Ignore pyproject.toml missing tool.black section

CHANGES.md

@@ -16,7 +16,12 @@
 
 ### Configuration
 
-<!-- Changes to how Black can be configured -->
+- _Black_ now ignores `pyproject.toml` that is missing a `tool.black` section when
+  discovering project root and configuration. Since _Black_ continues to use version
+  control as an indicator of project root, this is expected to primarily change behavior
+  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)
 
 ### Packaging
 

docs/usage_and_configuration/the_basics.md

@@ -456,10 +456,11 @@ of tools like [Poetry](https://python-poetry.org/),
 
 ### Where _Black_ looks for the file
 
-By default _Black_ looks for `pyproject.toml` starting from the common base directory of
-all files and directories passed on the command line. If it's not there, it looks in
-parent directories. It stops looking when it finds the file, or a `.git` directory, or a
-`.hg` directory, or the root of the file system, whichever comes first.
+By default _Black_ looks for `pyproject.toml` containing a `[tool.black]` section
+starting from the common base directory of all files and directories passed on the
+command line. If it's not there, it looks in parent directories. It stops looking when
+it finds the file, or a `.git` directory, or a `.hg` directory, or the root of the file
+system, whichever comes first.
 
 If you're formatting standard input, _Black_ will look for configuration starting from
 the current working directory.

src/black/files.py

@@ -42,6 +42,12 @@
     import colorama  # noqa: F401
 
 
+@lru_cache
+def _load_toml(path: Union[Path, str]) -> Dict[str, Any]:
+    with open(path, "rb") as f:
+        return tomllib.load(f)
+
+
 @lru_cache
 def find_project_root(
     srcs: Sequence[str], stdin_filename: Optional[str] = None
@@ -84,7 +90,9 @@ def find_project_root(
             return directory, ".hg directory"
 
         if (directory / "pyproject.toml").is_file():
-            return directory, "pyproject.toml"
+            pyproject_toml = _load_toml(directory / "pyproject.toml")
+            if "black" in pyproject_toml.get("tool", {}):
+                return directory, "pyproject.toml"
 
     return directory, "file system root"
 
@@ -117,8 +125,7 @@ def parse_pyproject_toml(path_config: str) -> Dict[str, Any]:
 
     If parsing fails, will raise a tomllib.TOMLDecodeError.
     """
-    with open(path_config, "rb") as f:
-        pyproject_toml = tomllib.load(f)
+    pyproject_toml = _load_toml(path_config)
     config: Dict[str, Any] = pyproject_toml.get("tool", {}).get("black", {})
     config = {k.replace("--", "").replace("-", "_"): v for k, v in config.items()}
 

tests/test_black.py

@@ -1668,9 +1668,9 @@ def test_find_project_root(self) -> None:
             src_dir.mkdir()
 
             root_pyproject = root / "pyproject.toml"
-            root_pyproject.touch()
+            root_pyproject.write_text("[tool.black]", encoding="utf-8")
             src_pyproject = src_dir / "pyproject.toml"
-            src_pyproject.touch()
+            src_pyproject.write_text("[tool.black]", encoding="utf-8")
             src_python = src_dir / "foo.py"
             src_python.touch()
 
@@ -1693,6 +1693,20 @@ def test_find_project_root(self) -> None:
                     (src_dir.resolve(), "pyproject.toml"),
                 )
 
+            src_sub = src_dir / "sub"
+            src_sub.mkdir()
+
+            src_sub_pyproject = src_sub / "pyproject.toml"
+            src_sub_pyproject.touch()  # empty
+
+            src_sub_python = src_sub / "bar.py"
+
+            # we skip src_sub_pyproject since it is missing the [tool.black] section
+            self.assertEqual(
+                black.find_project_root((src_sub_python,)),
+                (src_dir.resolve(), "pyproject.toml"),
+            )
+
     @patch(
         "black.files.find_user_pyproject_toml",
     )