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.

def login_url(self, org=None):
        """ 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.
        """
        if org is None:
            org = self.org
        else:
            org = self.keychain.get_org(org)
        return org.start_url