flake8-type-checking

Lets you know which imports to put in type-checking blocks.

For the imports you've already defined inside type-checking blocks, it can also help you manage forward references using PEP 484 or PEP 563 style references.

Codes

Code	Description
TC001	Move import into a type-checking block
TC002	Move third-party import into a type-checking block
TC003	Found multiple type checking blocks
TC004	Move import out of type-checking block. Import is used for more than type hinting.
TC005	Empty type-checking block

Forward reference codes

These code ranges are opt-in. They represent two different ways of solving the same problem, so please only choose one.

TC100 and TC101 manage forward references by taking advantage of postponed evaluation of annotations.

Code	Description
TC100	Add 'from __future__ import annotations' import
TC101	Annotation does not need to be a string literal

TC200 and TC201 manage forward references using string literals.

Code	Description
TC200	Annotation needs to be made into a string literal
TC201	Annotation does not need to be a string literal

To select one of the ranges, just specify the code in your flake8 config:

[flake8]
max-line-length = 80
max-complexity = 12
...
ignore = E501
select = C,E,F,W,..., TC, TC2  # or TC1
# alternatively:
enable-extensions = TC, TC2  # or TC1

Rationale

Good type hinting requires a lot of imports, which can increase the risk of import cycles in your project. The recommended way of preventing this problem is to use typing.TYPE_CHECKING blocks to guard these types of imports.

Both TC001 and TC002 help alleviate this problem; the reason there are two codes instead of one, is because the import cycles rarely occur from library/third-party imports, so this artificial split provides a way to filter down the total pool of imports for users that want to guard against import cycles, but don't want to manage every import in their projects this strictly.

Once imports are guarded, they will no longer be evaluated during runtime. The consequence of this is that these imports can no longer be treated as if they were imported outside the block. Instead we need to use forward references.

For Python version >= 3.7, there are actually two ways of solving this issue. You can either make your annotations string literals, or you can use a __futures__ import to enable postponed evaluation of annotations. See this excellent stackoverflow answer for a better explanation of the differences.

Installation

pip install flake8-type-checking

Examples

Bad code

models/a.py

from models.b import B

class A(Model):
    def foo(self, b: B): ...

models/b.py

from models.a import A

class B(Model):
    def bar(self, a: A): ...

Will result in these errors

>> a.py: TC002: Move third-party import 'models.b.B' into a type-checking block
>> b.py: TC002: Move third-party import 'models.a.A' into a type-checking block

and consequently trigger these errors if imports are purely moved into type-checking block, without proper forward reference handling

>> a.py: TC100: Add 'from __future__ import annotations' import
>> b.py: TC100: Add 'from __future__ import annotations' import

or

>> a.py: TC200: Annotation 'B' needs to be made into a string literal
>> b.py: TC200: Annotation 'A' needs to be made into a string literal

Good code

models/a.py

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from models.b import B

class A(Model):
    def foo(self, b: 'B'): ...

models/b.py

# TC1
from __future__ import annotations

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from models.a import A

class B(Model):
    def bar(self, a: A): ...

or

# TC2
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from models.a import A

class B(Model):
    def bar(self, a: 'A'): ...

As a pre-commit hook

You can run this flake8 plugin as a pre-commit hook:

- repo: https://github.com/pycqa/flake8
  rev: 3.9.2
  hooks:
    - id: flake8
      additional_dependencies: [ flake8-type-checking ]