Python: generates crc32 and adler32 checksum for big files

import zlib
import sys
import urllib2

def __zlib_csum(url, func):
    if isinstance(url, basestring if sys.version_info[0] < 3 else str):
        url = urllib2.Request(url)
    f = urllib2.urlopen(url)
    csum = None
    try:
        chunk = f.read(1024)
        if len(chunk)>0:
            csum = func(chunk)
            while True:
                chunk = f.read(1024)
                if len(chunk)>0:
                    csum = func(chunk, csum)
                else:
                    break
    finally:
        f.close()
    if csum is not None:
        csum = csum & 0xffffffff
    return csum
    

def crc32(url):
    return __zlib_csum(url, zlib.crc32)

def adler32(url):
    return __zlib_csum(url, zlib.adler32)

if __name__ == '__main__':
    print(hex(crc32('file:/tmp/111.zip')))
    print(hex(adler32('file:/tmp/111.zip')))