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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 get_groups(self, username):
 """ Get a user's groups :param username: 'key' attribute of the user :type username: string :rtype: list of groups """

        try:
            return self.users[username]['groups']
        except Exception as e:
            raise UserDoesntExist(username, self.backend_name)