fix: handle cross-drive ValueError in _adjust_args_and_chdir on Windows

terminalchai · terminalchai · commit 712770739912 · 2026-04-17T00:34:39.000+05:30
On Windows, os.path.relpath() raises ValueError when the path and the current working directory are on different drives (e.g. config on C: but the git repo is on D:). Introduce a small _try_relpath() helper that wraps os.path.relpath in a try/except ValueError and returns the original path unchanged when the drives differ. All four os.path.relpath calls in _adjust_args_and_chdir (config, files, commit_msg_filename, repo) are updated to use this helper, so every cross-drive combination is covered. Fixes #2530
diff --git a/pre_commit/main.py b/pre_commit/main.py
@@ -172,6 +172,16 @@ def _add_run_options(parser: argparse.ArgumentParser) -> None:
     )
 
 
+def _try_relpath(path: str) -> str:
+    """Return os.path.relpath(path), or path unchanged on Windows when path
+    and the current working directory are on different drives (which would
+    raise ValueError: path is on mount 'C:', start on mount 'D:')."""
+    try:
+        return os.path.relpath(path)
+    except ValueError:
+        return path
+
+
 def _adjust_args_and_chdir(args: argparse.Namespace) -> None:
     # `--config` was specified relative to the non-root working directory
     if os.path.exists(args.config):
@@ -188,15 +198,15 @@ def _adjust_args_and_chdir(args: argparse.Namespace) -> None:
     toplevel = git.get_root()
     os.chdir(toplevel)
 
-    args.config = os.path.relpath(args.config)
+    args.config = _try_relpath(args.config)
     if args.command in {'run', 'try-repo'}:
-        args.files = [os.path.relpath(filename) for filename in args.files]
+        args.files = [_try_relpath(filename) for filename in args.files]
         if args.commit_msg_filename is not None:
-            args.commit_msg_filename = os.path.relpath(
+            args.commit_msg_filename = _try_relpath(
                 args.commit_msg_filename,
             )
     if args.command == 'try-repo' and os.path.exists(args.repo):
-        args.repo = os.path.relpath(args.repo)
+        args.repo = _try_relpath(args.repo)
 
 
 def main(argv: Sequence[str] | None = None) -> int:
diff --git a/tests/main_test.py b/tests/main_test.py
@@ -89,6 +89,35 @@ def test_adjust_args_try_repo_repo_relative(in_git_dir):
         assert args.repo == 'foo'
 
 
+def test_try_relpath_returns_relpath_when_same_drive():
+    # Normal case: same drive / same filesystem -- should behave like relpath
+    result = main._try_relpath(os.path.abspath('.'))
+    assert result == os.path.relpath(os.path.abspath('.'))
+
+
+def test_try_relpath_returns_original_on_cross_drive_valueerror():
+    # On Windows, os.path.relpath raises ValueError when paths are on
+    # different drives (#2530).  _try_relpath must return the path unchanged.
+    abs_path = 'C:\\Users\\me\\.pre-commit-config.yaml'
+    with mock.patch('os.path.relpath', side_effect=ValueError('different mount')):
+        assert main._try_relpath(abs_path) == abs_path
+
+
+@pytest.mark.skipif(os.name != 'nt', reason='windows cross-drive only')
+def test_adjust_args_config_on_different_drive(in_git_dir):  # pragma: posix no cover
+    # Simulate a config whose absolute path is on a drive other than the git
+    # root so that os.path.relpath would raise ValueError.
+    abs_config = 'C:\\Users\\me\\.pre-commit-config.yaml'
+    with mock.patch('pre_commit.main.os.path.relpath', side_effect=ValueError('x')):
+        args = _args(config=abs_config)
+        # abspath is called before chdir, so mock exists to avoid a real chdir
+        with mock.patch('os.path.exists', return_value=False):
+            with mock.patch('pre_commit.git.get_root', return_value=str(in_git_dir)):
+                main._adjust_args_and_chdir(args)
+        # config must be preserved (not explode with ValueError)
+        assert args.config == abs_config
+
+
 FNS = (
     'autoupdate', 'clean', 'gc', 'hook_impl', 'install', 'install_hooks',
     'migrate_config', 'run', 'sample_config', 'uninstall',
