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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 parse_qs(self, qs):
 """ Parse query string, but enforce one instance of each variable. Return a dict with the variables on success Return None on parse error """

        qs_state = urllib2.urlparse.parse_qs(qs)
        ret = {}
        for qs_var, qs_value_list in qs_state.items():
            if len(qs_value_list) > 1:
                return None

            ret[qs_var] = qs_value_list[0]

        return ret