生成随机列表并终止它

Question

I made the following code which generates a random letter from list l every 3 seconds. The first part of my question is how do I terminate this process after 1 minute?

import threading
import random
x = []
def printit():
    threading.Timer(3.0, printit).start()
    l = ['a', 'b', 'c', 'd']
    secure_random = random.SystemRandom()
    print x.append(secure_random.choice(l))

printit()

moreover I want to make another list from those randomly chosen letters and call it x the code above does the job, there is a little problem: when I turn the code it starts printing None, and when I interrupt and print x it gives the list of letters and keeps printing None after that, this is because I don't know how to terminate the process after one minute.

Answer 1

You certainly shouldn't use threads for something like this.

Instead, just

import time
import random

start_time = time.time()
rng = random.SystemRandom()

while time.time() - start_time < 60:  # End after a minute
    print(rng.choice('abcd'))
    time.sleep(3)

Answer 2

Try this code if you still want to use threading:

import threading
import random
import time

x = []
start_time = time.time()


def printit():
    current_time = time.time()
    print(current_time - start_time)
    if current_time - start_time > 60.0:
        return
    threading.Timer(3.0, printit).start()
    l = ['a', 'b', 'c', 'd']
    secure_random = random.SystemRandom()
    x.append(secure_random.choice(l))

    print(x)

printit()

I tested this code in Python 3.6 you may want to change the print() to print to make it work in Python 2

Answer 3

if I did not understand your question wrong, adding

if x.length is 20:
     threading.Timer(3.0, printit).cancel()

at the end of your function may help