<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sha1(self):
 """Return a sha1 hash of the model items. :rtype: str """

        sha1 = hashlib.sha1(''.join(['%s:%s' % (k,v) for k,v in self.items()]))
        return str(sha1.hexdigest())