Extract u-boot multi-file image in Python

uboot_extract.py

#!/usr/bin/env python

import sys

def toNumber(buf):
    """ Convert string in number """
    size = 0
    for b in buf: size=size*256+ord(b)
    return size

if len(sys.argv)!=2:
    sys.stdout.write('Usage %s uboot_image_file\n' % sys.argv[0])
    sys.exit(1)

f = open(sys.argv[1],'rb')
f.seek(64) # skip image header

parts = []

# get file size of all images
while True:
    buf = f.read(4)
    size = toNumber(buf)
    if size==0: break
    parts.append(size)

i = 0
for size in parts:
    pattern = 'file%d.img' % i
    p = open(pattern, 'wb')
    buf = f.read(size)
    p.write(buf)
    p.close()
    if size%4 != 0:
        # 4 byte padding
        f.read(4-(size%4))
    i+=1

f.close()

TypeError: ord() expected string of length 1, but int found