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.


from multimethod import 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 can't be found, 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.

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__)

The functools.singledispatch style syntax introduced in Python 3.4 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

def func(*args):

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

Overloads dispatch on annotated predicates (Python >=3.5 required). 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 overload

def func(obj):
   # optional base implementation, i.e., no annotations always match

def func(obj: isa(str)):

def func(obj: str.isdigit):

See tests for more example usage.


$ pip install multimethod


100% branch coverage.

$ pytest [--cov]



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


  • Multimethods can be defined inside a class


  • Optimized dispatching
  • Support for functools.singledispatch syntax


  • Dispatch on Python 3 annotations