Get revrange from env vars with -r :PRE-COMMIT:

Fixes #103

.pre-commit-hooks.yaml

@@ -1,7 +1,7 @@
 - id: darker
   name: darker
   description: "Black reformatting to Python files only in regions changed since last commit"
-  entry: darker
+  entry: "darker -r :PRE-COMMIT:"
   language: python
   language_version: python3
   require_serial: true

CHANGES.rst

@@ -5,6 +5,9 @@ These features will be included in the next release:
 
 Added
 -----
+- Get revision range from pre-commit_'s ``PRE_COMMIT_FROM_REF`` and
+  ``PRE_COMMIT_TO_REF`` environment variables when using the ``--revision :PRE-COMMIT:``
+  option
 - Configure a pre-commit hook for Darker itself
 
 Fixed

src/darker/git.py

@@ -1,6 +1,7 @@
 """Helpers for listing modified files and getting unmodified content from Git"""
 
 import logging
+import os
 import re
 import sys
 from dataclasses import dataclass
@@ -22,8 +23,11 @@
 
 
 # A colon is an invalid character in tag/branch names. Use that in the special value for
-# denoting the working tree as one of the "revisions" in revision ranges.
+# - denoting the working tree as one of the "revisions" in revision ranges
+# - referring to the `PRE_COMMIT_FROM_REF` and `PRE_COMMIT_TO_REF` environment variables
+#   for determining the revision range
 WORKTREE = ":WORKTREE:"
+PRE_COMMIT_FROM_TO_REFS = ":PRE-COMMIT:"
 
 
 def git_get_content_at_revision(path: Path, revision: str, cwd: Path) -> List[str]:
@@ -89,6 +93,20 @@ def parse(cls, revision_range: str) -> "RevisionRange":
         RevisionRange(rev1='a', rev2=':WORKTREE:', use_common_ancestor=True)
 
         """
+        if revision_range == PRE_COMMIT_FROM_TO_REFS:
+            try:
+                return cls(
+                    os.environ["PRE_COMMIT_FROM_REF"],
+                    os.environ["PRE_COMMIT_TO_REF"],
+                    use_common_ancestor=True,
+                )
+            except KeyError:
+                logger.error(
+                    "The environment variables PRE_COMMIT_FROM_REF and"
+                    " PRE_COMMIT_TO_REF must be defined when using -r / --revision"
+                    " :PRE-COMMIT:"
+                )
+                sys.exit(123)
         match = COMMIT_RANGE_RE.match(revision_range)
         if match:
             rev1, range_dots, rev2 = match.groups()

src/darker/tests/test_git.py

@@ -2,6 +2,7 @@
 
 # pylint: disable=redefined-outer-name
 
+import os
 from pathlib import Path
 from unittest.mock import patch
 
@@ -15,6 +16,7 @@
     should_reformat_file,
 )
 from darker.tests.conftest import GitRepoFixture
+from darker.tests.helpers import raises_if_exception
 
 
 @pytest.mark.parametrize(
@@ -271,6 +273,27 @@ def test_git_get_modified_files_revision_range(
     assert {path.name for path in result} == expect
 
 
+@pytest.mark.parametrize(
+    "environ, expect",
+    [
+        ({}, SystemExit),
+        ({"PRE_COMMIT_FROM_REF": "old"}, SystemExit),
+        ({"PRE_COMMIT_TO_REF": "new"}, SystemExit),
+        ({"PRE_COMMIT_FROM_REF": "old", "PRE_COMMIT_TO_REF": "new"}, ["old", "new"]),
+    ],
+)
+def test_revisionrange_parse_pre_commit(environ, expect):
+    """RevisionRange.parse(':PRE-COMMIT:') gets the range from environment variables"""
+    with patch.dict(os.environ, environ), raises_if_exception(expect):
+
+        result = RevisionRange.parse(":PRE-COMMIT:")
+
+        expect_rev1, expect_rev2 = expect
+        assert result.rev1 == expect_rev1
+        assert result.rev2 == expect_rev2
+        assert result.use_common_ancestor
+
+
 edited_linenums_differ_cases = pytest.mark.parametrize(
     "context_lines, expect",
     [