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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 fillna(self, value):
 """Returns Index with missing values replaced with value. Parameters value : {int, float, bytes, bool} Scalar value to replace missing values with. Returns ------- Index With missing values replaced. """

        if not is_scalar(value):
            raise TypeError('Value to replace with is not a valid scalar')

        return Index(weld_replace(self.weld_expr,
                                  self.weld_type,
                                  default_missing_data_literal(self.weld_type),
                                  value),
                     self.dtype,
                     self.name)