Why does codecs.StreamWriter accept text data stream?

[infoagee]

Bug report

Checklist

• [x] I am confident this is a bug in CPython, not a bug in a third-party project
• [X] I have searched the CPython issue tracker, and am confident this bug has not been reported before

CPython versions tested on:

3.8

Operating systems tested on:

Windows

Output from running 'python -VV' on the command line:

No response

A clear and concise description of the bug:

import sys, codecs
a = codecs.getwriter('cp850')
b = a(sys.stdout.buffer)
c = a(sys.stdout)
b.write('A')
c.write('A')  # error!

Now that b is enough to write text in cp850, why can one construct a werid and buggy c? Then, why does codecs.StreamWriter accept text data stream such as sys.stdout?

[zougloub]

Not a bug: cp850 is a codec that encodes from str to bytes, and sys.stdout is a str stream.