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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 url(self):
        "Returns the URL that can be visited to obtain a verifier code"
        # The authorize url is always api.xero.com
        query_string = {'oauth_token': self.oauth_token}

        if self.scope:
            query_string['scope'] = self.scope

        url = XERO_BASE_URL + AUTHORIZE_URL + '?' + \
              urlencode(query_string)
        return url