code: def _enum_from_op_string(op_string):
      
      try:
          return _COMPARISON_OPERATORS[op_string]
      except KeyError:
          choices = ", ".join(sorted(_COMPARISON_OPERATORS.keys()))
          msg = _BAD_OP_STRING.format(op_string, choices)
          raise ValueError(msg)
docs: Convert a string representation of a binary operator to an enum.
  
      These enums come from the protobuf message definition
      ``StructuredQuery.FieldFilter.Operator``.
  
      Args:
          op_string (str): A comparison operation in the form of a string.
              Acceptable values are ``<``, ``<=``, ``==``, ``>=``
              and ``>``.
  
      Returns:
          int: The enum corresponding to ``op_string``.
  
      Raises:
          ValueError: If ``op_string`` is not a valid operator.