Reads a .png file and outputs its sections

readpng.py

#!/usr/bin/env python2
import sys, binascii, struct
def islower(c):
	return (c.upper() != c)
def countin(s, c):
	i = 0
	for b in s:
		if b == c:
			i += 1
	return i
if len(sys.argv) < 2:
	print("Usage: %s <png file>" % sys.argv[0])
else:
	try:
		f = open(sys.argv[1], "rb")
	except Exception as e:
		print(repr(e))
		exit(1)
	si = "\x89PNG\r\n\x1a\n"
	if f.read(len(si)) != si:
		print("Not a PNG file")
		f.close()
		exit(1)
	print("Name\tFlags\t\t\t\tLength\tStored CRC\tComputed CRC\tCRC Status")
	while True:
		chklen = f.read(4)
		if chklen == "" or chklen == "\0\0\0\0":
			f.close()
			exit(0)
		chklen = struct.unpack("!L", chklen)[0]
		chktype = f.read(4)
		chkdata = f.read(chklen)
		chkcrc = f.read(4)
		realcrc = struct.pack("!L", binascii.crc32(chktype + chkdata) & 0xffffffff)
		del chkdata
		ancillary_bit = islower(chktype[0])
		private_bit = islower(chktype[1])
		reserved_bit = islower(chktype[2])
		safe_to_copy_bit = islower(chktype[3])
		bits = ""
		if ancillary_bit:
			bits += "Ancillary "
		if private_bit:
			bits += "Private "
		if reserved_bit:
			bits += "Reserved "
		if safe_to_copy_bit:
			bits += "Safe-to-copy "
		crcstatus = "Good"
		if chkcrc != realcrc:
			crcstatus = "Bad"

		print("%s\t%s%s%lu\t%s\t%s\t%s" % (chktype, bits, '\t'*(4-countin(bits, " ")), chklen, binascii.b2a_hex(chkcrc), binascii.b2a_hex(realcrc), crcstatus))
	f.close()