“Python Expert” Newsletter (Sep 22, 2021): Learning Corner

class.py

def foo(self, _):
    pass

class T:
    from json import dumps
    from itertools import repeat
    foo = foo

    def bar(self, _):
        pass

if __name__ == '__main__':
    obj = T()
    print(
        f'{obj.bar("") = }',
        f'{obj.foo("") = }',
        #  f'{obj.dumps("") = }',
        f'{obj.repeat("") = }',
        sep='\n'
    )

    from json import dumps
    from itertools import repeat
    print(
        f'{dumps.__get__  = }',
        #  f'{repeat.__get__ = }',
    )

classmethod.py

from dataclasses import dataclass
from types import FunctionType
from functools import wraps

@dataclass
class instancemethod:
    f : FunctionType
    def __get__(self, instance, _):
        return wraps(self.f)(lambda *args, **kwargs: self.f(instance, *args, **kwargs))

@dataclass
class classmethod_:
    f : FunctionType
    def __get__(self, _, owner):
        return wraps(self.f)(lambda *args, **kwargs: self.f(owner, *args, **kwargs))

@dataclass
class staticmethod_:
    f : FunctionType
    def __get__(self, _, __):
        return self.f
        
#  # or you could consider `staticmethod` to be
#  #   the absence of the descriptor protocol…
#  @dataclass
#  class staticmethod_:
#      f : FunctionType
#      __call__ = lambda s, *args, **kwargs: s.f(*args, **kwargs)

@dataclass
class property_:
    f : FunctionType
    def __get__(self, instance, _):
        return self.f(instance)

class T:
    @instancemethod
    def foo(self):
        return f'T.foo({self})'

    @classmethod_
    def bar(cls):
        return f'T.foo({cls})'

    @staticmethod_
    def baz():
        return f'T.foo()'

    @property_
    def quux(self):
        return f'T.foo({self})'

if __name__ == '__main__':
    obj = T()
    print(f'{obj.foo() = }')
    print(f'{obj.bar() = }')
    print(f'{obj.baz() = }')
    print(f'{obj.quux  = }')

As you can see:

• the descriptor protocol is the mechanism by which instance methods, @classmethod, @staticmethod, and @property all work!

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