Fix matching of absolute paths in --include

CHANGES.md

@@ -18,6 +18,8 @@
 
 <!-- Changes to how Black can be configured -->
 
+- Fix a bug in the matching of absolute path names in `--include` (#3976)
+
 ### Packaging
 
 <!-- Changes to how Black is packaged, such as dependency requirements -->

src/black/files.py

@@ -361,6 +361,10 @@ def gen_python_files(
         if normalized_path is None:
             continue
 
+        normalized_path = "/" + normalized_path
+        if child.is_dir():
+            normalized_path += "/"
+
         if child.is_dir():
             # If gitignore is None, gitignore usage is disabled, while a Falsey
             # gitignore is when the directory doesn't have a .gitignore file.

tests/test_black.py

@@ -2388,6 +2388,27 @@ def test_empty_include(self) -> None:
         # Setting exclude explicitly to an empty string to block .gitignore usage.
         assert_collected_sources(src, expected, include="", exclude="")
 
+    def test_include_absolute_path(self) -> None:
+        path = DATA_DIR / "include_exclude_tests"
+        src = [path]
+        expected = [
+            Path(path / "b/dont_exclude/a.pie"),
+        ]
+        assert_collected_sources(
+            src, expected, root=path, include=r"^/b/dont_exclude/a\.pie$", exclude=""
+        )
+
+    def test_exclude_absolute_path(self) -> None:
+        path = DATA_DIR / "include_exclude_tests"
+        src = [path]
+        expected = [
+            Path(path / "b/dont_exclude/a.py"),
+            Path(path / "b/.definitely_exclude/a.py"),
+        ]
+        assert_collected_sources(
+            src, expected, root=path, include=r"\.py$", exclude=r"^/b/exclude/a\.py$"
+        )
+
     def test_extend_exclude(self) -> None:
         path = DATA_DIR / "include_exclude_tests"
         src = [path]