
When packing a value x using one of the integer formats ('b', 'B', 'h', 'H', 'i', 'I', 'l', 'L', 'q', 'Q'), if x is outside the valid range for that format then struct.error is raised.
Changed in version 3.1: Previously, some of the integer formats wrapped out-of-range values and raised DeprecationWarning instead of struct.error.
The 'p' format character encodes a “Pascal string”, meaning a short variable-length string stored in a fixed number of bytes, given by the count. The first byte stored is the length of the string, or 255, whichever is smaller. The bytes of the string follow. If the string passed in to pack() is too long (longer than the count minus 1), only the leading count-1 bytes of the string are stored. If the string is shorter than count-1, it is padded with null bytes so that exactly count bytes in all are used. Note that for unpack(), the 'p' format character consumes count bytes, but that the string returned can never contain more than 255 bytes.
For the '?' format character, the return value is either True or False. When packing, the truth value of the argument object is used. Either 0 or 1 in the native or standard bool representation will be packed, and any non-zero value will be True when unpacking.
Note
All examples assume a native byte order, size, and alignment with a big-endian machine.
A basic example of packing/unpacking three integers:
Unpacked fields can be named by assigning them to variables or by wrapping the result in a named tuple:
The ordering of format characters may have an impact on size since the padding needed to satisfy alignment requirements is different:
The following format 'llh0l' specifies two pad bytes at the end, assuming longs are aligned on 4-byte boundaries:
This only works when native size and alignment are in effect; standard size and alignment does not enforce any alignment.
See also
Packed binary storage of homogeneous data.
Packing and unpacking of XDR data.
The struct module also defines the following type:
Return a new Struct object which writes and reads binary data according to the format string format. Creating a Struct object once and calling its methods is more efficient than calling the struct functions with the same format since the format string only needs to be compiled once.
Note
The compiled versions of the most recent format strings passed to Struct and the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single Struct instance.
Compiled Struct objects support the following methods and attributes:
Identical to the pack() function, using the compiled format. (len(result) will equal size.)
Identical to the pack_into() function, using the compiled format.
Identical to the unpack() function, using the compiled format. The buffer’s size in bytes must equal size.
Identical to the unpack_from() function, using the compiled format. The buffer’s size in bytes, starting at position offset, must be at least size.
Identical to the iter_unpack() function, using the compiled format. The buffer’s size in bytes must be a multiple of size.
New in version 3.4.
The format string used to construct this Struct object.
Changed in version 3.7: The format string type is now str instead of bytes.
The calculated size of the struct (and hence of the bytes object produced by the pack() method) corresponding to format.


Return the size of the struct (and hence of the bytes object produced by pack(format, ...)) corresponding to the format string format.


Exception raised on various occasions; argument is a string describing what is wrong.


Iteratively unpack from the buffer buffer according to the format string format. This function returns an iterator which will read equally-sized chunks from the buffer until all its contents have been consumed. The buffer’s size in bytes must be a multiple of the size required by the format, as reflected by calcsize().
Each iteration yields a tuple as specified by the format string.
New in version 3.4.


Return a bytes object containing the values v1, v2, … packed according to the format string format. The arguments must match the values required by the format exactly.


Pack the values v1, v2, … according to the format string format and write the packed bytes into the writable buffer buffer starting at position offset. Note that offset is a required argument.


Return a new Struct object which writes and reads binary data according to the format string format. Creating a Struct object once and calling its methods is more efficient than calling the struct functions with the same format since the format string only needs to be compiled once.
Note
The compiled versions of the most recent format strings passed to Struct and the module-level functions are cached, so programs that use only a few format strings needn’t worry about reusing a single Struct instance.
Compiled Struct objects support the following methods and attributes:
Identical to the pack() function, using the compiled format. (len(result) will equal size.)
Identical to the pack_into() function, using the compiled format.
Identical to the unpack() function, using the compiled format. The buffer’s size in bytes must equal size.
Identical to the unpack_from() function, using the compiled format. The buffer’s size in bytes, starting at position offset, must be at least size.
Identical to the iter_unpack() function, using the compiled format. The buffer’s size in bytes must be a multiple of size.
New in version 3.4.
The format string used to construct this Struct object.
Changed in version 3.7: The format string type is now str instead of bytes.
The calculated size of the struct (and hence of the bytes object produced by the pack() method) corresponding to format.


