Protocol Checker —
Does It Quack?
Check It Live.

Protocols in the Standard Library

typing ships several ready-made protocols. You never need to define these yourself — just import and check against them.

A custom Speaker protocol, three classes — two that satisfy it, one that doesn't — and a loop that checks each with isinstance.

from typing import Protocol, runtime_checkable


@runtime_checkable
class Speaker(Protocol):
    def speak(self) -> str:
        ...


class Duck:
    def speak(self):
        return "Quack"


class Dog:
    def speak(self):
        return "Woof"


class Rock:
    pass


class BrokenDuck:
    speak = "Quack"


for obj in (Duck(), Dog(), Rock(), BrokenDuck()):
    name = type(obj).__name__
    ok = isinstance(obj, Speaker)
    print(f"{name:12s} satisfies Speaker: {ok}")

print("-" * 40)

from collections.abc import Iterable, Sized, Callable, Hashable

class Bag:
    def __init__(self, *items):
        self._items = list(items)
    def __iter__(self):
        return iter(self._items)
    def __len__(self):
        return len(self._items)

bag = Bag(1, 2, 3)
for proto in (Iterable, Sized, Hashable, Callable):
    name = proto.__name__
    print(f"Bag is {name:12s}: {isinstance(bag, proto)}")

print(isinstance([1, 2], Iterable), isinstance("hi", Sized))
print(isinstance(42, Hashable), isinstance(lambda: 1, Callable))
print(isinstance(3.14, Iterable))

🔍 Deep dive — what each check actually does

📖 Line-by-line walkthrough

1. @runtime_checkable + class Speaker(Protocol) — declares a protocol. Without @runtime_checkable, isinstance would raise TypeError; only static type checking would work.
2. def speak(self) -> str: ... — the required shape. The body is just ... because it's never called; it exists only to declare the contract.
3. Duck, Dog, Rock — three classes with no inheritance relationship. Two have speak(); one doesn't.
4. BrokenDuck with speak = "Quack" — speak is a plain string attribute, not a method. In modern Python the runtime check rejects it.
5. The first loop — isinstance(obj, Speaker) returns True only for objects with a proper speak attribute. Notice there's no explicit inheritance anywhere.
6. from collections.abc import Iterable, Sized, Callable, Hashable — Python's built-in protocols. These are the same ones used to describe the standard library's own types.
7. class Bag — a plain class that defines __iter__ and __len__. It doesn't inherit from Iterable, but it still satisfies the protocol by structural typing.
8. The second loop — isinstance(bag, proto) for four protocols. Iterable and Sized pass because the methods exist.
9. The last three prints — demonstrate protocols on built-in objects: lists, strings, ints, lambdas, floats.
10. isinstance(3.14, Iterable) → False — floats aren't iterable. The check works on any object, not just your own classes.

⚠️ Common mistakes

🧪 Try it yourself