Protocols in Python

Structural typing: anything with the right methods satisfies the protocol.

from typing import Protocol, runtime_checkable


@runtime_checkable
class Sized(Protocol):
    def __len__(self) -> int:
        ...


class Tour:
    def __init__(self, steps):
        self.steps = steps

    def __len__(self):
        return len(self.steps)


def describe(item: Sized) -> str:
    return f"{len(item)} items"


print(describe([1, 2, 3]), describe(Tour([1, 2])))
print(isinstance(Tour([]), Sized), isinstance(3, Sized))

How it works

1. No inheritance is required, only matching methods.
2. runtime_checkable allows isinstance against it.
3. This is how duck typing gets a name a checker understands.

Keywords and builtins used here

• Sized
• Tour
• class
• def
• describe
• int
• isinstance
• len
• print
• return
• self
• str

The run, in numbers

Lines

23

Characters to type

397

Tokens

121

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 66 seconds.

Type this snippet

Step 2 of 4 in Typing in depth, step 42 of 53 in Pythonic Python.

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