Python single dispatch

Python single dispatch (from functools.singledispatch) provides function overloading based on the type of the first argument. A single function name dispatches to different implementations depending on what type is passed, making type-specific behavior declarative and extensible without large if/elif chains.

Use single dispatch to handle different types elegantly, or to allow plugins to register handlers for new types.

This example shows function dispatch by type without conditional chains.

# run: python3 singledispatch.py
# description: single dispatch for type-based function overloading
 
from functools import singledispatch
 
@singledispatch
def process(arg):
    """Default implementation for unknown types."""
    raise TypeError(f"Cannot process {type(arg)}")
 
@process.register(int)
def _(value: int):
    return value * 2
 
@process.register(str)
def _(value: str):
    return value.upper()
 
@process.register(list)
def _(value: list):
    return sum(value)
 
@process.register(dict)
def _(value: dict):
    return len(value)
 
# Dispatch based on argument type
print(f"process(5) = {process(5)}")
print(f"process('hello') = {process('hello')}")
print(f"process([1, 2, 3]) = {process([1, 2, 3])}")
print(f"process({{'a': 1, 'b': 2}}) = {process({'a': 1, 'b': 2})}")
 
# Plugin system: register new types
class Custom:
    def __init__(self, value):
        self.value = value
 
@process.register(Custom)
def _(obj: Custom):
    return obj.value
 
print(f"process(Custom(42)) = {process(Custom(42))}")
 
# Check registered types
print(f"\nRegistered types: {process.registry}")
 
# Dispatch with inheritance
class Animal:
    pass
 
class Dog(Animal):
    pass
 
@singledispatch
def speak(animal):
    print(f"Unknown animal: {animal}")
 
@speak.register(Dog)
def _(dog):
    print("Woof!")
 
@speak.register(Animal)
def _(animal):
    print("Generic animal sound")
 
speak(Dog())
speak(Animal())
speak("not an animal")  # uses default
 
# Method dispatch (Python 3.8+)
from functools import singledispatchmethod
 
class Formatter:
    @singledispatchmethod
    def format(self, arg):
        raise TypeError(f"Cannot format {type(arg)}")
 
    @format.register(int)
    def _(self, value: int):
        return f"Integer: {value}"
 
    @format.register(str)
    def _(self, value: str):
        return f"String: {value}"
 
fmt = Formatter()
print(f"\nFormatter: {fmt.format(123)}")
print(f"Formatter: {fmt.format('hello')}")

Default implementation:

• Base function with @singledispatch decorator
• Called when no registered type matches
• Usually raises NotImplementedError or provides fallback

Registering handlers:

• @func.register(type): register handler for type
• Can also use func.register(type, handler) syntax
• Handler receives all original arguments

Type hierarchy:

• Dispatch uses MRO (method resolution order)
• If type hierarchy exists, most specific type is used
• If multiple types in MRO are registered, most specific wins

Method dispatch:

• @singledispatchmethod: for class methods
• Dispatches on type of first argument after self
• Useful for multi-type handling in classes

Limitations:

• Only dispatches on first argument type
• For multiple arguments, need multipledispatch library (third-party)
• Not true overloading (Python doesn't support it natively)

Use cases:

• Serialization/deserialization for different types
• Visitor pattern without subclassing
• Plugin system where plugins register handlers
• Type-specific formatting, processing, or validation

vs if/elif chains:

• Single dispatch: cleaner, more extensible, declarative
• if/elif: simpler for two or three types, slightly faster
• Single dispatch better for design; if/elif better for quick scripts

Registry inspection:

• func.registry: dict of registered types and handlers
• func.register(type, func): register if you have handler function
• Useful for debugging, listing available handlers

Forward references:

• Can't register type before class definition
• Use string annotations or register after class defined
• Or use func.register('TypeName') in Python 3.10+