def tail(f, lines=20):
    total_lines_wanted = lines

    block_size = 1024
    f.seek(0, 2)
    block_end_byte = f.tell()
    lines_to_go = total_lines_wanted
    block_number = -1
    blocks = []
    while lines_to_go > 0 and block_end_byte > 0:
        if block_end_byte - block_size > 0:
            f.seek(block_number * block_size, 2)
            blocks.append(f.read(block_size))
        else:
            f.seek(0, 0)
            blocks.append(f.read(block_end_byte))
        lines_found = blocks[-1].count(b'\n')
        lines_to_go -= lines_found
        block_end_byte -= block_size
        block_number -= 1
    all_read_text = b''.join(reversed(blocks))
    return b'\n'.join(all_read_text.splitlines()[-total_lines_wanted:]).decode()