Add support for black config

src/darker/__main__.py

@@ -3,7 +3,7 @@
 import logging
 import sys
 from pathlib import Path
-from typing import Iterable, List, Set
+from typing import Iterable, List, Optional, Set
 
 from darker.black_diff import diff_and_get_opcodes, opcodes_to_chunks, run_black
 from darker.chooser import choose_lines
@@ -20,7 +20,9 @@
 MAX_CONTEXT_LINES = 1000
 
 
-def format_edited_parts(srcs: Iterable[Path], isort: bool) -> None:
+def format_edited_parts(
+    srcs: Iterable[Path], isort: bool, config: Optional[str]
+) -> None:
     """Black (and optional isort) formatting for chunks with edits since the last commit
 
     1. run isort on each edited file
@@ -40,6 +42,7 @@ def format_edited_parts(srcs: Iterable[Path], isort: bool) -> None:
 
     :param srcs: Directories and files to re-format
     :param isort: ``True`` to also run ``isort`` first on each changed file
+    :param config: Path to the black configuration file (optional)
 
     """
     remaining_srcs: Set[Path] = set(srcs)
@@ -65,7 +68,7 @@ def format_edited_parts(srcs: Iterable[Path], isort: bool) -> None:
                 continue
 
             # 4. run black
-            edited, formatted = run_black(src)
+            edited, formatted = run_black(src, config)
             logger.debug("Read %s lines from edited file %s", len(edited), src)
             logger.debug("Black reformat resulted in %s lines", len(formatted))
 
@@ -134,7 +137,7 @@ def main(argv: List[str] = None) -> None:
         exit(1)
 
     paths = {Path(p) for p in args.src}
-    format_edited_parts(paths, args.isort)
+    format_edited_parts(paths, args.isort, args.config)
 
 
 if __name__ == "__main__":

src/darker/black_diff.py

@@ -74,22 +74,44 @@
 
 import logging
 from difflib import SequenceMatcher
+from functools import lru_cache
 from pathlib import Path
-from typing import Generator, List, Tuple
+from typing import Dict, Generator, List, Optional, Tuple, Union
 
-from black import FileMode, format_str
+from black import FileMode, format_str, read_pyproject_toml
+from click import Command, Context, Option
 
 logger = logging.getLogger(__name__)
 
 
-def run_black(src: Path) -> Tuple[List[str], List[str]]:
+@lru_cache(maxsize=1)
+def read_black_config(
+    src: Path, value: Optional[str]
+) -> Dict[str, Union[bool, int, set]]:
+    """Read the black configuration from pyproject.toml"""
+    command = Command("main")
+
+    context = Context(command)
+    context.params["src"] = (str(src),)
+
+    parameter = Option(("--config",))
+
+    read_pyproject_toml(context, parameter, value)
+
+    return context.default_map or {}
+
+
+def run_black(src: Path, config: Optional[str]) -> Tuple[List[str], List[str]]:
     """Run the black formatter for the contents of the given Python file
 
     Return lines of the original file as well as the formatted content.
 
     """
+    defaults = read_black_config(src, config)
+    mode = FileMode(**defaults)
+
     src_contents = src.read_text()
-    dst_contents = format_str(src_contents, mode=FileMode())
+    dst_contents = format_str(src_contents, mode=mode)
     return src_contents.splitlines(), dst_contents.splitlines()
 
 

src/darker/command_line.py

@@ -30,6 +30,9 @@ def parse_command_line(argv: List[str]) -> Namespace:
     parser.add_argument(
         "-i", "--isort", action="store_true", help="".join(isort_help),
     )
+    parser.add_argument(
+        "-c", "--config", metavar="PATH", help="Read configuration from PATH."
+    )
     parser.add_argument(
         "-v",
         "--verbose",

src/darker/tests/test_black_diff.py

@@ -1,8 +1,10 @@
+from pathlib import Path
 from textwrap import dedent
 
+import pytest
 from black import FileMode, format_str
 
-from darker.black_diff import diff_and_get_opcodes, opcodes_to_chunks
+from darker.black_diff import diff_and_get_opcodes, opcodes_to_chunks, read_black_config
 
 FUNCTIONS2_PY = dedent(
     '''\
@@ -147,3 +149,15 @@ def test_mixed():
             ['    print("Inner defs should breathe a little.")'],
         ),
     ]
+
+
+@pytest.mark.parametrize("config,line_length", ((None, 79), ("custom.toml", 99)))
+def test_black_config(tmpdir, config, line_length):
+    tmpdir = Path(tmpdir)
+    src = tmpdir / "src.py"
+    toml = tmpdir / (config or "pyproject.toml")
+
+    toml.write_text("[tool.black]\nline-length = {}\n".format(line_length))
+
+    config = read_black_config(src, config and str(toml))
+    assert config == {"line_length": line_length}