The format string used to construct this Struct object.
Changed in version 3.7: The format string type is now str instead of bytes.


Identical to the iter_unpack() function, using the compiled format. The buffer’s size in bytes must be a multiple of size.
New in version 3.4.


Identical to the pack() function, using the compiled format. (len(result) will equal size.)


Identical to the pack_into() function, using the compiled format.


The calculated size of the struct (and hence of the bytes object produced by the pack() method) corresponding to format.


Identical to the unpack() function, using the compiled format. The buffer’s size in bytes must equal size.


Identical to the unpack_from() function, using the compiled format. The buffer’s size in bytes, starting at position offset, must be at least size.


Unpack from the buffer buffer (presumably packed by pack(format, ...)) according to the format string format. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes must match the size required by the format, as reflected by calcsize().


Unpack from buffer starting at position offset, according to the format string format. The result is a tuple even if it contains exactly one item. The buffer’s size in bytes, starting at position offset, must be at least the size required by the format, as reflected by calcsize().


Source code: Lib/subprocess.py
The subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several older modules and functions:
Information about how the subprocess module can be used to replace these modules and functions can be found in the following sections.
See also
PEP 324 – PEP proposing the subprocess module
The recommended approach to invoking subprocesses is to use the run() function for all use cases it can handle. For more advanced use cases, the underlying Popen interface can be used directly.
The run() function was added in Python 3.5; if you need to retain compatibility with older versions, see the Older high-level API section.
Run the command described by args. Wait for command to complete, then return a CompletedProcess instance.
The arguments shown above are merely the most common ones, described below in Frequently Used Arguments (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the Popen constructor - most of the arguments to this function are passed through to that interface. (timeout, input, check, and capture_output are not.)
If capture_output is true, stdout and stderr will be captured. When used, the internal Popen object is automatically created with stdout=PIPE and stderr=PIPE. The stdout and stderr arguments may not be supplied at the same time as capture_output. If you wish to capture and combine both streams into one, use stdout=PIPE and stderr=STDOUT instead of capture_output.
The timeout argument is passed to Popen.communicate(). If the timeout expires, the child process will be killed and waited for. The TimeoutExpired exception will be re-raised after the child process has terminated.
The input argument is passed to Popen.communicate() and thus to the subprocess’s stdin. If used it must be a byte sequence, or a string if encoding or errors is specified or text is true. When used, the internal Popen object is automatically created with stdin=PIPE, and the stdin argument may not be used as well.
If check is true, and the process exits with a non-zero exit code, a CalledProcessError exception will be raised. Attributes of that exception hold the arguments, the exit code, and stdout and stderr if they were captured.
If encoding or errors are specified, or text is true, file objects for stdin, stdout and stderr are opened in text mode using the specified encoding and errors or the io.TextIOWrapper default. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.
If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process’ environment. It is passed directly to Popen.
Examples:
New in version 3.5.
Changed in version 3.6: Added encoding and errors parameters
Changed in version 3.7: Added the text parameter, as a more understandable alias of universal_newlines. Added the capture_output parameter.
The return value from run(), representing a process that has finished.
The arguments used to launch the process. This may be a list or a string.
Exit status of the child process. Typically, an exit status of 0 indicates that it ran successfully.
A negative value -N indicates that the child was terminated by signal N (POSIX only).
Captured stdout from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stdout was not captured.
If you ran the process with stderr=subprocess.STDOUT, stdout and stderr will be combined in this attribute, and stderr will be None.
Captured stderr from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stderr was not captured.
If returncode is non-zero, raise a CalledProcessError.
New in version 3.5.
Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that the special file os.devnull will be used.
New in version 3.3.
Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that a pipe to the standard stream should be opened. Most useful with Popen.communicate().
Special value that can be used as the stderr argument to Popen and indicates that standard error should go into the same handle as standard output.
Base class for all other exceptions from this module.
New in version 3.3.
Subclass of SubprocessError, raised when a timeout expires while waiting for a child process.
Command that was used to spawn the child process.
Timeout in seconds.
Output of the child process if it was captured by run() or check_output(). Otherwise, None.
