A package with lisp-like generic functions.
gf lets you write generic functions generic functions with multi-methods, that dispatch on all their arguments.
>>> from gf import generic, method >>> add = generic() >>> type(add) <type 'function'>
Lets define add for two integers:
>>> @method(int, int) ... def add(n0, n1): ... return n0 + n1
Lets test it:
>>> add(1, 2) 3
Calling add with instances of other types fails:
>>> add("Hello ", "World") Traceback (most recent call last): ... NotImplementedError: Generic '__main__.add' has no implementation for type(s): __builtin__.str, __builtin__.str
Of course add can also by defined for two strings:
>>> @method(basestring, basestring) ... def add(s0, s1): ... return s0 + s1
Now our hello world example works:
>>> add("Hello ", "World") 'Hello World'
add can also be defined for a string and an integer:
>>> @method(basestring, int) ... def add(s, n): ... return s + str(n)
Thus we can add a string and a number:
>>> add("You ", 2) 'You 2'
Python’s Special Methods
gf.Object implements (nearly) all of the special instance methods of a python object as a generic function. The package includes a rational number implementation that makes heavy use of this feature:
@method(object, Rational) def __add__(a, b): """Add an object and a rational number. `a` is converted to a `Rational` and then both are added.""" return Rational(a) + b @method(Rational, object) def __add__(a, b): """Add a rational number and an object. `b` is converted to a `Rational` and then both are added.""" return a + Rational(b)
gf.Object also has a more Smalltalk means of overwriting object.__str__ and object.__repr__ using a file like object. Again the rational example is instructive about its usage.
@method(Rational, Writer) def __out__(rational, writer): """Write a nice representation of the rational. Denominators that equal 1 are not printed.""" writer("%d", rational.numerator) if rational.denominator != 1: writer(" / %d", rational.denominator) @method(Rational, Writer) def __spy__(rational, writer): """Write a debug representation of the rational.""" writer("%s(", rational.__class__.__name__) if rational.numerator != 0: writer("%r", rational.numerator) if rational.denominator != 1: writer(", %r", rational.denominator) writer(")")
A short sketch of the changes done in each release.
The following was fixed in Release 0.1.4:
Fixed an issue with variadic methods. Sometimes definitions of variadic methods added after the method was already called where not added.
Specified and implemented a precedence rule for overlapping variadic methods of generic functions.
Improved generated documentation for variadic methods.
Fixed the markup of some notes in the documentation.
The following was changed in Release 0.1.3:
Added variadic methods, e.g. multi-methods with a variable number of arguments.
Improved the long description text a bit and fixed bug in its markup.
Fixed invalid references in the long description.
The following was changed in Release 0.1.2:
Added a generic functions for gf.Object.__call__.
Added a gf.go.FinalizingMixin.
gf.generic now also accepts a type.
Improved the exception information for ambiguous calls.
Fixed some documentation glitches.
This was the initial release.
Guido van Rossum created the core of this package. I just renamed some things and added some convenience stuff. Thank you Guido!
© 2006 Python Software Foundation. © 2006-2013 Gerald Klix.
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