The following code displays "Xbc" using io, and "bc" using _pyio (or an unbuffered file, e.g. io.FileIO):
-------------
import _pyio, io

with io.BytesIO(b'abc') as raw:
    #with _pyio.BufferedRandom(raw) as f:
    with io.BufferedRandom(raw) as f:
        f.write(b"X")
        print("pos?", f.tell(), raw.tell())
        print(f.read())
-------------

I expect .write() to change the file position, and so "bc" must be the correct result, not "Wbc".

_pyio.BufferedRandom overrides its write method, whereas io.BufferedRandom doesn't.

I already noticed the implement difference of BufferedRandom.write(), but I don't remember if I reported it or not!?