Alias for output, for symmetry with stderr.
Stderr output of the child process if it was captured by run(). Otherwise, None.
New in version 3.3.
Changed in version 3.5: stdout and stderr attributes added
Subclass of SubprocessError, raised when a process run by check_call() or check_output() returns a non-zero exit status.
Exit status of the child process. If the process exited due to a signal, this will be the negative signal number.
Command that was used to spawn the child process.
Output of the child process if it was captured by run() or check_output(). Otherwise, None.
Alias for output, for symmetry with stderr.
Stderr output of the child process if it was captured by run(). Otherwise, None.
Changed in version 3.5: stdout and stderr attributes added
To support a wide variety of use cases, the Popen constructor (and the convenience functions) accept a large number of optional arguments. For most typical use cases, many of these arguments can be safely left at their default values. The arguments that are most commonly needed are:
args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be True (see below) or else the string must simply name the program to be executed without specifying any arguments.
stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE, DEVNULL, an existing file descriptor (a positive integer), an existing file object, and None. PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. With the default settings of None, no redirection will occur; the child’s file handles will be inherited from the parent. Additionally, stderr can be STDOUT, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.
If encoding or errors are specified, or text (also known as universal_newlines) is true, the file objects stdin, stdout and stderr will be opened in text mode using the encoding and errors specified in the call or the defaults for io.TextIOWrapper.
For stdin, line ending characters '\n' in the input will be converted to the default line separator os.linesep. For stdout and stderr, all line endings in the output will be converted to '\n'. For more information see the documentation of the io.TextIOWrapper class when the newline argument to its constructor is None.
If text mode is not used, stdin, stdout and stderr will be opened as binary streams. No encoding or line ending conversion is performed.
New in version 3.6: Added encoding and errors parameters.
New in version 3.7: Added the text parameter as an alias for universal_newlines.
Note
The newlines attribute of the file objects Popen.stdin, Popen.stdout and Popen.stderr are not updated by the Popen.communicate() method.
If shell is True, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ~ to a user’s home directory. However, note that Python itself offers implementations of many shell-like features (in particular, glob, fnmatch, os.walk(), os.path.expandvars(), os.path.expanduser(), and shutil).
Changed in version 3.3: When universal_newlines is True, the class uses the encoding locale.getpreferredencoding(False) instead of locale.getpreferredencoding(). See the io.TextIOWrapper class for more information on this change.
Note
Read the Security Considerations section before using shell=True.
These options, along with all of the other options, are described in more detail in the Popen constructor documentation.
The underlying process creation and management in this module is handled by the Popen class. It offers a lot of flexibility so that developers are able to handle the less common cases not covered by the convenience functions.
Execute a child program in a new process. On POSIX, the class uses os.execvp()-like behavior to execute the child program. On Windows, the class uses the Windows CreateProcess() function. The arguments to Popen are as follows.
args should be a sequence of program arguments or else a single string or path-like object. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.
An example of passing some arguments to an external program as a sequence is:
On POSIX, if args is a string, the string is interpreted as the name or path of the program to execute. However, this can only be done if not passing arguments to the program.
Note
It may not be obvious how to break a shell command into a sequence of arguments, especially in complex cases. shlex.split() can illustrate how to determine the correct tokenization for args:
Note in particular that options (such as -input) and arguments (such as eggs.txt) that are separated by whitespace in the shell go in separate list elements, while arguments that need quoting or backslash escaping when used in the shell (such as filenames containing spaces or the echo command shown above) are single list elements.
On Windows, if args is a sequence, it will be converted to a string in a manner described in Converting an argument sequence to a string on Windows. This is because the underlying CreateProcess() operates on strings.
Changed in version 3.6: args parameter accepts a path-like object if shell is False and a sequence containing path-like objects on POSIX.
Changed in version 3.8: args parameter accepts a path-like object if shell is False and a sequence containing bytes and path-like objects on Windows.
The shell argument (which defaults to False) specifies whether to use the shell as the program to execute. If shell is True, it is recommended to pass args as a string rather than as a sequence.
On POSIX with shell=True, the shell defaults to /bin/sh. If args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, Popen does the equivalent of:
