https://github.com/python/cpython/commit/0bfee223909adc113ece862cb8d7d807c35e201c
commit: 0bfee223909adc113ece862cb8d7d807c35e201c
branch: main
author: Cody Maloney <[email protected]>
committer: StanFromIreland <[email protected]>
date: 2026-10-29T13:52:42Z
summary:
gh-160781: Fix `StringIO.truncate()` rejecting sizes above `2**31 - 1` on
Windows (#160784)
files:
A Misc/NEWS.d/next/Library/2026-10-27-15-27-32.gh-issue-160781.kARuHu.rst
M Lib/test/test_io/test_memoryio.py
M Modules/_io/stringio.c
diff --git a/Lib/test/test_io/test_memoryio.py
b/Lib/test/test_io/test_memoryio.py
index 6a286e5a62ef44c..349c7281230eeba 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -1150,6 +1150,12 @@ def __index__(self):
with self.assertRaises(ValueError):
stream.truncate(Size())
+ # Sizes above 2**31 - 1 must work where C long is 32-bit.
+ if sys.maxsize > 2**31:
+ stream = self.ioclass("abc")
+ self.assertEqual(stream.truncate(2**31), 2**31)
+ self.assertEqual(stream.getvalue(), "abc")
+
# XXX: For the Python version of io.StringIO, this is highly
# dependent on the encoding used for the underlying buffer.
def test_widechar(self):
diff --git
a/Misc/NEWS.d/next/Library/2026-10-27-15-27-32.gh-issue-160781.kARuHu.rst
b/Misc/NEWS.d/next/Library/2026-10-27-15-27-32.gh-issue-160781.kARuHu.rst
new file mode 100644
index 000000000000000..1f1e25d8b461b4e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-27-15-27-32.gh-issue-160781.kARuHu.rst
@@ -0,0 +1,2 @@
+Fix :class:`io.StringIO` implementation of :meth:`io.IOBase.truncate`
+rejecting sizes larger than ``2**31 - 1`` on Windows.
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 558fd57aa93c954..7800a4fcfca4dd8 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -490,7 +490,7 @@ _io_StringIO_truncate_impl(stringio *self, PyObject *pos)
size = self->pos;
}
else {
- size = PyLong_AsLong(pos);
+ size = PyNumber_AsSsize_t(pos, PyExc_OverflowError);
if (size == -1 && PyErr_Occurred()) {
return NULL;
}
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