Why do we need import TYPE_CHECKING in models?

fastapi / full-stack-fastapi-template

Full stack, modern web application template. Using FastAPI, React, SQLModel, PostgreSQL, Docker, GitHub Actions, automatic HTTPS and more.

MIT License

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Why do we need import TYPE_CHECKING in models? #406

Open BEEugene opened 3 years ago

BEEugene commented 3 years ago

Hi!

This is a great template project. Why should we import type_checking in models? from typing import TYPE_CHECKING https://github.com/tiangolo/full-stack-fastapi-postgresql/blob/490c554e23343eec0736b06e59b2108fdd057fdc/%7B%7Bcookiecutter.project_slug%7D%7D/backend/app/app/models/item.py

As far as I understood this import is needed for linter, but how it works? Does it somehow check database consistency?

haviduck commented 3 years ago

its for mypy, my guess is to lint relations while blocking with noqa

raphant commented 3 years ago

I think it's primarily for auto-completion while avoiding circular imports. In the link you provided, from .user import User # noqa: F401 will only import User for the sake of type checking. It will not execute any code from the .user module.