On Windows with shell=True, the COMSPEC environment variable specifies the default shell. The only time you need to specify shell=True on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy). You do not need shell=True to run a batch file or console-based executable.
Note
Read the Security Considerations section before using shell=True.
bufsize will be supplied as the corresponding argument to the open() function when creating the stdin/stdout/stderr pipe file objects:
Changed in version 3.3.1: bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In versions prior to Python 3.2.4 and 3.3.1 it incorrectly defaulted to 0 which was unbuffered and allowed short reads. This was unintentional and did not match the behavior of Python 2 as most code expected.
The executable argument specifies a replacement program to execute. It is very seldom needed. When shell=False, executable replaces the program to execute specified by args. However, the original args is still passed to the program. Most programs treat the program specified by args as the command name, which can then be different from the program actually executed. On POSIX, the args name becomes the display name for the executable in utilities such as ps. If shell=True, on POSIX the executable argument specifies a replacement shell for the default /bin/sh.
Changed in version 3.6: executable parameter accepts a path-like object on POSIX.
Changed in version 3.8: executable parameter accepts a bytes and path-like object on Windows.
stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are PIPE, DEVNULL, an existing file descriptor (a positive integer), an existing file object, and None. PIPE indicates that a new pipe to the child should be created. DEVNULL indicates that the special file os.devnull will be used. With the default settings of None, no redirection will occur; the child’s file handles will be inherited from the parent. Additionally, stderr can be STDOUT, which indicates that the stderr data from the applications should be captured into the same file handle as for stdout.
If preexec_fn is set to a callable object, this object will be called in the child process just before the child is executed. (POSIX only)
Warning
The preexec_fn parameter is not safe to use in the presence of threads in your application. The child process could deadlock before exec is called. If you must use it, keep it trivial! Minimize the number of libraries you call into.
Note
If you need to modify the environment for the child use the env parameter rather than doing it in a preexec_fn. The start_new_session parameter can take the place of a previously common use of preexec_fn to call os.setsid() in the child.
Changed in version 3.8: The preexec_fn parameter is no longer supported in subinterpreters. The use of the parameter in a subinterpreter raises RuntimeError. The new restriction may affect applications that are deployed in mod_wsgi, uWSGI, and other embedded environments.
If close_fds is true, all file descriptors except 0, 1 and 2 will be closed before the child process is executed. Otherwise when close_fds is false, file descriptors obey their inheritable flag as described in Inheritance of File Descriptors.
On Windows, if close_fds is true then no handles will be inherited by the child process unless explicitly passed in the handle_list element of STARTUPINFO.lpAttributeList, or by standard handle redirection.
Changed in version 3.2: The default for close_fds was changed from False to what is described above.
Changed in version 3.7: On Windows the default for close_fds was changed from False to True when redirecting the standard handles. It’s now possible to set close_fds to True when redirecting the standard handles.
pass_fds is an optional sequence of file descriptors to keep open between the parent and child. Providing any pass_fds forces close_fds to be True. (POSIX only)
Changed in version 3.2: The pass_fds parameter was added.
If cwd is not None, the function changes the working directory to cwd before executing the child. cwd can be a string, bytes or path-like object. In particular, the function looks for executable (or for the first item in args) relative to cwd if the executable path is a relative path.
Changed in version 3.6: cwd parameter accepts a path-like object on POSIX.
Changed in version 3.7: cwd parameter accepts a path-like object on Windows.
Changed in version 3.8: cwd parameter accepts a bytes object on Windows.
If restore_signals is true (the default) all signals that Python has set to SIG_IGN are restored to SIG_DFL in the child process before the exec. Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. (POSIX only)
Changed in version 3.2: restore_signals was added.
If start_new_session is true the setsid() system call will be made in the child process prior to the execution of the subprocess. (POSIX only)
Changed in version 3.2: start_new_session was added.
If group is not None, the setregid() system call will be made in the child process prior to the execution of the subprocess. If the provided value is a string, it will be looked up via grp.getgrnam() and the value in gr_gid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)
Availability: POSIX
New in version 3.9.
If extra_groups is not None, the setgroups() system call will be made in the child process prior to the execution of the subprocess. Strings provided in extra_groups will be looked up via grp.getgrnam() and the values in gr_gid will be used. Integer values will be passed verbatim. (POSIX only)
Availability: POSIX
