code: def fingerprint(self):
      
      if self._fingerprint_memo is None:
        self._fingerprint_memo = self._compute_fingerprint()
      return self._fingerprint_memo
docs: A memoized sha1 hexdigest hashing the contents of this PayloadField
  
      The fingerprint returns either a string or None.  If the return is None, consumers of the
      fingerprint may choose to elide this PayloadField from their combined hash computation.
  
      :API: public