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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 auth(self, username, password):
 """ Check authentication against the backend :param username: 'key' attribute of the user :type username: string :param password: password of the user :type password: string :rtype: boolean (True is authentication success, False otherwise) """

        if username not in self.users:
            return False
        elif self.users[username][self.pwd_attr] == password:
            return True
        return False