Find the last occurrence of given character in file

rfind_file.py

import os
import sys

def rfind_file(filename, value, blocksize, maxsize=0):
    fd = os.open(filename, os.O_RDONLY)
    try:
        size   = os.lseek(fd, 0, os.SEEK_END)
        if maxsize < 0:
            size = max(size - maxsize, 0)
        elif maxsize > 0:
            size = min(size, maxsize)
        offset = ((size - 1) // blocksize) * blocksize
        while True:
            assert os.lseek(fd, offset, os.SEEK_SET) == offset
            data    = os.read(fd, blocksize)
            assert len(data) == min(blocksize, size - offset)
            index   = data.rfind(value)
            if index >= 0: return offset + index
            offset -= blocksize
            if offset < 0: return -1
    finally:
        os.close(fd)

if __name__ == '__main__':
    argc      = len(sys.argv)
    filename  =     sys.argv[1]
    char      =     sys.argv[2].encode().decode('unicode_escape').encode()
    assert len(char) == 1, repr(char)
    value     = char[0]
    blocksize = int(sys.argv[3]) if argc > 3 else 65536
    maxsize   = int(sys.argv[4]) if argc > 4 else 0
    res       = rfind_file(filename, value, blocksize, maxsize)
    print(res)
    if res < 0:
        sys.exit(1)