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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 replace_missing_value(self, str_in):
 # type: (Text) -> Text """ returns 'str_in' if it is not equals to the 'sentinel' as defined in the missingValue section of the schema. Else it will return the 'replaceWith' value. :param str_in: :return: str_in or the missingValue replacement value """

        if self.missing_value is None:
            return str_in
        elif self.missing_value.sentinel == str_in:
            return self.missing_value.replace_with
        else:
            return str_in