code: def assert_keys_exist(self, caller, *keys):
          
          assert keys, ("*keys parameter must be specified.")
          for key in keys:
              self.assert_key_exists(key, caller)
docs: Assert that context contains keys.
  
          Args:
              keys: validates that these keys exists in context
              caller: string. calling function or module name - this used to
                      construct error messages
  
          Raises:
              KeyNotInContextError: When key doesn't exist in context.