Types

Protocols

Protocol describes required behavior for structural typing.

A protocol names required behavior. The ellipsis marks the method body as intentionally unspecified, similar to an interface declaration.

Source

from typing import Protocol

class Greeter(Protocol):
    def greet(self) -> str:
        ...

print(Greeter.__name__)

Output

Greeter

A class can satisfy the protocol without inheriting from it. Person has a compatible greet() method, so it has the right shape for static type checkers.

Source

class Person:
    def __init__(self, name):
        self.name = name

    def greet(self):
        return f"hello {self.name}"

print(Person("Ada").greet())

Output

hello Ada

Use the protocol as an annotation at the API boundary. The function only cares that the object can greet; it does not care about the concrete class.

Source

def welcome(greeter: Greeter):
    print(greeter.greet())

welcome(Person("Ada"))

Output

hello Ada

Notes

Protocols are for structural typing: compatibility by shape rather than explicit inheritance.
Type checkers understand protocols; normal runtime method calls still do the work.
Prefer inheritance when shared implementation matters, and protocols when only required behavior matters.

Run the complete example

Expected output

hello Ada
Greeter

Execution time appears here after you run the example.

Protocols

See also

Run the complete example

Example code

Expected output