“Python Expert” Newsletter (Sep 15, 2021): Learning Corner

getattr.py

class A:
    x = 1

class B(A):
    y = 20

class D:
    def __get__(self, instance, owner):
        return 50_000

class C(B):
    z = 300
    def __init__(self):
        self.w = 4_000

    abc = D()

def getattr_(obj, attr):
    # __getattribute__

    if attr in obj.__dict__:
        return obj.__dict__[attr]

    for cls in type(obj).__mro__:
        if attr in cls.__dict__:
            rv = cls.__dict__[attr]
            if hasattr(rv, '__get__'):
                return rv.__get__(obj, cls)
            return rv

    # __getattr__

if __name__ == '__main__':
    obj = C()

    print(
        f'{getattr (obj, "x")   = :,}',
        f'{getattr_(obj, "x")   = :,}',

        f'{getattr (obj, "y")   = :,}',
        f'{getattr_(obj, "y")   = :,}',

        f'{getattr (obj, "z")   = :,}',
        f'{getattr_(obj, "z")   = :,}',

        f'{getattr (obj, "w")   = :,}',
        f'{getattr_(obj, "w")   = :,}',

        f'{getattr (obj, "abc") = :,}',
        f'{getattr_(obj, "abc") = :,}',

        sep='\n'
    )

As you can see:

• the descriptor protocol is fairly straightforward, and has a number of uses
• as a consequence of mechanisms such as __slots__ it is not easy to write a complete, pure-Python implementation of __getattr__
• however, fundamentally, __getattr__ is a simple protocol—a sequence of steps for locating the desired attribute

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