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Multiple argument dispatching.

Project description

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Multimethod provides a decorator for adding multiple argument dispatching to functions. The decorator creates a multimethod object as needed, 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.

def func(x: float, y: int):

Alternatively, functions can be explicitly registered in the same style as functools.singledispatch. This syntax is also compatible with mypy, which by default checks that each name is defined once.

def _(x: bool, y: bool):

@func.register(object, bool)
@func.register(bool, object)
def _(x, y):  # stackable without annotations

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

Multimethods support any types that satisfy the issubclass relation, including abstract base classes in and typing. Subscripted generics are provisionally supported:

  • Union[...]
  • Mapping[...] - the first key-value pair is checked
  • Tuple[...] - all args are checked
  • Iterable[...] - the first arg is checked

Naturally checking subscripts is slower, but the implementation is optimized, cached, and bypassed if no subscripts are in use in the multimethod.

Dispatch resolution details:

  • If an exact match isn't registered, the next closest method is called (and cached).
  • If the issubclass relation is ambiguous, mro position is used as a tie-breaker.
  • If there are still ambiguous methods - or none - a custom TypeError is raised.
  • Default and keyword-only parameters may be annotated, but won't affect dispatching.
  • A skipped annotation is equivalent to : object, which implicitly supports methods by leaving self blank.
  • If no types are specified, it will inherently match all arguments.


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

def func(obj: isa(str)):

def func(obj: str.isalnum):

def func(obj: str.isdigit):


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:
    def bar(self, x: str):

    def bar(self, x: int):


% pip install multimethod


100% branch coverage.

% pytest [--cov]



  • Postponed evaluation of nested annotations
  • Variable-length tuples of homogeneous type
  • Ignore default and keyword-only parameters
  • Resolved ambiguous Union types
  • Fixed an issue with name collision when defining a multimethod
  • Resolved dispatch errors when annotating parameters with meta-types such as type


  • Python >=3.6 required
  • Expanded support for subscripted type hints


  • Python 3 required
  • Support for subscripted ABCs


  • Support for typing generics
  • Stricter dispatching consistent with singledispatch


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


  • Missing annotations default to object
  • Removed deprecated dispatch stacking


  • 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

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