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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 singleton(func):
 """ This decorator allows you to make sure that a function is called once and only once. Note that recursive functions will still work. WARNING: Not thread-safe!!! """


    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        if wrapper.__call_state__ == CALLED:
            return
        ret = func(*args, **kwargs)
        wrapper.__call_state__ = CALLED
        return ret

    def reset():
        wrapper.__call_state__ = NOT_CALLED

    wrapper.reset = reset
    reset()

    # save original func to be able to patch and restore multiple times from
    # unit tests
    wrapper.__original_func = func
    return wrapper