Multiple Dispatch

A relatively sane approach to multiple dispatch in Python.

There exists several implementations of multiple dispatch (see links below). This implementation is efficient, relatively complete, and performs static analysis to avoid common issues.

Example

>>> from multipledispatch import dispatch

>>> @dispatch(int, int)
... def add(x, y):
...     return x + y

>>> @dispatch(object, object)
... def add(x, y):
...     return "%s + %s" % (x, y)

>>> add(1, 2)
3

>>> add(1, 'hello')
'1 + hello'

What this does

• Dispatches on all non-keyword arguments
• Supports inheritance
• Supports instance methods
• Supports union types, e.g. (int, float)
• Supports builtin abstract classes, e.g. Iterator, Number, ...
• Caches for fast repeated lookup
• Identifies possible ambiguities at function definition time
• Provides hints to resolve ambiguities when they occur

What this doesn't do

• Vararg dispatch

@dispatch([int])
def add(*args):
    ...

• Diagonal dispatch

a = arbitrary_type()
@dispatch(a, a)
def are_same_type(x, y):
    return True

• Respect namespaces

Installation and Dependencies

multipledispatch is on the Python Package Index (PyPI):

pip install multipledispatch

or

easy_install multipledispatch

multipledispatch supports Python 2.6+ and Python 3.2+ with a common codebase. It is pure Python and requires no dependencies beyond the standard library.

It is, in short, a light weight dependency.

License

New BSD. See License.

Links

• Five-minute Multimethods in Python by Guido
• `multimethods package on PyPI <https://pypi.python.org/pypi/multimethods>`__
• Clojure Protocols
• Julia methods docs
• Karpinksi notebook: *The Design Impact of Multiple Dispatch*
• Wikipedia article