hashlib md5 is slow

I need md5 checksums in part of a program. I created a small test script just to see how this works. The script is almost twice as slow as calling md5sum directly. Anyway I can tweak this or should I just use subprocess to call md5sum?

#!/usr/bin/python

import sys
import hashlib

filename=sys.argv[1]

class md5_checksum(object):
    def __init___(self):
        pass
    def create_checksum(self, x):
        self.x=x
        self.x=hashlib.md5(open(self.x,'rb').read()).hexdigest()
    def return_checksum(self):
        return self.x

m=md5_checksum()
m.create_checksum(filename)

print "The checksum for file '" + filename + "' is",     m.return_checksum() + '.'