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gh-150942: Speed up IOBase.readlines #155304

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gh-150942: Speed up IOBase.readlines #155304

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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-07-00-40-00.gh-issue-150942.demitryfly.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Improve the performance of :meth:`io.IOBase.readlines` by avoiding
unnecessary reference-count operations while building the result list.
5 changes: 2 additions & 3 deletions Modules/_io/iobase.c
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@
#include "Python.h"
#include "pycore_call.h" // _PyObject_CallMethod()
#include "pycore_fileutils.h" // _PyFile_Flush
#include "pycore_list.h" // _PyList_AppendTakeRef()
#include "pycore_long.h" // _PyLong_GetOne()
#include "pycore_object.h" // _PyType_HasFeature()
#include "pycore_pyerrors.h" // _PyErr_ChainExceptions1()
Expand Down Expand Up @@ -753,12 +754,10 @@ _io__IOBase_readlines_impl(PyObject *self, Py_ssize_t hint)
break; /* StopIteration raised */
}

if (PyList_Append(result, line) < 0) {
Py_DECREF(line);
if (_PyList_AppendTakeRef((PyListObject *)result, line) < 0) {
goto error;
}
line_length = PyObject_Size(line);
Py_DECREF(line);
if (line_length < 0) {
goto error;
}
Expand Down