Support index diffs against the empty tree#2155
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Pull request overview
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Adds IndexFile.diff(NULL_TREE) support so callers can diff the index against an “empty tree” (useful for initial index contents), and introduces a regression test to validate expected diff output.
Changes:
- Extends
IndexFile.diff()to accept and handleNULL_TREEas theotherdiff target. - Implements a custom
git diff --cached <empty-tree>invocation to produce raw/patch output for the NULL_TREE case. - Adds a test covering initial index diffing against
NULL_TREE, including path filtering and patch creation.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.
| File | Description |
|---|---|
| test/test_index.py | Adds coverage for diffing an initial index against NULL_TREE and verifying raw + patch output. |
| git/index/base.py | Implements IndexFile.diff(NULL_TREE) by invoking git diff --cached against the empty-tree SHA. |
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| if other is self.INDEX: | ||
| return git_diff.DiffIndex() | ||
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| if other is git_diff.NULL_TREE: |
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| args: List[Union[PathLike, str]] = [ | ||
| "--cached", | ||
| "4b825dc642cb6eb9a060e54bf8d69288fbee4904", | ||
| "--abbrev=40", | ||
| "--full-index", | ||
| ] |
| ) | ||
| index = diff_method(self.repo, proc) | ||
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| proc.wait() |
| index.write() | ||
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| index = IndexFile(repo) | ||
| assert not index.diff(None) |
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Related to #2025.
IndexFile.diff()is documented as accepting the same targets asDiffable.diff(), includingNULL_TREE, but the index-specific implementation rejectedNULL_TREEbefore it could produce a diff. That leaves no direct API path for inspecting staged entries for an initial commit against the empty tree.This adds explicit
IndexFile.diff(NULL_TREE)handling usinggit diff --cachedagainst the empty tree object. I keptIndexFile.diff(None)as the existing index-vs-working-tree comparison, since that behavior is documented in the quick docs and existing tests.The regression test creates a new repository, stages a file, writes the index, constructs a fresh
IndexFile, and verifies thatindex.diff(NULL_TREE)reports the staged addition whileindex.diff(None)remains empty because the working tree matches the index.Validation:
.venv/bin/python -m pytest --no-cov test/test_index.py::TestIndex::test_index_file_diff_null_tree_with_initial_index -q.venv/bin/python -m ruff check git/index/base.py test/test_index.pyAI assistance disclosure: OpenAI GPT-5 helped draft and check this change; I reviewed the implementation and tests before submitting.