add a test

Author: hauntsaninja

src/black/__init__.py

@@ -469,12 +469,8 @@ def main(  # noqa: C901
         out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
         ctx.exit(1)
 
-    if stdin_filename is not None:
-        src_with_stdin_filename = tuple(stdin_filename if s == "-" else s for s in src)
-    else:
-        src_with_stdin_filename = src
     root, method = (
-        find_project_root(src_with_stdin_filename) if code is None else (None, None)
+        find_project_root(src, stdin_filename) if code is None else (None, None)
     )
     ctx.obj["root"] = root
 

src/black/files.py

@@ -39,7 +39,9 @@
 
 
 @lru_cache()
-def find_project_root(srcs: Sequence[str]) -> Tuple[Path, str]:
+def find_project_root(
+    srcs: Sequence[str], stdin_filename: Optional[str] = None
+) -> Tuple[Path, str]:
     """Return a directory containing .git, .hg, or pyproject.toml.
 
     That directory will be a common parent of all files and directories
@@ -52,6 +54,8 @@ def find_project_root(srcs: Sequence[str]) -> Tuple[Path, str]:
     the second element as a string describing the method by which the
     project root was discovered.
     """
+    if stdin_filename is not None:
+        srcs = tuple(stdin_filename if s == "-" else s for s in srcs)
     if not srcs:
         srcs = [str(Path.cwd().resolve())]
 

tests/test_black.py

@@ -1396,6 +1396,12 @@ def test_find_project_root(self) -> None:
                 (src_dir.resolve(), "pyproject.toml"),
             )
 
+            with change_directory(test_dir):
+                self.assertEqual(
+                    black.find_project_root(("-",), stdin_filename="../whatever.py"),
+                    (root.resolve(), "pyproject.toml"),
+                )
+
     @patch(
         "black.files.find_user_pyproject_toml",
     )