Duck Types and Protocols

typing.Protocolとは何かを知りたくてこの部分だけ読んだ

https://realpython.com/python-type-checking/#duck-types-and-protocols

例：len関数のtype hint

ref: len()関数がオブジェクトの長さを手にいれる仕組み（PyCon JP 2017）

The answer hides behind the academic sounding term structural subtyping.

One way to categorize type systems is by whether they are nominal or structural:

nominal

In a nominal system, comparisons between types are based on names and declarations.

「nominalなシステムでは、型の比較は名前と宣言に基づく」

The Python type system is mostly nominal, where an int can be used in place of a float because of their subtype relationship.

「Pythonの型システムはほとんどがnominal」

例として「サブタイプ関係により、intはfloatの代わりに使える」

structural

In a structural system, comparisons between types are based on structure.

「structuralなシステムでは、型の比較は構造に基づく」

You could define a structural type Sized that includes all instances that define .__len__(), irrespective of their nominal type.

nominalな型に関わらず、.__len__()を定義する全てのインスタンスを含む構造的な型Sizedを定義できる

https://docs.python.org/ja/3/library/typing.html#typing.Sized

https://docs.python.org/ja/3/library/collections.abc.html#collections.abc.Sized

__len__() メソッドを提供するクラスの ABC です。 (ABC for classes that provide the __len__() method.)

code:typing_len_function.py

from typing import Sized

def len(obj: Sized) -> int:

return obj.__len__()

You can also define your own protocols. This is done by inheriting from Protocol and defining the function signatures (with empty function bodies) that the protocol expects.

「Protocolを継承し、そのプロトコルが期待する関数のシグネチャを（空の関数の本体と一緒に）定義することで、独自のプロトコルを定義できる」

Other examples of protocols 👉 Protocols and structural subtyping (Predefined protocols)