fix unc path bug as stdin-filename in Windows python 3.8

CHANGES.md

@@ -29,6 +29,7 @@
   for users in a monorepo setup (desirably). If you wish to preserve previous behavior,
   simply add an empty `[tool.black]` to the previously discovered `pyproject.toml`
   (#4204)
+- Fix UNC path as stdin-filename in Windows Python 3.8 (#4210)
 
 ### Packaging
 

src/black/__init__.py

@@ -749,7 +749,12 @@ def get_sources(
 
         # Compare the logic here to the logic in `gen_python_files`.
         if is_stdin or path.is_file():
-            root_relative_path = get_root_relative_path(path, root, report)
+            # From https://stackoverflow.com/questions/42513056/how-to-get-absolute-path-of-a-pathlib-path-object  # noqa: B950
+            # suggest to use resolve instead of absolute prior to Python 3.11
+            # to get the absolute path.
+            # This change is to fix bug:
+            # https://github.com/psf/black/issues/4209
+            root_relative_path = get_root_relative_path(path.resolve(), root, report)
 
             if root_relative_path is None:
                 continue
@@ -772,7 +777,7 @@ def get_sources(
                 continue
 
             if is_stdin:
-                path = Path(f"{STDIN_PLACEHOLDER}{str(path)}")
+                path = Path(f"{STDIN_PLACEHOLDER}{str(path.resolve())}")
 
             if path.suffix == ".ipynb" and not jupyter_dependencies_are_installed(
                 warn=verbose or not quiet

tests/test_black.py

@@ -43,6 +43,7 @@
 from black import Feature, TargetVersion
 from black import re_compile_maybe_verbose as compile_pattern
 from black.cache import FileData, get_cache_dir, get_cache_file
+from black.const import STDIN_PLACEHOLDER
 from black.debug import DebugVisitor
 from black.mode import Mode, Preview
 from black.output import color_diff, diff
@@ -2623,6 +2624,31 @@ def test_symlinks(self) -> None:
         outside_root_symlink.resolve.assert_called_once()
         ignored_symlink.resolve.assert_not_called()
 
+    def test_unc_path(self) -> None:
+        if system() != "Windows":
+            return
+
+        stdin_filename = "\\\\?\\D:\\git\\OLive\\setup.py"
+        root = Path("D:\\git\\OLive")
+        collected = black.get_sources(
+            root=root,
+            src=("-",),
+            quiet=False,
+            verbose=False,
+            include=DEFAULT_INCLUDE,
+            exclude=DEFAULT_EXCLUDE,
+            extend_exclude=None,
+            force_exclude=None,
+            report=black.Report(),
+            stdin_filename=stdin_filename,
+        )
+        assert len(collected) == 1
+        expected_path = Path(
+            f"{STDIN_PLACEHOLDER}{str(Path(stdin_filename).resolve())}"
+        )
+        (collected_item,) = collected
+        assert collected_item == expected_path
+
     def test_get_sources_with_stdin_symlink_outside_root(
         self,
     ) -> None: