Run a function until it first returns True, then do not run it again

Question

I am working on a module which receives a continuous flow of numerical inputs. The goal is to detect the first time the array of inputs goes above a certain pre-set threshold value. In other words, I need to run a comparison function until the point when the threshold is reached; then that function needs to be "switched off."

My idea was to approach the problem with decorators as I'm aware they can effectively be used for running a function only once and never again, which is a bit similar to what I am trying to achieve.

In the below case the continuous flow of numerical inputs are: 12, 19, 82, 92, 26, .... In this case the expected output would be:

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
...

At the moment however above_threshold gets called in every loop and I haven't succeeded "switching off" the function using the decorator.

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

I don't know of any way to have the decorator/wrapper not be called once a condition is reached, but it's pretty simple to turn it into a no-op once a condition is reached, which by your first comment is what you seem to want the code to do.

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)

Result:

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
...

The interesting bit here is that you need somewhere to store the state...the fact that the threshold has been reached. The way I do this in decorators is to attach the state to the wrapper function.

I changed your logic a bit so that the wrapped function isn't called twice on each wrapper invocation. This was producing duplicate lines of output that prevented matching your requested output.