Method Resolution

Multiple dispatch selects the function from the types of the inputs.

def f(x):           # increment integers
    return x + 1

def f(x):           # decrement floats
    return x - 1
>>> f(1)            # 1 is an int, so increment
>>> f(1.0)          # 1.0 is a float, so decrement

Union Types

Similarly to the builtin isinstance operation you specify multiple valid types with a tuple.

@dispatch((list, tuple))
def f(x):
    """ Apply ``f`` to each element in a list or tuple """
    return [f(y) for y in x]
>>> f([1, 2, 3])
[2, 3, 4]

>>> f((1, 2, 3))
[2, 3, 4]

Abstract Types

You can also use abstract classes like Iterable and Number in place of union types like (list, tuple) or (int, float).

from collections import Iterable

# @dispatch((list, tuple))
def f(x):
    """ Apply ``f`` to each element in an Iterable """
    return [f(y) for y in x]

Selecting Specific Implementations

If multiple valid implementations exist then we use the most specific one. In the following example we build a function to flatten nested iterables.

def flatten(L):
    return sum([flatten(x) for x in L], [])

def flatten(x):
    return [x]
>>> flatten([1, 2, 3])
[1, 2, 3]

>>> flatten([1, [2], 3])
[1, 2, 3]

>>> flatten([1, 2, (3, 4), [[5]], [(6, 7), (8, 9)]])
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Because strings are iterable they too will be flattened

>>> flatten([1, 'hello', 3])
[1, 'h', 'e', 'l', 'l', 'o', 3]

We avoid this by specializing flatten to str. Because str is more specific than Iterable this function takes precedence for strings.

def flatten(s):
    return s
>>> flatten([1, 'hello', 3])
[1, 'hello', 3]

The multipledispatch project depends on Python’s issubclass mechanism to determine which types are more specific than others.

Multiple Inputs

All of these rules apply when we introduce multiple inputs.

@dispatch(object, object)
def f(x, y):
    return x + y

@dispatch(object, float)
def f(x, y):
    """ Square the right hand side if it is a float """
    return x + y**2
>>> f(1, 10)

>>> f(1.0, 10.0)


However ambiguities arise when different implementations of a function are equally valid

@dispatch(float, object)
def f(x, y):
    """ Square left hand side if it is a float """
    return x**2 + y
>>> f(2.0, 10.0)

Which result do we expect, 2.0**2 + 10.0 or 2.0 + 10.0**2? The types of the inputs satisfy three different implementations, two of which have equal validity

input types:    float, float
Option 1:       object, object
Option 2:       object, float
Option 3:       float, object

Option 1 is strictly less specific than either options 2 or 3 so we discard it. Options 2 and 3 however are equally specific and so it is unclear which to use.

To resolve issues like this multipledispatch inspects the type signatures given to it and searches for ambiguities. It then raises a warning like the following:

multipledispatch/ AmbiguityWarning:
Ambiguities exist in dispatched function f

The following signatures may result in ambiguous behavior:
    [object, float], [float, object]

Consider making the following additions:

@dispatch(float, float)
def f(...)

This warning occurs when you write the function and guides you to create an implementation to break the ambiguity. In this case, a function with signature (float, float) is more specific than either options 2 or 3 and so resolves the issue. To avoid this warning you should implement this new function before the others.

@dispatch(float, float)
def f(x, y):

@dispatch(float, object)
def f(x, y):

@dispatch(object, float)
def f(x, y):

If you do not resolve ambiguities by creating more specific functions then one of the competing functions will be selected pseudo-randomly.