While condition in python doest stop the loop when true

I made a loop that needs to stop once the list U has no values in it. Once it reaches that point, the loop doesnt stop and it gives an error, because there is a value by 0 (the len(U) in the last loop)

def ATC(instance,K):
    pi=[]
    U = [i for i in range(instance.jobs)]
    t=0
    p= sum(instance.pt) / len(U)
    while len(U)!=0:
        I=[]
        for j in U:
            I.append((instance.w[j]/instance.pt[j])*exp(-(max(instance.dd[j]-instance.pt[j]-t,0))/(K*p)))
        k = np.argmax(I)        
        pi.append(U[k])         
        U.pop(k)                
        print(U,len(U))
        t = instance.Cmax(pi)         
        p = sum([instance.pt[i] for i in U])/len(U) ################################
        obj = instance.SumWjTj(pi)
    return pi,obj

enter image description here

Here the print of the list U and len(U) during the loop, showing it reaches 0 but the while condition doesnt work:

[0, 1, 2, 3, 4, 5, 7, 8, 9] 9
[0, 1, 2, 4, 5, 7, 8, 9] 8
[0, 1, 4, 5, 7, 8, 9] 7
[0, 1, 4, 7, 8, 9] 6
[0, 1, 4, 7, 9] 5
[0, 1, 4, 7] 4
[1, 4, 7] 3
[4, 7] 2
[7] 1
[] 0

Using a If len(U)!=0: p = sum([instance.pt[i] for i in U])/len(U) works, but I don't understand why the while isn't enough

Solution

The loop doesn't exit as soon as len(U) is zero and will go on to complete that current iteration of the loop. So, in your case, once U is empty it will still try do the lines:

t = instance.Cmax(pi)         
p = sum([instance.pt[i] for i in U])/len(U) ################################
obj = instance.SumWjTj(pi)

before it re-tests for len(U)!=0 and exits. So, when calculating p it'll try and divide by zero, which I assume is the error you are getting.

Instead you could put a break clause in your loop like:

while True:
    I=[]
    for j in U:
        I.append((instance.w[j]/instance.pt[j])*exp(-(max(instance.dd[j]-instance.pt[j]-t,0))/(K*p)))
    k = np.argmax(I)        
    pi.append(U[k])         
    U.pop(k)                
    print(U,len(U))

    # break out the loop here
    if len(U) == 0:
        break

    t = instance.Cmax(pi)         
    p = sum([instance.pt[i] for i in U])/len(U) ################################
    obj = instance.SumWjTj(pi)