bytearray.resize() must not clear the object on error (memory allocation failure) · Issue #157242 · python/cpython · GitHub

Python 3.15 reimplemented bytearray using an internal bytes object (to implement the bytearray.take_bytes() method). _PyBytes_Resize() is used internally to resize the bytearray object. Problem: if _PyBytes_Resize() fails, the bytearray object is cleared.

On Python 3.14, the bytearray is not cleared on resize failure.

The following example works on Python 3.14 but fails on Python 3.15.

import _testcapi

data = b'some data'
ba = bytearray(data)
try:
    try:
        _testcapi.set_nomemory(0)
        ba.resize(1024)
    except MemoryError:
        pass
    else:
        raise AssertionError("expect MemoryError")
finally:
    _testcapi.remove_mem_hooks()
if ba != data:
    raise AssertionError(f"{ba} != {data}")

Linked PRs

pythongh-157242: Leave bytearray unchanged if resize() fails

The PyBytesWriter_Resize() function has a similar issue: on failure, writer->obj is set to NULL and the writer is left in an inconsistent state.

3rd bug: bytearray.take_bytes() also clears the bytearray if _PyBytes_Resize() fails. At least, it's not a regression since the method is new in Python 3.15 :-)

gh-157242: Fix PyBytesWriter_Resize() on MemoryError (#157243)

pythongh-157242: Do not close io.BytesIO on MemoryError

gh-157242: Leave bytearray unchanged if resize() fails (#157340)

gh-157242: Fix set_nomemory() on Py_TRACE_REFS build (#157351)

gh-157242: Add more PyBytesWriter tests (#157352)

pythongh-157242: Fix PyBytesWriter_Resize() on MemoryError (python#15…

pythongh-157242: Leave bytearray unchanged if resize() fails (python#…

pythongh-157242: Fix set_nomemory() on Py_TRACE_REFS build (python#15…

pythongh-157242: Add more PyBytesWriter tests (python#157352)

gh-157242: Add _PyBytes_IsMutable() assertion (#157371)

pythongh-157242: Fix PyBytesWriter_Resize() on MemoryError (python#15…

pythongh-157242: Leave bytearray unchanged if resize() fails (python#…

pythongh-157242: Fix set_nomemory() on Py_TRACE_REFS build (python#15…

pythongh-157242: Add more PyBytesWriter tests (python#157352)

pythongh-157242: Do not close io.BytesIO on MemoryError (python#157344)

pythongh-157242: Allow negative grow in PyBytesWriter_Grow() (python#…

pythongh-157242: Add _PyBytes_IsMutable() assertion (python#157371)

gh-157242: Add support.inject_memory_error() function (#157431)

gh-157242: Fix typo in _PyBytes_Resize() (#157619)

pythongh-157242: Add byteswriter_check_consistency()

gh-157242: Add byteswriter_check_consistency() (#157723)

gh-157242: Use support.inject_memory_error_cm() in tests (#157811)

bytearray.resize() must not clear the object on error (memory allocation failure) · Issue #157242 · python/cpython · GitHub

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bytearray.resize() must not clear the object on error (memory allocation failure) #157242

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bytearray.resize() must not clear the object on error (memory allocation failure) · Issue #157242 · python/cpython · GitHub

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bytearray.resize() must not clear the object on error (memory allocation failure) #157242

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vstinner commented on Sep 10, 2026

vstinner commented on Sep 10, 2026

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