gh-150184: difflib C accelerator#150188
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Move the pure-Python implementation of difflib to Lib/_pydifflib.py and turn Lib/difflib.py into a thin shim that re-exports its public API. This mirrors the layout used by decimal/_pydecimal, datetime/_pydatetime, and pickle/_pickle, where the public module dispatches to a faster C implementation when available and the pure-Python module is preserved as a self-contained reference for alternative Python implementations. No public behaviour change. ``Match`` is constructed with ``module='difflib'`` so its qualified name matches the public module. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
blhsing
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AA-Turner,
corona10,
emmatyping and
erlend-aasland
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Introduce Modules/_difflibmodule.c, a heap-type C extension that implements __init__, set_seqs/set_seq1/set_seq2, find_longest_match, get_matching_blocks, get_opcodes, and ratio for SequenceMatcher. The inner DP loop and the full Ratcliff-Obershelp recursion run on int32 label arrays with zero Python C-API calls in the hot path; codepoint- keyed lookup tables short-circuit per-element dict probes for str and bytes inputs. Output is bit-identical to the pure-Python implementation including tie-breaks. Lib/difflib.py grows a small subclass that inherits the slow-path methods (quick_ratio, real_quick_ratio, get_grouped_opcodes) from the pure-Python class; this is a no-op when the accelerator is not built. Build wiring: configure.ac registers the module via PY_STDLIB_MOD_SIMPLE and Modules/Setup.stdlib.in references _difflibmodule.c. configure must be regenerated with autoreconf before this lands. Typical workloads run 5-25x faster than pure Python; the bytes path up to ~70x. See Lib/test/test_difflib.py for cross-implementation tests. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
When the _difflib C accelerator is built, programmatically generate a parallel ``*_PurePython`` TestCase for each existing test class so the same suite covers both implementations. Pure-Python coverage is obtained by patching ``difflib.SequenceMatcher`` to ``_pydifflib.SequenceMatcher`` in setUp / restoring it in tearDown; internal helpers like ``unified_diff`` and ``ndiff`` resolve ``SequenceMatcher`` on ``difflib`` at call time, so patching the module attribute covers the whole pipeline. This mirrors the dual-implementation test pattern used by test_decimal (C* / Py* class pairs) without requiring every existing test method to be parameterised. ``test_html_diff`` also gets a single-line fix: it depended on ``HtmlDiff._default_prefix`` starting at 0, which only held because it ran first. Resetting the counter at the top of the test makes it order-independent. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Sorry but this needs a discussion. Open a PR on your fork if you want to show a PoC |
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In addition, after a quick glance, there seem to be many paths that fail to check NULL after calling the C API. |
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#150184