Improve handling of root to get_sources

This is a little more type safe and a little cleaner

src/black/__init__.py

@@ -560,9 +560,10 @@ def main(  # noqa: C901
             content=code, fast=fast, write_back=write_back, mode=mode, report=report
         )
     else:
+        assert root is not None  # root is only None if code is not None
         try:
             sources = get_sources(
-                ctx=ctx,
+                root=root,
                 src=src,
                 quiet=quiet,
                 verbose=verbose,
@@ -615,7 +616,7 @@ def main(  # noqa: C901
 
 def get_sources(
     *,
-    ctx: click.Context,
+    root: Path,
     src: Tuple[str, ...],
     quiet: bool,
     verbose: bool,
@@ -628,7 +629,6 @@ def get_sources(
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""
     sources: Set[Path] = set()
-    root = ctx.obj["root"]
 
     using_default_exclude = exclude is None
     exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES) if exclude is None else exclude
@@ -645,7 +645,7 @@ def get_sources(
 
         if is_stdin or p.is_file():
             normalized_path: Optional[str] = normalize_path_maybe_ignore(
-                p, ctx.obj["root"], report
+                p, root, report
             )
             if normalized_path is None:
                 if verbose:
@@ -674,7 +674,9 @@ def get_sources(
 
             sources.add(p)
         elif p.is_dir():
-            p = root / normalize_path_maybe_ignore(p, ctx.obj["root"], report)
+            p_relative = normalize_path_maybe_ignore(p, root, report)
+            assert p_relative is not None
+            p = root / p_relative
             if verbose:
                 out(f'Found input source directory: "{p}"', fg="blue")
 
@@ -686,7 +688,7 @@ def get_sources(
             sources.update(
                 gen_python_files(
                     p.iterdir(),
-                    ctx.obj["root"],
+                    root,
                     include,
                     exclude,
                     extend_exclude,

tests/test_black.py

@@ -507,13 +507,11 @@ def _mocked_calls() -> bool:
         with patch("pathlib.Path.iterdir", return_value=target_contents), patch(
             "pathlib.Path.cwd", return_value=working_directory
         ), patch("pathlib.Path.is_dir", side_effect=mock_n_calls([True])):
-            ctx = FakeContext()
             # Note that the root folder (project_root) isn't the folder
             # named "root" (aka working_directory)
-            ctx.obj["root"] = project_root
             report = MagicMock(verbose=True)
             black.get_sources(
-                ctx=ctx,
+                root=project_root,
                 src=("./child",),
                 quiet=False,
                 verbose=True,
@@ -2163,7 +2161,7 @@ def assert_collected_sources(
     src: Sequence[Union[str, Path]],
     expected: Sequence[Union[str, Path]],
     *,
-    ctx: Optional[FakeContext] = None,
+    root: Optional[Path] = None,
     exclude: Optional[str] = None,
     include: Optional[str] = None,
     extend_exclude: Optional[str] = None,
@@ -2179,7 +2177,7 @@ def assert_collected_sources(
     )
     gs_force_exclude = None if force_exclude is None else compile_pattern(force_exclude)
     collected = black.get_sources(
-        ctx=ctx or FakeContext(),
+        root=root or THIS_DIR,
         src=gs_src,
         quiet=False,
         verbose=False,
@@ -2215,20 +2213,16 @@ def test_gitignore_used_as_default(self) -> None:
             base / "b/.definitely_exclude/a.pyi",
         ]
         src = [base / "b/"]
-        ctx = FakeContext()
-        ctx.obj["root"] = base
-        assert_collected_sources(src, expected, ctx=ctx, extend_exclude=r"/exclude/")
+        assert_collected_sources(src, expected, root=base, extend_exclude=r"/exclude/")
 
     def test_gitignore_used_on_multiple_sources(self) -> None:
         root = Path(DATA_DIR / "gitignore_used_on_multiple_sources")
         expected = [
             root / "dir1" / "b.py",
             root / "dir2" / "b.py",
         ]
-        ctx = FakeContext()
-        ctx.obj["root"] = root
         src = [root / "dir1", root / "dir2"]
-        assert_collected_sources(src, expected, ctx=ctx)
+        assert_collected_sources(src, expected, root=root)
 
     @patch("black.find_project_root", lambda *args: (THIS_DIR.resolve(), None))
     def test_exclude_for_issue_1572(self) -> None:
@@ -2334,27 +2328,21 @@ def test_gitignore_that_ignores_subfolders(self) -> None:
         # If gitignore with */* is in root
         root = Path(DATA_DIR / "ignore_subfolders_gitignore_tests" / "subdir")
         expected = [root / "b.py"]
-        ctx = FakeContext()
-        ctx.obj["root"] = root
-        assert_collected_sources([root], expected, ctx=ctx)
+        assert_collected_sources([root], expected, root=root)
 
         # If .gitignore with */* is nested
         root = Path(DATA_DIR / "ignore_subfolders_gitignore_tests")
         expected = [
             root / "a.py",
             root / "subdir" / "b.py",
         ]
-        ctx = FakeContext()
-        ctx.obj["root"] = root
-        assert_collected_sources([root], expected, ctx=ctx)
+        assert_collected_sources([root], expected, root=root)
 
         # If command is executed from outer dir
         root = Path(DATA_DIR / "ignore_subfolders_gitignore_tests")
         target = root / "subdir"
         expected = [target / "b.py"]
-        ctx = FakeContext()
-        ctx.obj["root"] = root
-        assert_collected_sources([target], expected, ctx=ctx)
+        assert_collected_sources([target], expected, root=root)
 
     def test_empty_include(self) -> None:
         path = DATA_DIR / "include_exclude_tests"