<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_source_variable(self, source_id, variable, value):
 """ Change the value of a source variable. """

        source_id = int(source_id)
        return self._send_cmd("SET S[%d].%s=\"%s\"" % (
            source_id, variable, value))