How can I stop a While loop?

I wrote a while loop in a function, but don't know how to stop it. When it doesn't meet its final condition, the loop just go for ever. How can I stop it?

def determine_period(universe_array):
    period=0
    tmp=universe_array
    while True:
        tmp=apply_rules(tmp)#aplly_rules is a another function
        period+=1
        if numpy.array_equal(tmp,universe_array) is True:
            break    #i want the loop to stop and return 0 if the 
                     #period is bigger than 12
        if period>12:  #i wrote this line to stop it..but seems it 
                       #doesnt work....help..
            return 0
        else:   
            return period

just indent your code correctly:

def determine_period(universe_array):
    period=0
    tmp=universe_array
    while True:
        tmp=apply_rules(tmp)#aplly_rules is a another function
        period+=1
        if numpy.array_equal(tmp,universe_array) is True:
            return period
        if period>12:  #i wrote this line to stop it..but seems its doesnt work....help..
            return 0
        else:   
            return period

You need to understand that the break statement in your example will exit the infinite loop you've created with while True. So when the break condition is True, the program will quit the infinite loop and continue to the next indented block. Since there is no following block in your code, the function ends and don't return anything. So I've fixed your code by replacing the break statement by a return statement.

Following your idea to use an infinite loop, this is the best way to write it:

def determine_period(universe_array):
    period=0
    tmp=universe_array
    while True:
        tmp=apply_rules(tmp)#aplly_rules is a another function
        period+=1
        if numpy.array_equal(tmp,universe_array) is True:
            break
        if period>12:  #i wrote this line to stop it..but seems its doesnt work....help..
            period = 0
            break

    return period

def determine_period(universe_array):
    period=0
    tmp=universe_array
    while period<12:
        tmp=apply_rules(tmp)#aplly_rules is a another function
        if numpy.array_equal(tmp,universe_array) is True:
            break 
        period+=1

    return period

The is operator in Python probably doesn't do what you expect. Instead of this:

    if numpy.array_equal(tmp,universe_array) is True:
        break

I would write it like this:

    if numpy.array_equal(tmp,universe_array):
        break

The is operator tests object identity, which is something quite different from equality.

I would do it using a for loop as shown below :

def determine_period(universe_array):
    tmp = universe_array
    for period in xrange(1, 13):
        tmp = apply_rules(tmp)
        if numpy.array_equal(tmp, universe_array):
            return period
    return 0

来源：https://stackoverflow.com/questions/368545/how-can-i-stop-a-while-loop

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while-loop