Fix diff parsing to support mnemonicPrefix configuration #2092

MirrorDNA-Reflection-Protocol · 2025-12-07T15:58:32Z

Summary

Fixes #2013

When diff.mnemonicPrefix=true is set in git config, git uses different prefixes for diff paths instead of the standard a/ and b/:

c/ for commit
w/ for worktree
i/ for index
o/ for object
h/ for HEAD

Previously, the diff regex and decode_path() function only accepted a/ and b/ prefixes, causing create_patch=True diffs to fail parsing when users had this config enabled.

Reproduction

As described in #2013:

# With ~/.gitconfig containing:
# [diff]
#     mnemonicPrefix = true

repo = git.Repo('.')
ancestor_ref = repo.merge_base(repo.head, repo.refs['main'])
diff = ancestor_ref[0].diff(None, create_patch=True)  # Would fail to parse

Changes

Update re_header regex to accept [abciwoh]/ prefixes instead of just [ab]/
Update decode_path() assertion to accept all valid mnemonic prefixes
Add test case for mnemonicPrefix-style diffs

Testing

New test test_diff_mnemonic_prefix verifies parsing works with c/ and w/ prefixes
All existing diff tests pass (except one pre-existing failure on main unrelated to this change)

References

Git documentation: https://git-scm.com/docs/git-config#Documentation/git-config.txt-diffmnemonicPrefix
Issue: gitconfig: mnemonicPrefix conflict with create_patch #2013

Fixes gitpython-developers#2013 When diff.mnemonicPrefix=true is set in git config, git uses different prefixes for diff paths instead of the standard a/ and b/: - c/ for commit - w/ for worktree - i/ for index - o/ for object - h/ for HEAD Previously, the diff regex and decode_path() function only accepted a/ and b/ prefixes, causing create_patch=True diffs to fail parsing. Changes: - Update re_header regex to accept [abciwoh]/ prefixes - Update decode_path() assertion to accept all valid mnemonic prefixes - Add test case for mnemonicPrefix-style diffs

- Remove whitespace from blank lines in docstrings (ruff W293) - Use !r format specifier for bytes in f-string (mypy str-bytes-safe)

Copilot

Pull request overview

This PR fixes diff parsing to support Git's diff.mnemonicPrefix configuration option, which was causing parse failures when users enabled this feature. When diff.mnemonicPrefix=true is set, Git uses alternative path prefixes (c/, w/, i/, o/, h/) instead of the standard a/ and b/ prefixes in diff output.

Key changes:

Updated the re_header regex pattern to accept all valid mnemonic prefixes in addition to standard a/b prefixes
Modified decode_path() function to validate and strip all supported prefix types
Added test coverage for mnemonicPrefix-style diffs

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
git/diff.py	Updated regex pattern and decode_path() function to support c/, w/, i/, o/, and h/ prefixes alongside standard a/ and b/ prefixes
test/test_diff.py	Added test case verifying diff parsing works correctly with mnemonicPrefix-style c/ and w/ prefixes

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Copilot · 2025-12-08T18:43:58Z

git/diff.py


    # Precompiled regex.
+    # Note: The path prefixes support both default (a/b) and mnemonicPrefix mode
+    # which can use prefixes like c/ (commit), w/ (worktree), i/ (index), o/ (object)


The comment is missing the h/ prefix for HEAD. According to the PR description and the Git documentation, h/ is also a valid mnemonicPrefix. The comment should list all supported prefixes: c/ (commit), w/ (worktree), i/ (index), o/ (object), and h/ (HEAD).

Suggested change

# which can use prefixes like c/ (commit), w/ (worktree), i/ (index), o/ (object)

# which can use prefixes like c/ (commit), w/ (worktree), i/ (index), o/ (object), and h/ (HEAD)

Copilot · 2025-12-08T18:43:59Z

test/test_diff.py

+        - c/ for commit
+        - w/ for worktree
+        - i/ for index
+        - o/ for object


The docstring is missing the h/ prefix for HEAD. According to the PR description and Git documentation, h/ is also a valid mnemonicPrefix. The comment should list all supported prefixes: c/ (commit), w/ (worktree), i/ (index), o/ (object), and h/ (HEAD).

Suggested change

- o/ for object

- o/ for object

- h/ for HEAD

Copilot · 2025-12-08T18:43:59Z

test/test_diff.py

+    def test_diff_mnemonic_prefix(self):
+        """Test that diff parsing works with mnemonicPrefix enabled.
+
+        When diff.mnemonicPrefix=true is set in git config, git uses different
+        prefixes for diff paths:
+        - c/ for commit
+        - w/ for worktree
+        - i/ for index
+        - o/ for object
+
+        This addresses issue #2013 where the regex only matched [ab]/ prefixes.
+        """
+        # Create a diff with mnemonicPrefix-style c/ and w/ prefixes
+        # Using valid 40-char hex SHAs
+        diff_mnemonic = b"""diff --git c/.vscode/launch.json w/.vscode/launch.json
+index 1234567890abcdef1234567890abcdef12345678..abcdef1234567890abcdef1234567890abcdef12 100644
+--- c/.vscode/launch.json
+++ w/.vscode/launch.json
+@@ -1,3 +1,3 @@
+-old content
+new content
+"""
+        diff_proc = StringProcessAdapter(diff_mnemonic)
+        diffs = Diff._index_from_patch_format(self.rorepo, diff_proc)
+
+        # Should parse successfully (previously would fail or return empty)
+        self.assertEqual(len(diffs), 1)
+        diff = diffs[0]
+        # The path should be extracted correctly (without the c/ or w/ prefix)
+        self.assertEqual(diff.a_path, ".vscode/launch.json")
+        self.assertEqual(diff.b_path, ".vscode/launch.json")


The test only covers c/ and w/ prefixes but doesn't test the other valid mnemonicPrefix options (i/, o/, h/). Consider adding test cases for these prefixes as well to ensure the regex pattern [abciwoh] and the decode_path() function work correctly with all supported prefixes. This could be done either by adding more test data in this test or creating a parameterized test.

Byron

Thanks a lot for contributing! I particularly like the exhaustive documentation around the matter, and what I see looks good!

The test is also something I'd love to see improved, and I side with Copilot here.

… values Changes per reviewer (Byron) and Copilot suggestions: - Add h/ (HEAD) to the comment listing supported prefixes - Update test docstring to include h/ prefix - Expand test to cover all prefix combinations using subTest: - c/ (commit) vs w/ (worktree) - c/ (commit) vs i/ (index) - i/ (index) vs w/ (worktree) - o/ (object) vs w/ (worktree) - h/ (HEAD) vs i/ (index) - h/ (HEAD) vs w/ (worktree) This ensures the regex pattern [abciwoh] and decode_path() work correctly with all supported mnemonicPrefix values.

MirrorDNA-Reflection-Protocol added 2 commits December 7, 2025 21:28

Fix lint and mypy errors

6e77e26

- Remove whitespace from blank lines in docstrings (ruff W293) - Use !r format specifier for bytes in f-string (mypy str-bytes-safe)

Byron requested a review from Copilot December 8, 2025 18:40

Copilot started reviewing on behalf of Byron December 8, 2025 18:41 View session

Copilot AI reviewed Dec 8, 2025

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Byron requested changes Dec 8, 2025

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	# which can use prefixes like c/ (commit), w/ (worktree), i/ (index), o/ (object)
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MirrorDNA-Reflection-Protocol commented Dec 7, 2025

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