New in version 3.9.
If user is not None, the setreuid() system call will be made in the child process prior to the execution of the subprocess. If the provided value is a string, it will be looked up via pwd.getpwnam() and the value in pw_uid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)
Availability: POSIX
New in version 3.9.
If umask is not negative, the umask() system call will be made in the child process prior to the execution of the subprocess.
Availability: POSIX
New in version 3.9.
If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process’ environment.
Note
If specified, env must provide any variables required for the program to execute. On Windows, in order to run a side-by-side assembly the specified env must include a valid SystemRoot.
If encoding or errors are specified, or text is true, the file objects stdin, stdout and stderr are opened in text mode with the specified encoding and errors, as described above in Frequently Used Arguments. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.
New in version 3.6: encoding and errors were added.
New in version 3.7: text was added as a more readable alias for universal_newlines.
If given, startupinfo will be a STARTUPINFO object, which is passed to the underlying CreateProcess function. creationflags, if given, can be one or more of the following flags:
Popen objects are supported as context managers via the with statement: on exit, standard file descriptors are closed, and the process is waited for.
Popen and the other functions in this module that use it raise an auditing event subprocess.Popen with arguments executable, args, cwd, and env. The value for args may be a single string or a list of strings, depending on platform.
Changed in version 3.2: Added context manager support.
Changed in version 3.6: Popen destructor now emits a ResourceWarning warning if the child process is still running.
Changed in version 3.8: Popen can use os.posix_spawn() in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, Popen constructor using os.posix_spawn() no longer raise an exception on errors like missing program, but the child process fails with a non-zero returncode.
Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent.
The most common exception raised is OSError. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for OSError exceptions.
A ValueError will be raised if Popen is called with invalid arguments.
check_call() and check_output() will raise CalledProcessError if the called process returns a non-zero return code.
All of the functions and methods that accept a timeout parameter, such as call() and Popen.communicate() will raise TimeoutExpired if the timeout expires before the process exits.
Exceptions defined in this module all inherit from SubprocessError.
New in version 3.3: The SubprocessError base class was added.
Unlike some other popen functions, this implementation will never implicitly call a system shell. This means that all characters, including shell metacharacters, can safely be passed to child processes. If the shell is invoked explicitly, via shell=True, it is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities.
When using shell=True, the shlex.quote() function can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.
Instances of the Popen class have the following methods:
Check if child process has terminated. Set and return returncode attribute. Otherwise, returns None.
Wait for child process to terminate. Set and return returncode attribute.
If the process does not terminate after timeout seconds, raise a TimeoutExpired exception. It is safe to catch this exception and retry the wait.
Note
This will deadlock when using stdout=PIPE or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use Popen.communicate() when using pipes to avoid that.
Note
The function is implemented using a busy loop (non-blocking call and short sleeps). Use the asyncio module for an asynchronous wait: see asyncio.create_subprocess_exec.
Changed in version 3.3: timeout was added.
Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate and set the returncode attribute. The optional input argument should be data to be sent to the child process, or None, if no data should be sent to the child. If streams were opened in text mode, input must be a string. Otherwise, it must be bytes.
communicate() returns a tuple (stdout_data, stderr_data). The data will be strings if streams were opened in text mode; otherwise, bytes.
Note that if you want to send data to the process’s stdin, you need to create the Popen object with stdin=PIPE. Similarly, to get anything other than None in the result tuple, you need to give stdout=PIPE and/or stderr=PIPE too.
If the process does not terminate after timeout seconds, a TimeoutExpired exception will be raised. Catching this exception and retrying communication will not lose any output.
The child process is not killed if the timeout expires, so in order to cleanup properly a well-behaved application should kill the child process and finish communication:
Note
The data read is buffered in memory, so do not use this method if the data size is large or unlimited.
Changed in version 3.3: timeout was added.
Sends the signal signal to the child.
Do nothing if the process completed.
Note
On Windows, SIGTERM is an alias for terminate(). CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP.
Stop the child. On POSIX OSs the method sends SIGTERM to the child. On Windows the Win32 API function TerminateProcess() is called to stop the child.
