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# 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.

## Example

This example shows function dispatch by type without conditional chains.

```python
# 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')}")
```

## Common patterns

**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+