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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 field_types(self):
 """ Access the field_types :returns: twilio.rest.autopilot.v1.assistant.field_type.FieldTypeList :rtype: twilio.rest.autopilot.v1.assistant.field_type.FieldTypeList """

        if self._field_types is None:
            self._field_types = FieldTypeList(self._version, assistant_sid=self._solution['sid'], )
        return self._field_types