docs: def parse(cls, string):
          """Parses `string` and returns an :class:`Operator`
          object.
  
          :raises: :exc:`InvalidOperatorExpression` If `string` is not \
          a valid operator.
  
          Valid operators are ``==``, ``!=``, ``<``, ``>``, ``<=``, and ``>=``.
  
          """
          if string in STR_TO_OP_FUNC:
              return cls(STR_TO_OP_FUNC[string], string)
          else:
              raise InvalidOperatorExpression(string)