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diff --git a/README.txt b/README.txt new file mode 100644 index 0000000..b11459f --- /dev/null +++ b/README.txt @@ -0,0 +1,260 @@ +========================= +Trivial Generic Functions +========================= + +* New in 0.8: Source and tests are compatible with Python 3 (w/o ``setup.py``) + * 0.8.1: setup.py is now compatible with Python 3 as well +* New in 0.7: `Multiple Types or Objects`_ +* New in 0.6: `Inspection and Extension`_, and thread-safe method registration + +The ``simplegeneric`` module lets you define simple single-dispatch +generic functions, akin to Python's built-in generic functions like +``len()``, ``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 an `excellent 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. + + +Usage +----- + +Defining 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 Types +----------------------------- + +Defining 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 the ``when_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 class + +Methods 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 Objects +------------------------- + +As 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) + True + + +Defaults and Docs +----------------- + +You can obtain a function's default implementation using its ``default`` +attribute:: + + >>> @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 Extension +------------------------ + +You can find out if a generic function has a method for a type or object using +the ``has_object()`` and ``has_type()`` methods:: + + >>> move.has_object(zig) + True + >>> move.has_object(42) + False + + >>> move.has_type(X) + True + >>> move.has_type(float) + False + +Note that ``has_type()`` only queries whether there is a method registered for +the *exact* type, not subtypes or supertypes:: + + >>> class Z(X): pass + >>> move.has_type(Z) + False + +You can create a generic function that "inherits" from an existing generic +function by calling ``generic()`` on the existing function:: + + >>> move2 = generic(move) + >>> move(2101, "war") + In AD 2101, war was beginning. + +Any methods added to the new generic function override *all* methods 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 though ``move()`` has a method for type ``Y``, the method +defined for ``X`` in ``move2()`` takes precedence. This is because the +``move`` function is used as the ``default`` method of ``move2``, and ``move2`` +has no method for type ``Y``:: + + >>> move2.default is move + True + >>> move.has_type(Y) + True + >>> move2.has_type(Y) + False + + +Limitations +----------- + +* The first argument is always used for dispatching, and it must always be + passed *positionally* when 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 in ``RuleDispatch``, +however, which is somewhat of a pain. (Alternately I could use the +``BytecodeAssembler`` package 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.) + +.. _excellent alternative to the Visitor pattern: http://peak.telecommunity.com/DevCenter/VisitorRevisited + |