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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)