Stateful CJK codecs reset the codec at each call to encode() producing a valid but overlong output:

>>> import codecs
>>> encoder = codecs.getincrementalencoder('hz')()
>>> encoder.encode('\u804a') + encoder.encode('\u804a')
b'~{AD~}~{AD~}'
>>> '\u804a\u804a'.encode('hz')
b'~{ADAD~}'

Multibyte encodings: HZ and all encodings of the ISO 2022 family (e.g. iso-2022-jp).

Attached patch fixes this issue. I don't like how I added the tests, these tests may be moved somewhere else, but HZ codec doesn't have tests today (I opened issue #12057 for that), and ISO 2022 codecs don't have specific tests (test_multibytecodec is "Unit test for multibytecodec itself"). We should maybe also add tests specific to ISO 2022 first?

I hesitate to reset the codec on .encode(text, final=True), but UTF-8-SIG or UTF-16 don't reset the codec if final=True. io.TextIOWrapper only calls encoder.reset() on file.seek(0). On a seek to another position, it calls encoder.setstate(0).

See also issues #12016 and #12057.