“Python Expert” Newsletter (Sep 1, 2021): Learning Corner

protocols.py

from pandas import DataFrame

class Base:
    def __len__(self):
        return 1

class Derived(Base):
    def __len__(self):
        return super().__len__() + 1

class T:
    def __init__(self, data):
        self.data = data
    def __len__(self):
        return len(self.data)

if __name__ == '__main__':
    df = DataFrame({
        'a': range(10)
    })

    print(
        df['a'], # __getitem__
        df.a,    # __getattr__
    )

As you can see:

• almost every syntax or built-in function has an associated implementation point—a __-method, a bytecode instruction, a tp_ method—and these constitute a “protocol.”
• in Python, implementation of a protocol almost always involves dispatching to a constituent object or a base type.
• one of the more important and more interesting protocols is __getattr__

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