quopri.encode pure-Python fallback splits hexadecimal escapes at line boundaries

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The quopri.encode docstring promises: "Read 'input', apply quoted-printable encoding, and write to 'output'." The module docstring identifies the format as "quoted-printable transport encoding as per RFC 1521." Quoted-printable hexadecimal escapes must remain intact across soft line breaks.

Expected: Valid quoted-printable output that decodes to 74 ASCII A bytes followed by byte 0xff, with the =FF escape kept intact.

Actual: The output is 74 ASCII A bytes followed by b'==\nFF'; decoding yields those A bytes followed by b'=\nFF' instead of byte 0xff.

import binascii, io, quopri

s = b'A' * 74 + b'\xff'
if not isinstance(s, bytes) or b'\r' in s or b'\n' in s:
    print('REFUTATION REJECTED:', 'input is not a newline-free bytes payload')
else:
    quopri.b2a_qp = None
    out = io.BytesIO()
    quopri.encode(io.BytesIO(s), out, quotetabs=True, header=False)
    encoded = out.getvalue()
    actual = binascii.a2b_qp(encoded)
    expected = s
    if actual != expected:
        print('REFUTATION CONFIRMED:', s, (encoded, actual), expected)
    else:
        print('REFUTATION REJECTED:', 'independent decoding recovers the input')

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

REFUTATION CONFIRMED: b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\xff' (b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==\nFF', b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=\nFF') b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\xff'

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/quopri-20261003-042505-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).