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A package with lisp-like generic functions for python 3.

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)
<class 'function'>

Lets define add for two integers:

>>> @method()
... def add(n0: int, n1: int):
...     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()
... def add(s0: str, s1: str):
...     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()
... def add(s: str, n: int):
...     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:

def __add__(a: object, b: Rational):
    """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: Rational, b: object):
    """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.

def __out__(rational: Rational, writer: 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)

def __spy__(rational: Rational, writer: 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)


As usual gf3 can be installed with pip, like this:

pip install gf3


The whole documentation is available at in the following formats

HTML (Also servers as gf’s homepage)



A short sketch of the changes done in each release.

Release 0.2.4

The following was changed in Release 0.2.4:

  • The push-method accepts an identation string for identing writers.

  • The methods push and pop now accept arbitrary arguments in the general case.

  • Successfully tested the whole framework with Python 3.5.

Release 0.2.3

The following was changed in Release 0.2.3:

  • Fixed the long description.

  • Wrote some documentation about changing the implementation class of a generic function.

Release 0.2.2

The following was changed in Release 0.2.2:

  • Write more documentation. Especially documented the merge and the isgeneric functions.

  • Consistency between the long text and on PyPi and the documentation.

Release 0.2.1

Needed to bump the version information, because the homepage in the package-information was wrong [1] and a new upload was needed.

Release 0.2.0

The following was changed in Release 0.2.0:

  • Ported the whole module to Python 3.6 and Python 3.7.

  • Exclusively uses parameter annotations to specify the types to dispatch on.

  • Added standard conforming default implementations for methods like __add__. All these methods now raise a proper TypeError instead of raising a NotImplementedError.

  • Added some means to write generic functions that dispatch types only. This is the generic function equivalent of a class-method.

  • Added some means to dispatch on single objects. This is the equivalent adding methods to class-instances [2].

  • The package name for PyPi is now gf3.

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^H^H^H^H oodles of convenience stuff. Thank you Guido!

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