运行一个函数，直到它首先返回 True，然后不再运行它

Question

我正在开发一个接收连续数字输入流的模块。目标是检测输入数组超过某个预设阈值的第一次时间。换句话说，我需要运行一个比较函数，直到达到阈值；那么该功能需要“关闭”。

我的想法是使用装饰器来解决问题，因为我知道它们可以有效地用于运行一个函数，并且永远不会再次运行，这与我想要实现的有点相似。

在以下情况下，数字输入的连续流是：12, 19, 82, 92, 26, ...。在这种情况下，预期的输出将是：

Rand. val:  12
above_threshold returns False
Rand. val:  19
above_threshold returns False
Rand. val:  82
above_threshold returns True 
Threshold has been reached! 
Comparison function above_threshold shouldn't be called any more.
Rand. val: 92
Rand. val: 26
...

目前，above_threshold 在每个循环中都被调用，我还没有成功地使用装饰器“关闭”该功能。

import time 
import random 

random.seed(12771)

threshold = 75

def run_until_first_true_reached(f):
    """
    Decorator that runs the function f until it first returns True. 
    After returning True once, it will stop running the wrapped function again.
    """
    def wrapper(*args, **kwargs):
        # If f is False
        if not f(*args, **kwargs):
            return f(*args, **kwargs)
        # If f is True
        else: 
            print("Threshold has been reached!")
            print("Comparison function above_threshold shouldn't be called any more.")

            # tried an empty "return" in this line but didn't solve the issue
    return wrapper 

@run_until_first_true_reached
def above_threshold(value, threshold): 
    if value > threshold:
        print("above_threshold returns True")
        return True 
    else:   
        print("above_threshold returns False")
        return False

# Modelling the continuous stream of inputs 
for _ in range(100): 

    rand_val = random.randint(1,100)
    print("Rand. val: ", rand_val)

    above_threshold(rand_val, threshold)

    time.sleep(1)

Answer 1

我不知道有什么方法可以让装饰器/包装器在达到条件后不被调用，但是一旦达到条件就将其变成无操作非常简单，您的第一条评论是您似乎希望代码执行的操作。

import time 
import random 

random.seed(12771)

threshold = 75

def run_until_first_true_reached(f):
    """
    Decorator that runs the function f until it first returns True. 
    After returning True once, it will stop running the wrapped function again.
    """

    def wrapper(*args, **kwargs):
        if not wrapper.reached:
            v = f(*args, **kwargs)
            # If f is False
            if not v:
                return v
            # If f is True
            else: 
                print("Threshold has been reached!")
                print("Comparison function above_threshold shouldn't be called any more.")
                wrapper.reached = True
        return None   #  ? or wahtever we want to return once the threshold is reached

    wrapper.reached = False
    return wrapper 

@run_until_first_true_reached
def above_threshold(value, threshold): 
    if value > threshold:
        print("above_threshold returns True")
        return True 
    else:   
        print("above_threshold returns False")
        return False

# Modelling the continuous stream of inputs 
for _ in range(100): 

    rand_val = random.randint(1,100)
    print("Rand. val: ", rand_val)

    above_threshold(rand_val, threshold)

    time.sleep(1)

结果：

Rand. val:  12
above_threshold returns False
Rand. val:  19
above_threshold returns False
Rand. val:  82
above_threshold returns True
Threshold has been reached!
Comparison function above_threshold shouldn't be called any more.
Rand. val:  92
Rand. val:  26
Rand. val:  18
Rand. val:  55
...

这里有趣的一点是，您需要在某个地方存储状态……已经达到阈值的事实。我在装饰器中这样做的方式是将状态附加到包装函数。

我稍微更改了您的逻辑，以便在每次包装器调用时不会调用包装函数两次。这会产生重复的输出行，导致无法匹配您请求的输出。

Answer 2

简单地使用这个想法，你想要什么：“打开和关闭”包装函数的调用。为此，我们可以使用父装饰器函数的变量作为状态标志：

import time 
import random 

random.seed(12771)

threshold = 75

def run_until_first_true_reached(f):
    """
    Decorator that runs the function f until it first returns True. 
    After returning True once, it will stop running the wrapped function again.
    """

    switch_on = True

    def wrapper(*args, **kwargs):
        nonlocal switch_on

        if switch_on:
            threshold_reached = f(*args, **kwargs)
            if threshold_reached:
                print("Threshold has been reached!")
                print("Comparison function above_threshold shouldn't be called any more.")
                switch_on = False

    return wrapper 

@run_until_first_true_reached
def above_threshold(value, threshold): 
    if value > threshold:
        print("above_threshold returns True")
        return True 
    else:   
        print("above_threshold returns False")
        return False

输出：

Rand. val:  12
above_threshold returns False
Rand. val:  19
above_threshold returns False
Rand. val:  82
above_threshold returns True
Threshold has been reached!
Comparison function above_threshold shouldn't be called any more.
Rand. val:  92
Rand. val:  26
Rand. val:  18
Rand. val:  55