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GeezerVison is showing github.com — gh-150942: Speed up IOBase.readlines by demitryfly · Pull Request #155304 · python/cpython

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@demitryfly

@demitryfly demitryfly commented Aug 6, 2026 •
edited by bedevere-app Bot
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This PR optimizes _io.IOBase.readlines() by using
_PyList_AppendTakeRef() when adding lines to the result list.

Previously, each line was added with PyList_Append(), which creates an
extra temporary reference. The new code transfers the existing reference
directly to the result list, avoiding an unnecessary reference-count
increment/decrement pair.

The order of operations is unchanged: the line is added before its length
is inspected, preserving the prior behavior.

Tests:

  • ./python.exe -m test --verbose test_io
  • Issue: Improve performance by using reference stealing methods #150942

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@python-cla-bot

python-cla-bot Bot commented Aug 6, 2026 •
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All commit authors signed the Contributor License Agreement.

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@bedevere-app

bedevere-app Bot commented Aug 6, 2026

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.
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@bedevere-app bedevere-app Bot added the awaiting review label Aug 6, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Aug 6, 2026
Improve performance by using reference stealing methods #150942
Open
@demitryfly
pythongh-150942: Speed up IOBase.readlines
c75bcab
@demitryfly
demitryfly force-pushed the gh-150942-iobase-readlines branch from f2b4ed4 to c75bcab Compare August 6, 2026 21:49
@Eclips4
Merge branch 'main' into pythongh-150942-iobase-readlines
12880e0
@Eclips4
Eclips4 requested a review from eendebakpt August 11, 2026 04:53
eendebakpt
eendebakpt reviewed Aug 11, 2026
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Comment thread Modules/_io/iobase.c

if (PyList_Append(result, line) < 0) {
Py_DECREF(line);
if (_PyList_AppendTakeRef((PyListObject *)result, line) < 0) {

@eendebakpt eendebakpt Aug 11, 2026

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The call to PyObject_Size(line) should be moved up. After the _PyList_AppendTakeRef we are no longer have a valid reference to line.

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Comment thread Modules/_io/iobase.c
/* XXX special-casing this made sense in the Python version in order
to remove the bytecode interpretation overhead, but it could
probably be removed here. */
PyObject *ret = PyObject_CallMethodObjArgs(result, &_Py_ID(extend),

@eendebakpt eendebakpt Aug 11, 2026

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The changes in this PR will not matter for calls like f.readlines() since that ends up in the fast path here. We could change the call here to PyObject_CallMethodOneArg which might be faster, but that is for another PR.

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Comment thread Misc/NEWS.d/next/Library/2026-08-07-00-40-00.gh-issue-150942.demitryfly.rst
@@ -0,0 +1,2 @@
Improve the performance of :meth:`io.IOBase.readlines` by avoiding

@eendebakpt eendebakpt Aug 11, 2026

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Can you provide benchmarks where this change has a positive effect for real-world cases? I suspect the change might be too minimal to warrant acceptance of the PR.

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  • 👍 1 reaction

@demitryfly demitryfly Oct 27, 2026

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The benchmark showed that this change has no significant effect, so I’ve decided to close this PR.

Thanks for the review!

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@StanFromIreland StanFromIreland added the pending The issue will be closed if no feedback is provided label Aug 15, 2026
@demitryfly demitryfly closed this Oct 27, 2026
@demitryfly
demitryfly deleted the gh-150942-iobase-readlines branch October 27, 2026 15:42
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@demitryfly @eendebakpt @StanFromIreland @Eclips4
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