Factory function for making unary operator methods for Filters.

def unary_operator(op):
    """
    Factory function for making unary operator methods for Filters.
    """
    valid_ops = {'~'}
    if op not in valid_ops:
        raise ValueError("Invalid unary operator %s." % op)

    def unary_operator(self):
        # This can't be hoisted up a scope because the types returned by
        # unary_op_return_type aren't defined when the top-level function is
        # invoked.
        if isinstance(self, NumericalExpression):
            return NumExprFilter.create(
                "{op}({expr})".format(op=op, expr=self._expr),
                self.inputs,
            )
        else:
            return NumExprFilter.create("{op}x_0".format(op=op), (self,))

    unary_operator.__doc__ = "Unary Operator: '%s'" % op
    return unary_operator