Extracting main icon from 64-bit PE (Windows EXE) files in Python 2.x using 7z (p7zip)

extract.py

#!/usr/bin/env python

import subprocess, tempfile, struct, shutil, os

HDR_FMT = '<hhh'
HDR_LEN = struct.calcsize(HDR_FMT)

def extract_64bit_ico(exefn, icofn):
    d = tempfile.mkdtemp()
    gis = os.path.join(d, '.rsrc', 'GROUP_ICON')
    iis = os.path.join(d, '.rsrc', 'ICON')
    entries = []
    try:
        subprocess.check_call(['7z', '-o' + d, 'x', exefn, '.rsrc/ICON', '.rsrc/GROUP_ICON'])
        with open(os.path.join(gis, sorted(os.listdir(gis))[0]), 'rb') as group:
            header = group.read(HDR_LEN)
            reserved, img_type, no_images = struct.unpack(HDR_FMT, header)
            pos = no_images * 16 + HDR_LEN
            with open(icofn, 'wb') as ico_out:
                ico_out.write(header)
                for _ in xrange(no_images):
                    hdr, rsrc_id = struct.unpack('<12sH', group.read(14))
                    fn = os.path.join(iis, str(rsrc_id))
                    if os.path.exists(fn):
                        with open(fn, 'rb') as png:
                            payload = png.read()
                    else:
                        with open(fn + '.ico', 'rb') as ico_in:
                            ico_in.seek(HDR_LEN)
                            hdr = ico_in.read(16)
                            payload = ico_in.read(struct.unpack('<I', hdr[8:12])[0])
                    entries.append(payload)
                    ico_out.write(hdr[:12] + struct.pack('<I', pos))
                    pos += len(payload)
                ico_out.write(''.join(entries))
    finally:
        shutil.rmtree(d)