code: def get_key(self, command, args):
          
          spec = COMMANDS.get(command.upper())
  
          if spec is None:
              raise UnroutableCommand('The command "%r" is unknown to the '
                                      'router and cannot be handled as a '
                                      'result.' % command)
  
          if 'movablekeys' in spec['flags']:
              raise UnroutableCommand('The keys for "%r" are movable and '
                                      'as such cannot be routed to a single '
                                      'host.')
  
          keys = extract_keys(args, spec['key_spec'])
          if len(keys) == 1:
              return keys[0]
          elif not keys:
              raise UnroutableCommand(
                  'The command "%r" does not operate on a key which means '
                  'that no suitable host could be determined.  Consider '
                  'using a fanout instead.')
  
          raise UnroutableCommand(
              'The command "%r" operates on multiple keys (%d passed) which is '
              'something that is not supported.' % (command, len(keys)))
docs: Returns the key a command operates on.