Package: simplegeneric
New in 0.8: Source and tests are compatible with Python 3 (w/osetup.py)0.8.1: setup.py is now compatible with Python 3 as wellNew in 0.7:Multiple Types or ObjectsNew in 0.6:Inspection and Extension, and thread-safe method registrationThesimplegenericmodule lets you define simple single-dispatch
generic functions, akin to Python’s built-in generic functions likelen(),iter()and so on.  However, instead of using
specially-named methods, these generic functions use simple lookup
tables, akin to those used by e.g.pickle.dump()and other
generic functions found in the Python standard library.As you can see from the above examples, generic functions are actually
quite common in Python already, but there is no standard way to create
simple ones.  This library attempts to fill that gap, as generic
functions are anexcellent alternative to the Visitor pattern, as
well as being a great substitute for most common uses of adaptation.This library tries to be the simplest possible implementation of generic
functions, and it therefore eschews the use of multiple or predicate
dispatch, as well as avoiding speedup techniques such as C dispatching
or code generation.  But it has absolutely no dependencies, other than
Python 2.4, and the implementation is just a single Python module of
less than 100 lines.UsageDefining and using a generic function is straightforward:>>> from simplegeneric import generic
>>> @generic
... def move(item, target):
...     """Default implementation goes here"""
...     print("what you say?!")

>>> @move.when_type(int)
... def move_int(item, target):
...     print("In AD %d, %s was beginning." % (item, target))

>>> @move.when_type(str)
... def move_str(item, target):
...     print("How are you %s!!" % item)
...     print("All your %s are belong to us." % (target,))

>>> zig = object()
>>> @move.when_object(zig)
... def move_zig(item, target):
...     print("You know what you %s." % (target,))
...     print("For great justice!")

>>> move(2101, "war")
In AD 2101, war was beginning.

>>> move("gentlemen", "base")
How are you gentlemen!!
All your base are belong to us.

>>> move(zig, "doing")
You know what you doing.
For great justice!

>>> move(27.0, 56.2)
what you say?!Inheritance and Allowed TypesDefining multiple methods for the same type or object is an error:>>> @move.when_type(str)
... def this_is_wrong(item, target):
...     pass
Traceback (most recent call last):
...
TypeError: <function move...> already has method for type <...'str'>

>>> @move.when_object(zig)
... def this_is_wrong(item, target): pass
Traceback (most recent call last):
  ...
TypeError: <function move...> already has method for object <object ...>And thewhen_type()decorator only accepts classes or types:>>> @move.when_type(23)
... def move_23(item, target):
...     print("You have no chance to survive!")
Traceback (most recent call last):
  ...
TypeError: 23 is not a type or classMethods defined for supertypes are inherited following MRO order:>>> class MyString(str):
...     """String subclass"""

>>> move(MyString("ladies"), "drinks")
How are you ladies!!
All your drinks are belong to us.Classic class instances are also supported (although the lookup process
is slower than for new-style instances):>>> class X: pass
>>> class Y(X): pass

>>> @move.when_type(X)
... def move_x(item, target):
...     print("Someone set us up the %s!!!" % (target,))

>>> move(X(), "bomb")
Someone set us up the bomb!!!

>>> move(Y(), "dance")
Someone set us up the dance!!!Multiple Types or ObjectsAs a convenience, you can now pass more than one type or object to the
registration methods:>>> @generic
... def isbuiltin(ob):
...     return False
>>> @isbuiltin.when_type(int, str, float, complex, type)
... @isbuiltin.when_object(None, Ellipsis)
... def yes(ob):
...     return True

>>> isbuiltin(1)
True
>>> isbuiltin(object)
True
>>> isbuiltin(object())
False
>>> isbuiltin(X())
False
>>> isbuiltin(None)
True
>>> isbuiltin(Ellipsis)
TrueDefaults and DocsYou can obtain a function’s default implementation using itsdefaultattribute:>>> @move.when_type(Y)
... def move_y(item, target):
...     print("Someone set us up the %s!!!" % (target,))
...     move.default(item, target)

>>> move(Y(), "dance")
Someone set us up the dance!!!
what you say?!help()and other documentation tools see generic functions as normal
function objects, with the same name, attributes, docstring, and module as
the prototype/default function:>>> help(move)
Help on function move:
...
move(*args, **kw)
    Default implementation goes here
...Inspection and ExtensionYou can find out if a generic function has a method for a type or object using
thehas_object()andhas_type()methods:>>> move.has_object(zig)
True
>>> move.has_object(42)
False

>>> move.has_type(X)
True
>>> move.has_type(float)
FalseNote thathas_type()only queries whether there is a method registered for
theexacttype, not subtypes or supertypes:>>> class Z(X): pass
>>> move.has_type(Z)
FalseYou can create a generic function that “inherits” from an existing generic
function by callinggeneric()on the existing function:>>> move2 = generic(move)
>>> move(2101, "war")
In AD 2101, war was beginning.Any methods added to the new generic function overrideallmethods in the
“base” function:>>> @move2.when_type(X)
... def move2_X(item, target):
...     print("You have no chance to survive make your %s!" % (target,))

>>> move2(X(), "time")
You have no chance to survive make your time!

>>> move2(Y(), "time")
You have no chance to survive make your time!Notice that even thoughmove()has a method for typeY, the method
defined forXinmove2()takes precedence.  This is because themovefunction is used as thedefaultmethod ofmove2, andmove2has no method for typeY:>>> move2.default is move
True
>>> move.has_type(Y)
True
>>> move2.has_type(Y)
FalseLimitationsThe first argument is always used for dispatching, and it must always be
passedpositionallywhen the function is called.Documentation tools don’t see the function’s original argument signature, so
you have to describe it in the docstring.If you have optional arguments, you must duplicate them on every method in
order for them to work correctly.  (On the plus side, it means you can have
different defaults or required arguments for each method, although relying on
that quirk probably isn’t a good idea.)These restrictions may be lifted in later releases, if I feel the need.  They
would require runtime code generation the way I do it inRuleDispatch,
however, which is somewhat of a pain.  (Alternately I could use theBytecodeAssemblerpackage to do the code generation, as that’s a lot easier
to use than string-based code generation, but that would introduce more
dependencies, and I’m trying to keep this simple so I can just
toss it into Chandler without a big footprint increase.)