- Circular imports for Python typing annotations

Circular imports for Python typing annotations

Posted on February 1, 2023 (Last modified on July 2, 2024) • 1 min read • 143 words

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Problem:

# ###############################################################
# models.py
from controllers import BookController

class Book:
    def get_controller(self) -> BookController:
        return BookController(self)

# ###############################################################
# controllers.py
from models import Book

class BookController:
    def __init__(self, book: Book) -> None:
        self.book = book

Do you see it? Try it out - it will generate a “circular import” Exception.

The solution is very nicely described in Adam Johnson’s blog post (credit where credit’s due).

Copy-pasted for myself (still, please visit Adam’s page!!, he explains it much better and with more detail), this is it:

# ###############################################################
# models.py
from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from controllers import BookController

class Book:
    def get_controller(self) -> BookController:
        return BookController(self)

# ###############################################################
# controllers.py
from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from models import Book

class BookController:
    def __init__(self, book: Book) -> None:
        self.book = book