Delegation in Python

delegation.py

from typing import Callable
import inspect
from typing import Protocol
from abc import abstractmethod


def _create_method(to_method_name: str, to_attr_name: str):
    # returns a closure that traps the name of the method to invoke and the attribute_name of the object that will act as receiver-self
    def new_method(self, *args, **kargs):
        inner_self = getattr(self, to_attr_name)
        to_method = getattr(inner_self, to_method_name) # bound method (to inner_self)
        return to_method(*args, **kargs)
    return new_method

# decorator with parameters, so it has to return a function (that will be invoked with the class being decorated)
# we don't create a new class, we add functions to the existing class and return it
def delegate_methods(method_names: list[str], to_attr_name: str):
    # decorator with parameters   
    # receives a list of method names to create and delegate from them to the corresponding method in the object indicated by to_attr_name
    def add_methods(cls):
        for method_name in method_names:
            setattr(cls, method_name, _create_method(method_name, to_attr_name))
        return cls
    return add_methods


def delegate_interface(interface_like, to_attr_name: str):
    # decorator with parameters
    # receives an "interface" for which methods we will create "delegator methods" to delegate from them to the corresponding method in the object indicated by to_attr_name
    def add_methods(cls):
        method_names = [name for name, func in inspect.getmembers(interface_like, predicate=inspect.isfunction) if name != "__init__"]
        for method_name in method_names:
            setattr(cls, method_name, _create_method(method_name, to_attr_name))
        return cls
    return add_methods

#########################################

print("- Testing delegate_methods")

class Formatter:
    def short_format(self, txt: str, prepend: str):
        return f"{prepend}{txt}"

    def long_format(self, txt: str, wrap: str):
        return f"{wrap}{txt}{wrap}"


@delegate_methods(["short_format", "long_format"], "formatter")
class TextHelper:
    def __init__(self, id_, formatter):
        self.id = id_
        self.formatter = formatter

    def beautify(self, txt) -> str:
        return f"beautifying {self}"

helper = TextHelper("aa", Formatter())
print(helper.long_format("hi", "||"))
print(helper.short_format("hi", "||"))    
    
print("##################################")

print("- Testing delegate_interface")

class Formattable(Protocol):
    @abstractmethod
    def short_format(self, txt: str, prepend: str):
        pass

    @abstractmethod
    def long_format(self, txt: str, wrap: str):
        pass

class FormattableImp(Formattable):
    def short_format(self, txt: str, prepend: str):
        return f"{prepend}{txt}"

    def long_format(self, txt: str, wrap: str):
        return f"{wrap}{txt}{wrap}"

@delegate_interface(Formattable, "formatter")
class TextHelper2:
    def __init__(self, id_, formatter):
        self.id = id_
        self.formatter = formatter

    def beautify(self, txt) -> str:
        return f"beautifying {self}"


helper = TextHelper2("aa", FormattableImp())
print(helper.long_format("hi", "||"))
print(helper.short_format("hi", "||"))