Add relpath test for Windows error coverage

pre_commit/main.py

@@ -165,14 +165,14 @@ def _adjust_args_and_chdir(args: argparse.Namespace) -> None:
     toplevel = git.get_root()
     os.chdir(toplevel)
 
+    # https://github.com/pre-commit/pre-commit/issues/2530
+    # `os.relpath` raises a ValueError on Windows when the paths given are on
+    # separate drives.
     try:
         args.config = os.path.relpath(args.config)
-    # https://github.com/pre-commit/pre-commit/issues/2530
-    # os.relpath will fail with a ValueError if the two directories are on
-    # two different drives on Windows and since the path is made relative
-    # for display purposes only we can ignore the error
     except ValueError:
         pass
+
     if args.command in {'run', 'try-repo'}:
         args.files = [os.path.relpath(filename) for filename in args.files]
         if args.commit_msg_filename is not None:

tests/main_test.py

@@ -27,6 +27,13 @@ def test_adjust_args_and_chdir_not_in_git_dir(in_tmpdir):
         main._adjust_args_and_chdir(_args())
 
 
+def test_adjust_args_and_chdir_relpath_to_different_drive():
+    args = _args(command='run', files=['f1', 'f2'])
+    with mock.patch.object(os.path, 'relpath', side_effect=ValueError):
+        with pytest.raises(ValueError):
+            main._adjust_args_and_chdir(args)
+
+
 def test_adjust_args_and_chdir_noop(in_git_dir):
     args = _args(command='run', files=['f1', 'f2'])
     main._adjust_args_and_chdir(args)