I realize there is another problem, and doing tricks with a bytes object won’t help that. BufferedWriter bypasses its own buffer for large writes:

>>> writer = BufferedWriter(Raw())
>>> large = bytearray(10000)
>>> writer.write(large)
10000
>>> written.tobytes()[:10]
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
>>> large[:5] = b"blaua"
>>> written.tobytes()[:10]
b'blaua\x00\x00\x00\x00\x00'

BufferedWriter is passing a view of the original input through, without any copying. Perhaps the simplest thing is to warn and prevent the user from accessing the buffer after write() returns. I suggested some imperfect ideas in Issue 15994. Maybe I should just close this as a duplicate.