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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