Outline

• What are protocols:
• Structural vs nominal typing
• Static duck typing

• Difference between ABCs and protocols

• Examples

  • Simple example
  • Extending protocols

• Use cases

  • Dynamically typed code
  • Library types

During this talk we will go over Protocols and how they can be employed to achieve better code. Take the following code snippet as an example:

from typing import Sized, Iterable, Iterator

class Bucket(Sized, Iterable[int]):
    ...
    def __len__(self) -> int: ...
    def __iter__(self) -> Iterator[int]: ...

The problem here is that you must explicitly inherit these bases classes to register them as subtypes of their parents.

This is particularly difficult to do with library types as the type objects may be hidden deep in the implementation of the library. Also, extensive use of ABCs might impose additional runtime costs.

Consider the following snippet as a solution:

from typing import Iterator, Iterable

class Bucket:
    ...
    def __len__(self) -> int: ...
    def __iter__(self) -> Iterator[int]: ...

def collect(items: Iterable[int]) -> int: ...
result: int = collect(Bucket())  # Passes type check

Protocols enable you to pass static type checking implicitly.