Summary

Fixes #2530

On Windows, os.path.relpath() raises ValueError when the path and the current working directory are on different drives — e.g. a config at C:\Users\me.pre-commit-config.yaml while the git repo lives on D:. This causes an unhandled traceback instead of a helpful error message.

Root cause

_adjust_args_and_chdir calls os.path.relpath() unconditionally on �rgs.config, �rgs.files, �rgs.commit_msg_filename, and �rgs.repo after doing os.chdir(toplevel). When the git root and the path being relativised are on different Windows drives, Python raises:

\
ValueError: path is on mount 'C:', start on mount 'D:'
\\

Fix

Introduce a small _try_relpath() helper (as suggested by @asottile in #2530 — option 2):

\\python
def _try_relpath(path: str) -> str:
try:
return os.path.relpath(path)
except ValueError:
return path
\\

All four os.path.relpath calls in _adjust_args_and_chdir are updated to use _try_relpath, so every cross-drive combination (config, files list, commit-msg filename, try-repo repo path) is covered. On POSIX and same-drive Windows the behaviour is identical to before.

Tests added

• est_try_relpath_returns_relpath_when_same_drive — normal case, same drive
• est_try_relpath_returns_original_on_cross_drive_valueerror — mocks os.path.relpath to raise ValueError, asserts original path is returned unchanged
• est_adjust_args_config_on_different_drive — Windows-only integration test for the full _adjust_args_and_chdir path