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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 _expanded_sql(self):
 """Get the expanded BigQuery SQL string of this UDF Returns The expanded SQL string of this UDF """

    if not self._sql:
      self._sql = UDF._build_udf(self._name, self._code, self._return_type, self._params,
                                 self._language, self._imports)
    return self._sql