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typing-only-third-party-import (TC002)

Added in 0.8.0 · Related issues · View source

Derived from the flake8-type-checking linter.

Fix is sometimes available.

What it does

Checks for third-party imports that are only used for type annotations, but aren't defined in a type-checking block.

Why is this bad?

Unused imports add a performance overhead at runtime, and risk creating import cycles. If an import is only used in typing-only contexts, it can instead be imported conditionally under an if TYPE_CHECKING: block to minimize runtime overhead.

If lint.flake8-type-checking.quote-annotations is set to true, annotations will be wrapped in quotes if doing so would enable the corresponding import to be moved into an if TYPE_CHECKING: block.

If a class requires that type annotations be available at runtime (as is the case for Pydantic, SQLAlchemy, and other libraries), consider using the lint.flake8-type-checking.runtime-evaluated-base-classes and lint.flake8-type-checking.runtime-evaluated-decorators settings to mark them as such.

If lint.future-annotations is set to true, from __future__ import annotations will be added if doing so would enable an import to be moved into an if TYPE_CHECKING: block. This takes precedence over the lint.flake8-type-checking.quote-annotations setting described above if both settings are enabled.

Example

from __future__ import annotations

import pandas as pd


def func(df: pd.DataFrame) -> int:
    return len(df)

Use instead:

from __future__ import annotations

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    import pandas as pd


def func(df: pd.DataFrame) -> int:
    return len(df)

Options

References