bio.py

import os
import io

FN = 'thefile'

with open(FN, 'w') as f:
    f.write(b'abcdefgh\n'
            b'ijlkmnop\n'
            b'qrstuwxyz\n')

with io.open(FN, mode='rb', buffering=4096) as f:
    assert isinstance(f, io.BufferedReader)
    f.readlines()

os.unlink(FN)

$ strace -e trace=open,read python /tmp/bio.py |& tail -3
open("thefile", O_RDONLY)               = 3
read(3, "abcdefgh\nijlkmnop\nqrstuwxyz\n", 4096) = 28
read(3, "", 4096)                       = 0

You should see the same behavior with buffering=1