Kills the child. On POSIX OSs the function sends SIGKILL to the child. On Windows kill() is an alias for terminate().
The following attributes are also available:
The args argument as it was passed to Popen – a sequence of program arguments or else a single string.
New in version 3.3.
If the stdin argument was PIPE, this attribute is a writeable stream object as returned by open(). If the encoding or errors arguments were specified or the universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stdin argument was not PIPE, this attribute is None.
If the stdout argument was PIPE, this attribute is a readable stream object as returned by open(). Reading from the stream provides output from the child process. If the encoding or errors arguments were specified or the universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stdout argument was not PIPE, this attribute is None.
If the stderr argument was PIPE, this attribute is a readable stream object as returned by open(). Reading from the stream provides error output from the child process. If the encoding or errors arguments were specified or the universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stderr argument was not PIPE, this attribute is None.
Warning
Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.
The process ID of the child process.
Note that if you set the shell argument to True, this is the process ID of the spawned shell.
The child return code, set by poll() and wait() (and indirectly by communicate()). A None value indicates that the process hasn’t terminated yet.
A negative value -N indicates that the child was terminated by signal N (POSIX only).
The STARTUPINFO class and following constants are only available on Windows.
Partial support of the Windows STARTUPINFO structure is used for Popen creation. The following attributes can be set by passing them as keyword-only arguments.
Changed in version 3.7: Keyword-only argument support was added.
A bit field that determines whether certain STARTUPINFO attributes are used when the process creates a window.
If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard input handle for the process. If STARTF_USESTDHANDLES is not specified, the default for standard input is the keyboard buffer.
If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard output handle for the process. Otherwise, this attribute is ignored and the default for standard output is the console window’s buffer.
If dwFlags specifies STARTF_USESTDHANDLES, this attribute is the standard error handle for the process. Otherwise, this attribute is ignored and the default for standard error is the console window’s buffer.
If dwFlags specifies STARTF_USESHOWWINDOW, this attribute can be any of the values that can be specified in the nCmdShow parameter for the ShowWindow function, except for SW_SHOWDEFAULT. Otherwise, this attribute is ignored.
SW_HIDE is provided for this attribute. It is used when Popen is called with shell=True.
A dictionary of additional attributes for process creation as given in STARTUPINFOEX, see UpdateProcThreadAttribute.
Supported attributes:
Sequence of handles that will be inherited. close_fds must be true if non-empty.
The handles must be temporarily made inheritable by os.set_handle_inheritable() when passed to the Popen constructor, else OSError will be raised with Windows error ERROR_INVALID_PARAMETER (87).
Warning
In a multithreaded process, use caution to avoid leaking handles that are marked inheritable when combining this feature with concurrent calls to other process creation functions that inherit all handles such as os.system(). This also applies to standard handle redirection, which temporarily creates inheritable handles.
New in version 3.7.
The subprocess module exposes the following constants.
The standard input device. Initially, this is the console input buffer, CONIN$.
The standard output device. Initially, this is the active console screen buffer, CONOUT$.
The standard error device. Initially, this is the active console screen buffer, CONOUT$.
Hides the window. Another window will be activated.
Specifies that the STARTUPINFO.hStdInput, STARTUPINFO.hStdOutput, and STARTUPINFO.hStdError attributes contain additional information.
Specifies that the STARTUPINFO.wShowWindow attribute contains additional information.
The new process has a new console, instead of inheriting its parent’s console (the default).
A Popen creationflags parameter to specify that a new process group will be created. This flag is necessary for using os.kill() on the subprocess.
This flag is ignored if CREATE_NEW_CONSOLE is specified.
A Popen creationflags parameter to specify that a new process will have an above average priority.
New in version 3.7.
A Popen creationflags parameter to specify that a new process will have a below average priority.
New in version 3.7.
A Popen creationflags parameter to specify that a new process will have a high priority.
New in version 3.7.
A Popen creationflags parameter to specify that a new process will have an idle (lowest) priority.
New in version 3.7.
A Popen creationflags parameter to specify that a new process will have an normal priority. (default)
New in version 3.7.
A Popen creationflags parameter to specify that a new process will have realtime priority. You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts system threads that manage mouse input, keyboard input, and background disk flushing. This class can be appropriate for applications that “talk” directly to hardware or that perform brief tasks that should have limited interruptions.
