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A package with lisp-like generic functions.

Project description


gf lets you write generic functions generic functions with multi-methods, that dispatch on all their arguments.

Simple Example

>>> 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)

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)


A short sketch of the changes done in each release.

Release 0.1.4

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.

Release 0.1.3

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.

Release 0.1.2

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.

Release 0.1.1

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!

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