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Base class for protocol classes. Protocol classes are defined as::

class Proto(Protocol):
    def meth(self) -> int:

Such classes are primarily used with static type checkers that recognize structural subtyping (static duck-typing), for example::

class C:
    def meth(self) -> int:
        return 0

def func(x: Proto) -> int:
    return x.meth()

func(C())  # Passes static type check

See PEP 544 for details. Protocol classes decorated with @typing_extensions.runtime act as simple-minded runtime protocol that checks only the presence of given attributes, ignoring their type signatures.

Protocol classes can be generic, they are defined as::

class GenProto(Protocol[T]):
    def meth(self) -> T: