How to Avoid Cyclic Imports in Python\'s Type Hinting?

Type Hinting in Python Without Cyclic Imports

Problem:

When splitting a large class into multiple files, cyclic imports can prevent type hinting from working properly. Specifically, importing the "main" class in a mixin class creates a circular dependency that hinders type checking.

Solution:

For Python 3.4:

Use the following code structure:

<code class="python"># main.py

import mymixin.py

class Main(object, MyMixin):
    def func1(self, xxx):
        ...

# mymixin.py

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from main import Main

class MyMixin(object):
    def func2(self: 'Main', xxx):  # Note the string type hint
        ...</code>

The TYPE_CHECKING constant ensures that the import of main.py is only evaluated during type checking, avoiding the cyclic import error.

For Python 3.7 :

Python 3.7 introduces a more elegant solution:

<code class="python"># main.py

import mymixin.py

class Main(object, MyMixin):
    def func1(self, xxx):
        ...

# mymixin.py

from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from main import Main

class MyMixin(object):
    def func2(self: Main, xxx):  # No need for a string type hint
        ...</code>

The future import annotations import enables string type hints and avoids evaluating them at runtime.

Additional Considerations:

• Mypy may require creating an abstract base class (ABC) inherited by both Main and MyMixin to ensure proper type checking.
• PyCharm's checker may also require a similar approach for maximum compatibility.

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