Python etc / typing.TYPE_CHECKING

Published: 2022-07-26.

Often, your type annotations will have circular dependencies. For example, Article has an attribute category: Category, and Category has attribute articles: list[Article]. If both classes are in the same file, adding from __future__ import annotations would solve the issue. But what if they are in different modules? Then you can hide imports that you need only for type annotations inside of the if TYPE_CHECKING block:

from __future__ import annotations
from dataclasses import dataclass
from typing import TYPE_CHECKING

  from .category import Category

class Article:
  category: Category

Fun fact: this constant is defined as TYPE_CHECKING = False. It won’t be executed at runtime, but the type checker is a static analyzer, it doesn’t care.