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os-path-dirname (PTH120)

Added in v0.0.231 · Related issues · View source

Derived from the flake8-use-pathlib linter.

Fix is sometimes available.

What it does

Checks for uses of os.path.dirname.

Why is this bad?

pathlib offers a high-level API for path manipulation, as compared to the lower-level API offered by os.path. When possible, using Path object methods such as Path.parent can improve readability over the os.path module's counterparts (e.g., os.path.dirname()).

Examples

import os

os.path.dirname(__file__)

Use instead:

from pathlib import Path

Path(__file__).parent

Fix Safety

This rule's fix is always marked as unsafe because the replacement is not always semantically equivalent to the original code. In particular, pathlib performs path normalization, which can alter the result compared to os.path.dirname. For example, this normalization:

  • Collapses consecutive slashes (e.g., "a//b" → "a/b").
  • Removes trailing slashes (e.g., "a/b/" → "a/b").
  • Eliminates "." (e.g., "a/./b" → "a/b").

As a result, code relying on the exact string returned by os.path.dirname may behave differently after the fix.

Additionally, the fix is marked as unsafe because os.path.dirname() returns str or bytes (AnyStr), while Path.parent returns a Path object. This change in return type can break code that uses the return value.

Known issues

While using pathlib can improve the readability and type safety of your code, it can be less performant than the lower-level alternatives that work directly with strings, especially on older versions of Python.

References