feat(config): Support --config in pyproject.toml

You can now specify `config` key in pyproject.toml which does the same
job as the CLI option `--config`. The path of config should be relative
to the CWD it is running from. If the file is not found or is invalid
i.e. it is a folder rather a TOML file it would raise
`click.exceptions.FileError`, and if the file can't be parsed it would
raise `tomli.TOMLDecodeError`. All black config keys in the custom
config should be registered under `[black]` and they would overwrite the
config specified in pyproject but not the one specified by `--config`
flag while running black with the CLI.

Closes psf#1826

.gitignore

@@ -1,11 +1,8 @@
 .venv
-.coverage
-.coverage.*
 _build
 .DS_Store
 .vscode
 docs/_static/pypi.svg
-.tox
 __pycache__
 black.egg-info
 build/
@@ -16,6 +13,13 @@ src/_black_version.py
 .eggs
 .dmypy.json
 *.swp
-.hypothesis/
 venv/
 .ipynb_checkpoints/
+
+# Unit test / coverage reports
+htmlcov/
+.tox/
+.coverage
+.coverage.*
+.hypothesis/
+.pytest_cache/

src/black/__init__.py

@@ -45,7 +45,12 @@
 from black.concurrency import cancel, shutdown, maybe_install_uvloop
 from black.output import dump_to_file, ipynb_diff, diff, color_diff, out, err
 from black.report import Report, Changed, NothingChanged
-from black.files import find_project_root, find_pyproject_toml, parse_pyproject_toml
+from black.files import (
+    find_project_root,
+    find_pyproject_toml,
+    parse_pyproject_toml,
+    parse_black_config_toml,
+)
 from black.files import gen_python_files, get_gitignore, normalize_path_maybe_ignore
 from black.files import wrap_stream_for_windows
 from black.parsing import InvalidInput  # noqa F401
@@ -106,7 +111,10 @@ def read_pyproject_toml(
     Returns the path to a successfully found and read configuration file, None
     otherwise.
     """
+    value_is_pyproject_config = False
+
     if not value:
+        value_is_pyproject_config = True
         value = find_pyproject_toml(ctx.params.get("src", ()))
         if value is None:
             return None
@@ -120,26 +128,44 @@ def read_pyproject_toml(
 
     if not config:
         return None
-    else:
-        # Sanitize the values to be Click friendly. For more information please see:
-        # https://github.com/psf/black/issues/1458
-        # https://github.com/pallets/click/issues/1567
-        config = {
-            k: str(v) if not isinstance(v, (list, dict)) else v
-            for k, v in config.items()
-        }
-
-    target_version = config.get("target_version")
+
+    if ctx.default_map:
+        config.update(ctx.default_map)
+
+    black_config = config.get("config")
+
+    if black_config:
+        black_config_path = Path(black_config).resolve()
+
+        try:
+            custom_black_config = parse_black_config_toml(str(black_config_path))
+        except (OSError, ValueError) as e:
+            raise click.FileError(
+                filename=str(black_config_path),
+                hint=f"Error reading configuration file: {e}",
+            ) from None
+        else:
+            # Do overwrite the clashing keys of pyproject.toml but don't
+            # overwrite them for `--config` passed through the CLI.
+            if value_is_pyproject_config:
+                config.update(custom_black_config)
+            else:
+                custom_black_config.update(config)
+                config = custom_black_config.copy()
+
+    # Sanitize the values to be Click friendly. For more information please see:
+    # https://github.com/psf/black/issues/1458
+    # https://github.com/pallets/click/issues/1567
+    default_map: Dict[str, Any] = {
+        k: str(v) if not isinstance(v, (list, dict)) else v for k, v in config.items()
+    }
+
+    target_version = default_map.get("target_version")
     if target_version is not None and not isinstance(target_version, list):
         raise click.BadOptionUsage(
             "target-version", "Config key target-version must be a list"
         )
 
-    default_map: Dict[str, Any] = {}
-    if ctx.default_map:
-        default_map.update(ctx.default_map)
-    default_map.update(config)
-
     ctx.default_map = default_map
     return value
 

src/black/files.py

@@ -99,6 +99,17 @@ def parse_pyproject_toml(path_config: str) -> Dict[str, Any]:
     return {k.replace("--", "").replace("-", "_"): v for k, v in config.items()}
 
 
+def parse_black_config_toml(path_config: str) -> Dict[str, Any]:
+    """Parse the custom black config provided in pyproject.toml.
+
+    If parsing fails, will raise a tomli.TOMLDecodeError
+    """
+    with open(path_config, encoding="utf8") as f:
+        pyproject_toml = tomli.load(f)  # type: ignore  # due to deprecated API usage
+    config = pyproject_toml.get("black", {})
+    return {k.replace("--", "").replace("-", "_"): v for k, v in config.items()}
+
+
 @lru_cache()
 def find_user_pyproject_toml() -> Path:
     r"""Return the path to the top-level user configuration for black.

tests/invalid_test.toml

@@ -0,0 +1,17 @@
+[tool.black]
+config = "tests/bazqux.toml"
+verbose = 1
+--check = "no"
+diff = "y"
+color = true
+line-length = 79
+target-version = ["py36", "py37", "py38"]
+exclude='\.pyi?$'
+include='\.py?$'
+
+[v1.0.0-syntax]
+# This shouldn't break Black.
+contributors = [
+  "Foo Bar <foo@example.com>",
+  { name = "Baz Qux", email = "bazqux@example.com", url = "https://example.com/bazqux" }
+]

tests/test.toml

@@ -1,4 +1,5 @@
 [tool.black]
+config = "tests/test_black.toml"
 verbose = 1
 --check = "no"
 diff = "y"

tests/test_black.py

@@ -1277,20 +1277,34 @@ def test_parse_pyproject_toml(self) -> None:
         self.assertEqual(config["exclude"], r"\.pyi?$")
         self.assertEqual(config["include"], r"\.py?$")
 
+    def test_parse_black_config(self) -> None:
+        test_black_toml_file = THIS_DIR / "test_black.toml"
+        config = black.parse_black_config_toml(str(test_black_toml_file))
+        self.assertEqual(config["line_length"], 88)
+
     def test_read_pyproject_toml(self) -> None:
         test_toml_file = THIS_DIR / "test.toml"
         fake_ctx = FakeContext()
         black.read_pyproject_toml(fake_ctx, FakeParameter(), str(test_toml_file))
         config = fake_ctx.default_map
+        self.assertEqual(config["config"], "tests/test_black.toml")
         self.assertEqual(config["verbose"], "1")
         self.assertEqual(config["check"], "no")
         self.assertEqual(config["diff"], "y")
         self.assertEqual(config["color"], "True")
+        # Test that pyproject.toml specified config won't overwrite the config
+        # passed through `--config`.
         self.assertEqual(config["line_length"], "79")
         self.assertEqual(config["target_version"], ["py36", "py37", "py38"])
         self.assertEqual(config["exclude"], r"\.pyi?$")
         self.assertEqual(config["include"], r"\.py?$")
 
+    def test_invalid_black_config(self) -> None:
+        test_toml_file = THIS_DIR / "invalid_test.toml"
+        fake_ctx = FakeContext()
+        with self.assertRaises(click.exceptions.FileError):
+            black.read_pyproject_toml(fake_ctx, FakeParameter(), str(test_toml_file))
+
     def test_find_project_root(self) -> None:
         with TemporaryDirectory() as workspace:
             root = Path(workspace)

tests/test_black.toml

@@ -0,0 +1,2 @@
+[black]
+line-length = 88