Choosing random items in a while loop does not behave as expected

Question

I would like to choose exactly n items from the Lists (P1,P2,P3). If they are not already in the result list (LIST) they should be appended/extended. If the while loop reaches n (here <=3) it should stop. But I get results with 4,6 items. Why ? Thank you.

from random import choice

P1 = ["a", "b","c", "d", "e","f","g","h","i","j"]
P2 = ["a","m","b","n","e","z","h","g","f","j"]
P3 = [("a","b"), ("c","e"), ("g","a"), ("m","j"), ("d","f")]

LIST = []

while len(LIST) <=3:
    c1, c2 = choice(P3)
    d = choice(P1)
    e = choice(P2)
    f = choice(P1)
    g = choice(P2)
    if c1 not in LIST and c2 not in LIST:
        LIST.extend([c1,c2])
        if d not in LIST:
            LIST.append(d)
        if e not in LIST:
            LIST.append(e) 
        if f not in LIST:
            LIST.append(f) 
        if g not in LIST:
            LIST.append(g) 
print LIST

Answer 1

That is because you check that condition at start of each loop.

At start of the loop condition will be 0, 1, 2 but in the loop you can insert up to 5 new elements, increasing it possibly up to 7.

Answer 2

The while executes the whole loop, and before starting the next iteration, it checks whether the condition is still true.

Inside of a single iteration of your loop, you extend the list by up to 6 elements.

If you really only want 3 elements, that you would have to check the condition every time you appended something, like so:

    while True: 
        c1,c2 = choice(P3)
        d = choice(P1)
        e = choice(P2)
        f = choice(P1)
        g = choice(P2)
        if c1 not in LIST and c2 not in LIST:
            LIST.append(c1)
            if len(LIST) >= 3:
                break
            LIST.append(c2)
            if len(LIST) >= 3:
                break
        if d not in LIST:
            LIST.append(d)
            if len(LIST) >= 3:
                break
        if e not in LIST:
            LIST.append(e) 
            if len(LIST) >= 3:
                break
        if f not in LIST:
            LIST.append(f) 
            if len(LIST) >= 3:
                break
        if g not in LIST:
            LIST.append(g) 
            if len(LIST) >= 3:
                break

Answer 3

You check if there are less/equal than 3 Elements in you list

    while len(LIST) <=3:

after that, you add up to 5 items before you check again.

so if your last loop got 3 items at its begining and it adds up to 5 items, your list can contain up to 8 Elements.