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.

def _enum_from_op_string(op_string):
    """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.
    """
    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)