code: def invalidation_hash(self, fingerprint_strategy=None):
      
      fingerprint_strategy = fingerprint_strategy or DefaultFingerprintStrategy()
      if fingerprint_strategy not in self._cached_fingerprint_map:
        self._cached_fingerprint_map[fingerprint_strategy] = self.compute_invalidation_hash(fingerprint_strategy)
      return self._cached_fingerprint_map[fingerprint_strategy]
docs: :API: public