Context extension fix

src/darker/__main__.py

@@ -22,9 +22,6 @@
 
 logger = logging.getLogger(__name__)
 
-# Maximum `git diff -U<context_lines> value to try with
-MAX_CONTEXT_LINES = 1000
-
 
 def format_edited_parts(
     srcs: Iterable[Path],
@@ -72,16 +69,19 @@ def format_edited_parts(
         edited_srcs = worktree_srcs
 
     for src_relative, edited_content in edited_srcs.items():
-        for context_lines in range(MAX_CONTEXT_LINES + 1):
+        max_context_lines = len(edited_content)
+        for context_lines in range(max_context_lines + 1):
             src = git_root / src_relative
             edited = edited_content.splitlines()
             head_lines = head_srcs[src_relative]
 
-            # 2. diff HEAD and worktree for all file & dir paths on the command line
+            # 2. diff HEAD and worktree for the file
             edited_opcodes = diff_and_get_opcodes(head_lines, edited)
 
             # 3. extract line numbers in each edited to-file for changed lines
-            edited_linenums = list(opcodes_to_edit_linenums(edited_opcodes))
+            edited_linenums = list(
+                opcodes_to_edit_linenums(edited_opcodes, context_lines)
+            )
             if (
                 isort
                 and not edited_linenums
@@ -123,7 +123,7 @@ def format_edited_parts(
                 # a partially re-formatted Python file which produces an identical AST.
                 # Try again with a larger `-U<context_lines>` option for `git diff`,
                 # or give up if `context_lines` is already very large.
-                if context_lines == MAX_CONTEXT_LINES:
+                if context_lines == max_context_lines:
                     raise
                 logger.debug(
                     "AST verification failed. "

src/darker/diff.py

@@ -107,13 +107,23 @@ def _validate_opcodes(opcodes: List[Tuple[str, int, int, int, int]]) -> None:
 
 
 def opcodes_to_edit_linenums(
-    opcodes: List[Tuple[str, int, int, int, int]]
+    opcodes: List[Tuple[str, int, int, int, int]], context_lines: int
 ) -> Generator[int, None, None]:
-    """Convert diff opcode to line number of edited lines in the destination file"""
+    """Convert diff opcodes to line numbers of edited lines in the destination file
+
+    :param opcodes: The diff opcodes to convert
+    :param context_lines: The number of lines before and after an edited line to mark
+                          edited as well
+
+    """
     _validate_opcodes(opcodes)
+    prev_chunk_end = 1
+    _tag, _i1, _i2, _j1, end = opcodes[-1]
     for tag, _i1, _i2, j1, j2 in opcodes:
         if tag != "equal":
-            yield from range(j1 + 1, j2 + 1)
+            chunk_end = min(j2 + 1 + context_lines, end + 1)
+            yield from range(max(j1 + 1 - context_lines, prev_chunk_end), chunk_end)
+            prev_chunk_end = chunk_end
 
 
 def opcodes_to_chunks(

src/darker/tests/test_diff.py

@@ -1,5 +1,7 @@
+from itertools import chain
 from textwrap import dedent
 
+import pytest
 from black import FileMode, format_str
 
 from darker.diff import (
@@ -141,7 +143,17 @@ def test_opcodes_to_chunks():
     ]
 
 
-def test_opcodes_to_edit_linenums():
-    edit_linenums = list(opcodes_to_edit_linenums(OPCODES))
+@pytest.mark.parametrize(
+    'context_lines, expect',
+    [
+        (0, [1, 4, 5, 13, 14, 17, 20, 22, 23, 27]),
+        (1, [[1, 6], [12, 24], [26, 28]]),
+        (2, [[1, 7], [11, 28]]),
+    ],
+)
+def test_opcodes_to_edit_linenums(context_lines, expect):
+    edit_linenums = list(opcodes_to_edit_linenums(OPCODES, context_lines))
+    expect_ranges = [[n, n] if isinstance(n, int) else n for n in expect]
+    expect_linenums = list(chain(*(range(n[0], n[1] + 1) for n in expect_ranges)))
 
-    assert edit_linenums == [1, 4, 5, 13, 14, 17, 20, 22, 23, 27]
+    assert edit_linenums == expect_linenums