BytesIO and StringIO issues with large sizes

Bug report

BytesIO and StringIO .truncate reject sizes above 2**31 - 1 on Windows. This was introduced in a move from Argument Clinic converters to hand conversion (BytesIO in Segfault/abort from calling BytesIO unshare_buffer in threads on a free-threaded build #132551, StringIO in fix thread safety of io.StringIO.truncate #133732).

BytesIO.write near sys.maxsize raises SystemError instead of OverflowError.

import io, sys
b = io.BytesIO(b"abc")
b.seek(sys.maxsize - 1)
b.write(b"x")

Found while working on gh-71448

CPython versions tested on:

CPython main branch, 3.14, 3.15

Operating systems tested on:

Linux, Windows

Linked PRs

gh-160781: Fix `StringIO.truncate()` rejecting sizes above `2**31 - 1…

gh-160781: Fix `BytesIO` overflow handling for large sizes (#160783)

[3.14] gh-160781: Fix `BytesIO` overflow handling for large sizes (GH…

[3.15] gh-160781: Fix `BytesIO` overflow handling for large sizes (GH…

gh-160781: Only test BytesIO write overflow on the C implementation (#…

[3.14] gh-160781: Only test BytesIO write overflow on the C implement…

[3.15] gh-160781: Only test BytesIO write overflow on the C implement…

[3.15] pythongh-160781: Fix `StringIO.truncate()` rejecting sizes abo…

[3.15] gh-160781: Fix `StringIO.truncate()` rejecting sizes above `2*…

FYI, I saw this error yesterday on some of the buildbots (32-bit platforms maybe):

ERROR: test_issue141311 (test.test_memoryio.PyBytesIOTest.test_issue141311)----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/pull_request.ware-debian-x86.installed/build/target/lib/python3.14/test/test_memoryio.py", line 571, in test_issue141311
    self.assertEqual(memio.getvalue(), b"")
                     ~~~~~~~~~~~~~~^^  File "/buildbot/buildarea/pull_request.ware-debian-x86.installed/build/target/lib/python3.14/_pyio.py", line 901, in getvalue
    return bytes(self._buffer)
OverflowError: byte string is too large

Perhaps this has already been resolved but I'm posting this in case it's still failing.

@nascheme

@nascheme thanks for letting us know, however, was it before or after 8dbc313 (can you please send a link to the run?).

Here's one of the failed builds, git revision c866403. It seems that 8dbc313 is not in that branch.

https://buildbot.python.org/#/builders/1285/builds/822

I'd presume it was fixed by it, if it pops up with that commit please let me know.

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BytesIO and StringIO issues with large sizes #160781

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BytesIO and StringIO issues with large sizes #160781

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Bug report

CPython versions tested on:

Operating systems tested on:

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nascheme commented on Oct 30, 2026

StanFromIreland commented on Oct 30, 2026

nascheme commented on Oct 30, 2026

StanFromIreland commented on Oct 30, 2026

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