base64.encode silently truncates output when the binary stream performs short writes

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: "Encode a file; input and output are binary files." — base64.encode documentation. Binary raw streams may return a write count smaller than the supplied buffer.

Expected: b'AA==\n'

Actual: b'A'

import base64, binascii, io

class PrefixWriter(io.RawIOBase):
    def __init__(self, limit):
        super().__init__()
        self.limit, self.data = limit, bytearray()
    def writable(self):
        return True
    def write(self, b):
        n = min(len(b), self.limit)
        self.data.extend(b[:n])
        return n

p, limit = b'\x00', 1
src, dst = io.BytesIO(p), PrefixWriter(limit)
if not (isinstance(p, bytes) and src.readable() and dst.writable() and limit > 0):
    print("REFUTATION REJECTED: invalid binary stream input")
else:
    base64.encode(src, dst)
    actual = bytes(dst.data)
    expected = binascii.b2a_base64(p)
    if actual != expected:
        print("REFUTATION CONFIRMED:", (p, limit), "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED: actual matches expected")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library base64:

REFUTATION CONFIRMED: (b'\x00', 1) actual: b'A' expected: b'AA==\n'

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/base64-20261003-020052-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).