quopri pure-Python encoder splits hexadecimal escapes at line boundaries

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python standard-library documentation, quopri.encodestring: “Encode the contents of s, a bytes object which contains data, and return the encoded data as a bytes object.” The quopri module documentation identifies the encoding as quoted-printable.

Expected: 74 ASCII A bytes followed by b'=\n=FFB', preserving the contiguous =FF escape.

Actual: 74 ASCII A bytes followed by b'==\nFFB', splitting the =FF escape across a soft line break.

import quopri

s = b'A' * 74 + b'\xffB'
if not isinstance(s, bytes):
    print('REFUTATION REJECTED: input is not bytes')
else:
    lines, line = [], b''
    for c in s:
        token = bytes([c]) if 33 <= c <= 126 and c != 61 else ('=%02X' % c).encode()
        if len(line + token) > 75:
            lines.append(line + b'=\n')
            line = b''
        line += token
    expected = b''.join(lines) + line
    quopri.b2a_qp = None  # Exercise the supported pure-Python fallback.
    actual = quopri.encodestring(s, quotetabs=True, header=False)
    if actual != expected:
        print('REFUTATION CONFIRMED:', s, actual, expected)
    else:
        print('REFUTATION REJECTED: output matches independently encoded expectation')

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

REFUTATION CONFIRMED: b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\xffB' b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==\nFFB' b'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=\n=FFB'

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-c3

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).