Counting number of bitmaps in an mbm file (See related posts)

# a helping function
from struct import unpack
def readL(f, pos=None):
    if pos is not None:
        f.seek(pos)
    return unpack('L', f.read(4))[0]

# the real counting function
def mbm_count(mbm_file):
    f = open(mbm_file, 'rb')
    mbm_type = readL(f)
    if mbm_type == 0x10000041:  # mbm on ROM (Z:)
        return readL(f) 
    elif mbm_type == 0x10000037L:  # mbm on file (normal)
        return readL(f, readL(f, 16)) # read at trailer
    else:       # what type is it?
        return None

>>> mbm_count('z:\\system\\data\\avkon.mbm')
503L
>>> mbm_count('E:\\system\\apps\\FExplorer\\FExplorer.mbm')
19L
>>>