code: def login_url(self, org=None):
          
          if org is None:
              org = self.org
          else:
              org = self.keychain.get_org(org)
          return org.start_url
docs: Returns the login url which will automatically log into the target
              Salesforce org.  By default, the org_name passed to the library
              constructor is used but this can be overridden with the org option
              to log into a different org.