Basic Usage

Multiple dispatch allows you to implement multiple methods for the same function, where the methods specify the types of their arguments:

from plum import dispatch


@dispatch
def f(x: str):
    return "This is a string!"
    

@dispatch
def f(x: int):
    return "This is an integer!"

>>> f("1")
'This is a string!'

>>> f(1)
'This is an integer!'

We haven’t implemented a method for floats, so in that case an exception will be raised:

>>> try: f(1.0)
... except Exception as e: print(f"{type(e).__name__}: {e}")
NotFoundLookupError: `f(1.0)` could not be resolved...

Instead of implementing a method for floats, let’s implement a method for all numbers:

from numbers import Number


@dispatch
def f(x: Number):
    return "This is a number!"

Since a float is a Number, f(1.0) will return "This is a number!". But an int is also a Number, so f(1) can either return "This is an integer!" or "This is a number!". The rule of multiple dispatch is that the most specific method is chosen:

>>> f(1)
'This is an integer!'

since an int is a Number, but a Number is not necessarily an int.

For a function f, all available methods can be obtained with f.methods:

>>> f.methods
List of 3 method(s):
    [0] f(x: str)                                                               
        <function f at ...> @ ...
    [1] f(x: int)                                                               
        <function f at ...> @ ...
    [2] f(x: numbers.Number)                                                    
        <function f at ...> @ ...

For an excellent and way more detailed overview of multiple dispatch, see the manual of the Julia Language.