New in version 3.7.
A Popen creationflags parameter to specify that a new process will not create a window.
New in version 3.7.
A Popen creationflags parameter to specify that a new process will not inherit its parent’s console. This value cannot be used with CREATE_NEW_CONSOLE.
New in version 3.7.
A Popen creationflags parameter to specify that a new process does not inherit the error mode of the calling process. Instead, the new process gets the default error mode. This feature is particularly useful for multithreaded shell applications that run with hard errors disabled.
New in version 3.7.
A Popen creationflags parameter to specify that a new process is not associated with the job.
New in version 3.7.
Prior to Python 3.5, these three functions comprised the high level API to subprocess. You can now use run() in many cases, but lots of existing code calls these functions.
Run the command described by args. Wait for command to complete, then return the returncode attribute.
Code needing to capture stdout or stderr should use run() instead:
To suppress stdout or stderr, supply a value of DEVNULL.
The arguments shown above are merely some common ones. The full function signature is the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.
Note
Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.
Changed in version 3.3: timeout was added.
Run command with arguments. Wait for command to complete. If the return code was zero then return, otherwise raise CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute.
Code needing to capture stdout or stderr should use run() instead:
To suppress stdout or stderr, supply a value of DEVNULL.
The arguments shown above are merely some common ones. The full function signature is the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.
Note
Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.
Changed in version 3.3: timeout was added.
Run command with arguments and return its output.
If the return code was non-zero it raises a CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute and any output in the output attribute.
This is equivalent to:
The arguments shown above are merely some common ones. The full function signature is largely the same as that of run() - most arguments are passed directly through to that interface. One API deviation from run() behavior exists: passing input=None will behave the same as input=b'' (or input='', depending on other arguments) rather than using the parent’s standard input file handle.
By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level.
This behaviour may be overridden by setting text, encoding, errors, or universal_newlines to True as described in Frequently Used Arguments and run().
To also capture standard error in the result, use stderr=subprocess.STDOUT:
New in version 3.1.
Changed in version 3.3: timeout was added.
Changed in version 3.4: Support for the input keyword argument was added.
Changed in version 3.6: encoding and errors were added. See run() for details.
New in version 3.7: text was added as a more readable alias for universal_newlines.
In this section, “a becomes b” means that b can be used as a replacement for a.
Note
All “a” functions in this section fail (more or less) silently if the executed program cannot be found; the “b” replacements raise OSError instead.
In addition, the replacements using check_output() will fail with a CalledProcessError if the requested operation produces a non-zero return code. The output is still available as the output attribute of the raised exception.
In the following examples, we assume that the relevant functions have already been imported from the subprocess module.
becomes:
becomes:
The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1.
Alternatively, for trusted input, the shell’s own pipeline support may still be used directly:
becomes:
Notes:
A more realistic example would look like this:
P_NOWAIT example:
P_WAIT example:
Vector example:
Environment example:
Return code handling translates as follows:
Note
If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed.
popen2.Popen3 and popen2.Popen4 basically work as subprocess.Popen, except that:
This module also provides the following legacy functions from the 2.x commands module. These operations implicitly invoke the system shell and none of the guarantees described above regarding security and exception handling consistency are valid for these functions.
Return (exitcode, output) of executing cmd in a shell.
Execute the string cmd in a shell with Popen.check_output() and return a 2-tuple (exitcode, output). The locale encoding is used; see the notes on Frequently Used Arguments for more details.
A trailing newline is stripped from the output. The exit code for the command can be interpreted as the return code of subprocess. Example:
Availability: POSIX & Windows.
Changed in version 3.3.4: Windows support was added.
The function now returns (exitcode, output) instead of (status, output) as it did in Python 3.3.3 and earlier. exitcode has the same value as returncode.
Return output (stdout and stderr) of executing cmd in a shell.
Like getstatusoutput(), except the exit code is ignored and the return value is a string containing the command’s output. Example:
Availability: POSIX & Windows.
Changed in version 3.3.4: Windows support added
On Windows, an args sequence is converted to a string that can be parsed using the following rules (which correspond to the rules used by the MS C runtime):
See also
Module which provides function to parse and escape command lines.


