Add Black --preview support

CHANGES.rst

@@ -10,6 +10,8 @@ Added
 - Display exit code in parentheses after error message.
 - Do not reformat renamed files.
 - CI workflow to post recent project activity in a discussion. Triggered manually.
+- The ``--preview`` configuration flag is now supported in the configuration files for
+  Darker and Black
 
 Fixed
 -----

README.rst

@@ -358,6 +358,9 @@ The following `command line arguments`_ can also be used to modify the defaults:
        Also convert string formatting to use f-strings using the ``flynt`` package
 -i, --isort
        Also sort imports using the ``isort`` package
+--preview
+       In Black, enable potentially disruptive style changes that may be added to Black
+       in the future
 -L CMD, --lint CMD
        Run a linter on changed files. ``CMD`` can be a name or path of the linter
        binary, or a full quoted command line with the command and options. Linters read
@@ -422,6 +425,8 @@ An example ``pyproject.toml`` configuration file:
    exclude = "test_*\.py"
    extend_exclude = "/generated/"
    force_exclude = ".*\.pyi"
+   preview = true                    # Only supported in [tool.black]
+
 
    [tool.isort]
    profile = "black"

src/darker/__main__.py

@@ -522,6 +522,8 @@ def main(  # noqa: C901,PLR0912,PLR0915
         black_config["skip_string_normalization"] = args.skip_string_normalization
     if args.skip_magic_trailing_comma is not None:
         black_config["skip_magic_trailing_comma"] = args.skip_magic_trailing_comma
+    if args.preview:
+        black_config["preview"] = args.preview
 
     paths, common_root = resolve_paths(args.stdin_filename, args.src)
     # `common_root` is now the common root of given paths,

src/darker/black_diff.py

@@ -76,6 +76,7 @@ class BlackConfig(TypedDict, total=False):
     line_length: int
     skip_string_normalization: bool
     skip_magic_trailing_comma: bool
+    preview: bool
 
 
 class BlackModeAttributes(TypedDict, total=False):
@@ -86,6 +87,7 @@ class BlackModeAttributes(TypedDict, total=False):
     string_normalization: bool
     is_pyi: bool
     magic_trailing_comma: bool
+    preview: bool
 
 
 def read_black_config(src: Tuple[str, ...], value: Optional[str]) -> BlackConfig:
@@ -109,6 +111,7 @@ def read_black_config(src: Tuple[str, ...], value: Optional[str]) -> BlackConfig
         "line_length",
         "skip_magic_trailing_comma",
         "skip_string_normalization",
+        "preview",
     ]:
         if key in raw_config:
             config[key] = raw_config[key]  # type: ignore
@@ -203,6 +206,8 @@ def run_black(src_contents: TextDocument, black_config: BlackConfig) -> TextDocu
         # ``black.Mode`` needs to be the opposite boolean of
         # ``skip-string-normalization``, hence the inverse boolean
         mode["string_normalization"] = not black_config["skip_string_normalization"]
+    if "preview" in black_config:
+        mode["preview"] = black_config["preview"]
 
     # The custom handling of empty and all-whitespace files below will be unnecessary if
     # https://github.com/psf/black/pull/2484 lands in Black.

src/darker/command_line.py

@@ -39,6 +39,7 @@ def make_argument_parser(require_src: bool) -> ArgumentParser:
     add_arg(hlp.CHECK, "--check", action="store_true")
     add_arg(hlp.FLYNT, "-f", "--flynt", action="store_true")
     add_arg(hlp.ISORT, "-i", "--isort", action="store_true")
+    add_arg(hlp.PREVIEW, "--preview", action="store_true")
     add_lint_arg(parser)
     add_arg(
         hlp.SKIP_STRING_NORMALIZATION,

src/darker/help.py

@@ -133,6 +133,11 @@ def get_extra_instruction(dependency: str) -> str:
     " `skip-magic-trailing-comma = true` from a Black configuration file."
 )
 
+PREVIEW = (
+    "In Black, enable potentially disruptive style changes that may be added to Black"
+    " in the future"
+)
+
 LINE_LENGTH = "How many characters per line to allow [default: 88]"
 
 TARGET_VERSION = (

src/darker/tests/test_command_line.py

@@ -207,6 +207,12 @@ def get_darker_help_output(capsys):
         expect_config=None,
         expect_modified=None,
     ),
+    dict(
+        argv=["--preview", "."],
+        expect_value=("preview", True),
+        expect_config=("preview", True),
+        expect_modified=("preview", True),
+    ),
     environ={},
 )
 def test_parse_command_line(
@@ -299,6 +305,19 @@ def test_parse_command_line_deprecated_option(
     assert {c.args[0] for c in warn.call_args_list} == expect_warn
 
 
+def test_parse_command_line_unknown_conffile_option(tmp_path, monkeypatch):
+    """`parse_command_line` warns about deprecated configuration options."""
+    monkeypatch.chdir(tmp_path)
+    config = {"unknown": "value", "preview": "true"}
+    (tmp_path / "pyproject.toml").write_text(toml.dumps({"tool": {"darker": config}}))
+    with pytest.raises(
+        ConfigurationError,
+        match=r"Invalid \[tool.darker\] keys in pyproject.toml: preview, unknown",
+    ):
+
+        parse_command_line(["-"])
+
+
 def test_help_description_without_isort_package(capsys):
     """``darker --help`` description shows how to add ``isort`` if it's not present"""
     with isort_present(False):
@@ -444,6 +463,12 @@ def test_help_with_flynt_package(capsys):
             target_versions={TargetVersion.PY39},
         ),
     ),
+    dict(
+        options=["--preview"],
+        expect=call(
+            preview=True,
+        ),
+    ),
 )
 def test_black_options(monkeypatch, tmpdir, git_repo, options, expect):
     """Black options from the command line are passed correctly to Black"""
@@ -553,6 +578,21 @@ def test_black_options(monkeypatch, tmpdir, git_repo, options, expect):
         options=["-t", "py39"],
         expect=call(target_versions={TargetVersion.PY39}),
     ),
+    dict(
+        config=["preview = true"],
+        options=[],
+        expect=call(preview=True),
+    ),
+    dict(
+        config=["preview = false"],
+        options=["--preview"],
+        expect=call(preview=True),
+    ),
+    dict(
+        config=["preview = true"],
+        options=["--preview"],
+        expect=call(preview=True),
+    ),
 )
 def test_black_config_file_and_options(git_repo, config, options, expect):
     """Black configuration file and command line options are combined correctly"""
@@ -644,6 +684,16 @@ def test_black_config_file_and_options(git_repo, config, options, expect):
             {"target_version": {"py39"}},
         ),
     ),
+    dict(
+        options=["--preview", "a.py"],
+        expect=(
+            Path("git_root"),
+            {Path("a.py")},
+            Exclusions(isort={"**/*"}, flynt={"**/*"}),
+            RevisionRange("HEAD", ":WORKTREE:"),
+            {"preview": True},
+        ),
+    ),
 )
 def test_options(git_repo, options, expect):
     """The main engine is called with correct parameters based on the command line