struct.pack_into corrupts an 's' value when source and destination are the same bytearray

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python struct documentation, struct.pack_into: "Pack the values v1, v2, ... according to the format string format and write the packed bytes into the writable buffer buffer starting at position offset." The format-character notes also state: "For the 's' format character, the count is interpreted as the length of the bytes."

Expected: b'A'

Actual: b'\x00'

import struct

b = bytearray(b'A')
fmt, offset = '1s', 0
given = repr((fmt, b, offset, b))
if not isinstance(b, bytearray) or len(b) != struct.calcsize(fmt):
    print('REFUTATION REJECTED: invalid input')
else:
    expected = bytes(b)  # Exactly one byte: no padding or truncation.
    struct.pack_into(fmt, b, offset, b)
    actual = bytes(b)
    if actual != expected:
        print('REFUTATION CONFIRMED:', given, 'actual=', actual, 'expected=', expected)
    else:
        print('REFUTATION REJECTED: actual equals documented expectation')

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library struct:

REFUTATION CONFIRMED: ('1s', bytearray(b'A'), 0, bytearray(b'A')) actual= b'\x00' expected= b'A'

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/struct-20261003-122508-c3

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).