Reading and interpreting data from a binary file in Python

Question

I want to read a file byte by byte and check if the last bit of each byte is set:

#!/usr/bin/python

def main():
    fh = open('/tmp/test.txt', 'rb')
    try:
        byte = fh.read(1)
        while byte != "":
            if (int(byte,16) & 0x01) is 0x01:
                print 1
            else:
                print 0
            byte = fh.read(1)
    finally:
        fh.close

    fh.close()

if __name__ == "__main__":
        main()

The error I get is:

Traceback (most recent call last):
  File "./mini_01.py", line 21, in <module>
    main()
  File "./mini_01.py", line 10, in main
    if (int(byte,16) & 0x01) is 0x01:
ValueError: invalid literal for int() with base 16: '\xaf'

Anyone an idea? I didn't succeed using the struct and the binascii modules.

Answer 1

Try using the bytearray type (Python 2.6 and later), it's much better suited to dealing with byte data. Your try block would be just:

ba = bytearray(fh.read())
for byte in ba:
    print byte & 1

or to create a list of results:

low_bit_list = [byte & 1 for byte in bytearray(fh.read())]

This works because when you index a bytearray you just get back an integer (0-255), whereas if you just read a byte from the file you get back a single character string and so need to use ord to convert it to an integer.

---

If your file is too big to comfortably hold in memory (though I'm guessing it isn't) then an mmap could be used to create the bytearray from a buffer:

import mmap
m = mmap.mmap(fh.fileno(), 0, access=mmap.ACCESS_READ)
ba = bytearray(m)