8 - Static duck typing via Protocol

Protocol is a good fit for use cases which only care about the behaviour and/or attributes of an object, not the actual type of the object. Protocols are great for defining interfaces, also in cases where (abstract) base classes are out of question. Available in Python 3.8+.

Read more

• typing.Protocol: https://docs.python.org/3/library/typing.html#typing.Protocol
• Mypy docs about protocols: https://mypy.readthedocs.io/en/stable/protocols.html

The code

import typing
import uuid
from dataclasses import dataclass


class InstanceWithId(typing.Protocol):
    @property
    def id(self) -> str:
        ...


def print_instance_id(instance: InstanceWithId) -> None:
    print(f"Received instance with id: {instance.id}")


@dataclass
class MyDataClass:
    id: str


class MyRegularClass:
    def __init__(self) -> None:
        self.id = str(uuid.uuid4())


class MyNamedTuple(typing.NamedTuple):
    id: str


class ClassWithoutId:
    ...


print_instance_id(MyDataClass(id="123"))
print_instance_id(MyRegularClass())
print_instance_id(MyNamedTuple(id="abc"))
print_instance_id(ClassWithoutId())  # mypy gives error