code: def api_version(self):
          
          # Lean on the cache first.
          if self._api_version is not None:
              return self._api_version
  
          # We don't have a cache. Build it.
          self._api_version = self._introspect_api_version(
              self.session.core_session,
              self.service_name
          )
          return self._api_version
docs: Returns API version introspected from the service data.
  
          If the data has been previously accessed, a memoized version of the
          API version is returned.
  
          :returns: The service's version
          :rtype: string