Python: Searching/reading binary data

Question

I'm reading in a binary file (a jpg in this case), and need to find some values in that file. For those interested, the binary file is a jpg and I'm attempting to pick out its dimensions by looking for the binary structure as detailed here.

I need to find FFC0 in the binary data, skip ahead some number of bytes, and then read 4 bytes (this should give me the image dimensions).

What's a good way of searching for the value in the binary data? Is there an equivalent of 'find', or something like re?

Answer 1

You could actually load the file into a string and use

s.find('\xff\xc0')

Answer 2

The re module does work with both string and binary data (str in Python 2 and bytes in Python 3), so you can use it as well as str.find for your task.

Answer 3

Well, obviously there is PIL The Image module has size as an attribute. If you are wanting to get the size exactly how you suggest and without loading the file you are going to have to go through it line by line. Not the nicest way to do it but it would work.

Answer 4

The bitstring module was designed for pretty much this purpose. For your case the following code (which I haven't tested) should help illustrate:

from bitstring import Bits
# Can initialise from files, bytes, etc.
s = Bits(filename='your_file')
# Search to Start of Frame 0 code on byte boundary
found = s.find('0xffc0', bytealigned=True)
if found:
    print("Found start code at byte offset %d." % found[0])
    s0f0, length, bitdepth, height, width = s.readlist('hex:16, uint:16, 
                                                        uint:8, 2*uint:16')
    print("Width %d, Height %d" % (width, height))