Multimethod provides a decorator for adding multiple argument dispatching to functions. The decorator finds the multimethod of the same name, creating it if necessary, and registers the function with its annotations.

There are several multiple dispatch libraries on PyPI. This one aims for simplicity and speed. With caching of argument types, it should be the fastest pure Python implementation possible.

Usage

multimethod

from multimethod import multimethod

@multimethod
def func(x: int, y: float):
    ...

func is now a multimethod which will delegate to the above function, when called with arguments of the specified types. Subsequent usage will register new types and functions to the existing multimethod of the same name. If an exact match isn't registered, the next closest method is called (and cached). Candidate methods are ranked based on their subclass relationships. If no matches are found, a custom TypeError is raised.

A strict flag can also be set on the multimethod object, in which case finding multiple matches also raises a TypeError. Keyword arguments can be used when calling, but won't affect the dispatching. If no annotations are specified, it will inherently match any aruments.

Multimethods are implemented as mappings from signatures to functions, and can be introspected as such.

method[type, ...]           # get registered function
method[type, ...] = func    # register function by explicit types
method.register(func)       # decorator to register annotated function (with any __name__)

Multimethods support any types that satisfy the issubclass relation, including abstract base classes in collections and typing.

multidispatch

The functools.singledispatch style syntax is also supported. This requires creating a multidispatch object explicitly, and consequently doesn't rely on the name matching. The register method returns a decorator for given types, thereby supporting Python 2 and stacking of multiple signatures.

from multimethod import multidispatch

@multidispatch
def func(*args):
    ...

@func.register(object, int)
@func.register(int, object)
def _(*args):
    ...

overload

Overloads dispatch on annotated predicates. Each predicate is checked in the reverse order of registration.

The implementation is separate from multimethod due to the different performance characteristics. Instead a simple isa predicate is provided for checking instance type.

from multimethod import isa, overload

@overload
def func(obj: isa(str)):
    ...

@overload
def func(obj: str.isalnum):
    ...

@overload
def func(obj: str.isdigit):
    ...

multimeta

In Python 3, use metaclass=multimeta to create a class with a special namespace which converts callables to multimethods, and registers duplicate callables with the original.

from multimethod import multimeta

class Foo(metaclass=multimeta):
    def bar(self, x: str):
        ...

    def bar(self, x: int):
        ...

Equivalent to:

from multimethod import multimethod

class Foo:
    @multimethod
    def bar(self, x: str):
        ...

    @bar.register
    def bar(self, x: int):
        ...

Installation

$ pip install multimethod

Tests

100% branch coverage.

$ pytest [--cov]

Changes

1.1

• Fix for Python 2 typing backport
• Metaclass for automatic multimethods

1.0

• Missing annotations default to object
• Removed deprecated dispatch stacking

0.7

• Forward references allowed in type hints
• Register method
• Overloads with predicate dispatch

0.6

• Multimethods can be defined inside a class

0.5

• Optimized dispatching
• Support for functools.singledispatch syntax

0.4

• Dispatch on Python 3 annotations