Python 3.8+Annotations & typingterm

typing.Protocol

protocol

Defines an interface by structure (duck typing). A class is compatible if it implements every method of the Protocol, with no explicit inheritance.

Syntax

from typing import Protocol
class MyProtocol(Protocol):
    def method(self) -> type: ...

Protocol earns its keep when you need to type third-party classes you cannot make inherit from an ABC: compatibility comes from the set of methods, not from ancestry. By default it is a purely static affair — isinstance against a protocol raises TypeError until the protocol is decorated with @runtime_checkable, and even then only the presence of the attributes is checked, never their signatures.

Examples

from typing import Protocol
class Drawable(Protocol):
    def draw(self) -> None: ...
class Circle:
    def draw(self) -> None: print('O')
class Square:
    def draw(self) -> None: print('[]')
def render(shape: Drawable) -> None:
    shape.draw()
render(Circle())  # → O
render(Square())  # → []

from typing import Protocol, runtime_checkable
@runtime_checkable
class Drawable(Protocol):
    def draw(self) -> None: ...
class Circle:
    def draw(self) -> None: print('O')
print(isinstance(Circle(), Drawable))  # → True — наследовать не нужно
print(isinstance(42, Drawable))  # → False

See also

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