io.BytesIO же == open(file, 'rb')
?
>>> open(os.devnull)
<_io.TextIOWrapper name='nul' mode='r' encoding='cp1251'>
>>> open(os.devnull, 'rb')
<_io.BufferedReader name='nul'>
>>> open(os.devnull, 'rb', 0)
<_io.FileIO name='nul' mode='rb' closefd=True>
>>> open(os.devnull, 'r', 0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: can't have unbuffered text I/O
>>>