A Popen creationflags parameter to specify that a new process will have an above average priority.
New in version 3.7.


A Popen creationflags parameter to specify that a new process will have a below average priority.
New in version 3.7.


Run the command described by args. Wait for command to complete, then return the returncode attribute.
Code needing to capture stdout or stderr should use run() instead:
To suppress stdout or stderr, supply a value of DEVNULL.
The arguments shown above are merely some common ones. The full function signature is the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.
Note
Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.
Changed in version 3.3: timeout was added.


Subclass of SubprocessError, raised when a process run by check_call() or check_output() returns a non-zero exit status.
Exit status of the child process. If the process exited due to a signal, this will be the negative signal number.
Command that was used to spawn the child process.
Output of the child process if it was captured by run() or check_output(). Otherwise, None.
Alias for output, for symmetry with stderr.
Stderr output of the child process if it was captured by run(). Otherwise, None.
Changed in version 3.5: stdout and stderr attributes added


Command that was used to spawn the child process.


Output of the child process if it was captured by run() or check_output(). Otherwise, None.


Exit status of the child process. If the process exited due to a signal, this will be the negative signal number.


Stderr output of the child process if it was captured by run(). Otherwise, None.


Alias for output, for symmetry with stderr.


Run command with arguments. Wait for command to complete. If the return code was zero then return, otherwise raise CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute.
Code needing to capture stdout or stderr should use run() instead:
To suppress stdout or stderr, supply a value of DEVNULL.
The arguments shown above are merely some common ones. The full function signature is the same as that of the Popen constructor - this function passes all supplied arguments other than timeout directly through to that interface.
Note
Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.
Changed in version 3.3: timeout was added.


Run command with arguments and return its output.
If the return code was non-zero it raises a CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute and any output in the output attribute.
This is equivalent to:
The arguments shown above are merely some common ones. The full function signature is largely the same as that of run() - most arguments are passed directly through to that interface. One API deviation from run() behavior exists: passing input=None will behave the same as input=b'' (or input='', depending on other arguments) rather than using the parent’s standard input file handle.
By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level.
This behaviour may be overridden by setting text, encoding, errors, or universal_newlines to True as described in Frequently Used Arguments and run().
To also capture standard error in the result, use stderr=subprocess.STDOUT:
New in version 3.1.
Changed in version 3.3: timeout was added.
Changed in version 3.4: Support for the input keyword argument was added.
Changed in version 3.6: encoding and errors were added. See run() for details.
New in version 3.7: text was added as a more readable alias for universal_newlines.


The return value from run(), representing a process that has finished.
The arguments used to launch the process. This may be a list or a string.
Exit status of the child process. Typically, an exit status of 0 indicates that it ran successfully.
A negative value -N indicates that the child was terminated by signal N (POSIX only).
Captured stdout from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stdout was not captured.
If you ran the process with stderr=subprocess.STDOUT, stdout and stderr will be combined in this attribute, and stderr will be None.
Captured stderr from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stderr was not captured.
If returncode is non-zero, raise a CalledProcessError.
New in version 3.5.


The arguments used to launch the process. This may be a list or a string.


If returncode is non-zero, raise a CalledProcessError.


Exit status of the child process. Typically, an exit status of 0 indicates that it ran successfully.
A negative value -N indicates that the child was terminated by signal N (POSIX only).


Captured stderr from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stderr was not captured.


Captured stdout from the child process. A bytes sequence, or a string if run() was called with an encoding, errors, or text=True. None if stdout was not captured.
If you ran the process with stderr=subprocess.STDOUT, stdout and stderr will be combined in this attribute, and stderr will be None.


A Popen creationflags parameter to specify that a new process is not associated with the job.
New in version 3.7.


A Popen creationflags parameter to specify that a new process does not inherit the error mode of the calling process. Instead, the new process gets the default error mode. This feature is particularly useful for multithreaded shell applications that run with hard errors disabled.
New in version 3.7.


The new process has a new console, instead of inheriting its parent’s console (the default).


A Popen creationflags parameter to specify that a new process group will be created. This flag is necessary for using os.kill() on the subprocess.
This flag is ignored if CREATE_NEW_CONSOLE is specified.


A Popen creationflags parameter to specify that a new process will not create a window.
New in version 3.7.


A Popen creationflags parameter to specify that a new process will not inherit its parent’s console. This value cannot be used with CREATE_NEW_CONSOLE.
New in version 3.7.


Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that the special file os.devnull will be used.
New in version 3.3.


Return output (stdout and stderr) of executing cmd in a shell.
Like getstatusoutput(), except the exit code is ignored and the return value is a string containing the command’s output. Example:
Availability: POSIX & Windows.
Changed in version 3.3.4: Windows support added


Return (exitcode, output) of executing cmd in a shell.
Execute the string cmd in a shell with Popen.check_output() and return a 2-tuple (exitcode, output). The locale encoding is used; see the notes on Frequently Used Arguments for more details.
A trailing newline is stripped from the output. The exit code for the command can be interpreted as the return code of subprocess. Example:
Availability: POSIX & Windows.
Changed in version 3.3.4: Windows support was added.
The function now returns (exitcode, output) instead of (status, output) as it did in Python 3.3.3 and earlier. exitcode has the same value as returncode.


A Popen creationflags parameter to specify that a new process will have a high priority.
New in version 3.7.


A Popen creationflags parameter to specify that a new process will have an idle (lowest) priority.
New in version 3.7.


A Popen creationflags parameter to specify that a new process will have an normal priority. (default)
New in version 3.7.


Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that a pipe to the standard stream should be opened. Most useful with Popen.communicate().