once()

Ensure a function is only called once, returning None on subsequent calls. Thread-safe implementation that matches your test expectations.

Implementation

Args: func: The function to wrap Returns: A function that will only execute once, then return None

Example

                            wrapped_func = once(lambda: "called")
wrapped_func()  # Returns "called"
                        

Expected output: 'called'

Alternative usage:

wrapped_func() # Returns None

Expected output: None

Source Code

                        def once(func: Callable) -> Callable:
    result = [None]
    has_been_called = [False]
    lock = threading.Lock()

    def once_func(*args, **kwargs):
        with lock:  # Thread-safe check-and-set
            if not has_been_called[0]:
                result[0] = func(*args, **kwargs)
                has_been_called[0] = True
                return result[0]  # Return the actual result on first call
            else:
                return None  # Return None on subsequent calls

    return once_func
                    