Python:ProxyProperties

lastmod 2025-04-25created 2025-04-251 edits

Did you know that a class attribute can be used as both a method and an attribute?!?

This is not a built-in Python feature, but an example of what you can do by making good use of Python's magic methods and descriptors.

Code

from typing import Callable, Generic, TypeVar, ParamSpec, Self

P = ParamSpec("P")
R = TypeVar("R")
T = TypeVar("T")

class ProxyProperty(Generic[P, R]):
    func: Callable[P, R]
    instance: object

    def __init__(self, func: Callable[P, R]) -> None:
        self.func = func

    def __get__(self, instance: object, _=None) -> Self:
        self.instance = instance
        return self

    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> R:
        return self.func(self.instance, *args, **kwargs)

    def __repr__(self) -> str:
        return self.func(self.instance)

def proxy_property(func: Callable[P, R]) -> ProxyProperty[P, R]:
    return ProxyProperty(func)

class Container:
    @proxy_property
    def value(self, val: int = 5) -> str:
        return f"The value is: {val}"

# Example usage
c = Container()
print(c.value)      # Returns: The value is: 5
print(c.value(7))   # Returns: The value is: 7

How does this work? It works through Python's Descriptor Protocol¹.

1. The __get__ method transforms the ProxyProperty object into a descriptor.
2. When you access c.value, Python calls __get__ which returns self (the descriptor instance).
3. The __repr__ method handles property access (returning default values).
4. The __call__ method handles method calls with parameters.

This creates a dual-purpose attribute that can be read directly or called like a function!

The advantage of this class is that it lets you build intuitive APIs where an attribute needs configuration or should have sensible defaults but can still be customized.

See also

• Python

Favorite site

• Python Descriptors: An Introduction – Real Python
• Demystifying Python’s Descriptor Protocol • DeepSource
• Descriptor Guide — Python 3.13.3 documentation
• Proxies and Wrappers — wrapt 1.17.2 documentation
• [Recommended] Advanced Python features | GeekNews
  • [Original] 14 Advanced Python Features | Edward Li's Blog ²

References