Add formatter command line and config option

The only supported value for now is `black`.

Fixes #562

src/darker/command_line.py

@@ -79,6 +79,13 @@ def make_argument_parser(require_src: bool) -> ArgumentParser:
  metavar="VERSION",
  choices=[v.name.lower() for v in TargetVersion],
  )
+ add_arg(
+ "Formatter to use for reformatting code",
+ "--formatter",
+ default="black",
+ choices=["black"],
+ metavar="FORMATTER",
+ )
  return parser
 
 

src/darker/config.py

@@ -24,6 +24,7 @@ class DarkerConfig(BaseConfig, total=False):
  skip_magic_trailing_comma: bool
  line_length: int
  target_version: str
+ formatter: str
 
 
 class OutputMode:

src/darker/tests/test_command_line.py

@@ -183,6 +183,30 @@ def get_darker_help_output(capsys):
  expect_config=("target_version", "py37"),
  expect_modified=("target_version", "py37"),
  ),
+ dict(
+ argv=["--formatter", "black", "."],
+ expect_value=("formatter", "black"),
+ expect_config=("formatter", "black"),
+ expect_modified=("formatter", ...),
+ ),
+ dict(
+ argv=["--formatter=black", "."],
+ expect_value=("formatter", "black"),
+ expect_config=("formatter", "black"),
+ expect_modified=("formatter", ...),
+ ),
+ dict(
+ argv=["--formatter", "rustfmt", "."],
+ expect_value=SystemExit,
+ expect_config=None,
+ expect_modified=None,
+ ),
+ dict(
+ argv=["--formatter=rustfmt", "."],
+ expect_value=SystemExit,
+ expect_config=None,
+ expect_modified=None,
+ ),
  dict(
  argv=["--target-version", "py39", "."],
  expect_value=("target_version", "py39"),