How can I fix an IndexError regarding the random choice method?

Question

I am getting an IndexError and I'm unsure why.

cards = [2, 3, 4, 5, 6, 7, 8, 9, 10 ,11, 12, 13, 14,
         2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
         2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
         2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
USER = []
CPU = []
selected = 1
while selected < 26:
    CPU.append(cards.pop(random.choice(cards)))
    USER.append(cards.pop(random.choice(cards)))
    selected = selected + 1

I want the items inside the list cards to be appended into the lists USER and CPU. It worked originally but after working on something else it stopped working. I looked up why but the pop index says it's out of range, but I don't understand why.

The first argument passed to list pop method must be a valid *index* of the list, not the value of some element in the list (per the [documentation](https://docs.python.org/3/library/stdtypes.html#mutable-sequence-types)). — martineau, Apr 05 '22 at 18:53

score 3 · Answer 1 · edited Apr 05 '22 at 18:36

3

The problem is that random.choice() returns a random value from the list, but the argument to cards.pop() must be an index. When the list becomes shorter than 14 elements, some of the values will be invalid indexes and you'll get an error.

A simpler way to do this is to just shuffle the cards list and then assign half to each player.

random.shuffle(cards)
CPU = cards[:26]
USER = cards[26:]

edited Apr 05 '22 at 18:36

azro

53,056
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answered Apr 05 '22 at 18:36

Barmar

741,623
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@Solidpanda2191: Also note that this code, besides being way faster than the way you're trying to do things, also happens to be very similar to the way real cards would be manipulated to achieve the same goal. – martineau Apr 05 '22 at 19:10

score 0 · Answer 2 · answered Apr 05 '22 at 18:36

0

I'd change the while line into: while not cards. If cards is an empty list, this returns false and exits the loop. Please keep in mind that since You're taking more than one card out of the list, this may fail if 52/(takings per iteration) gives a non-0 remainder.

answered Apr 05 '22 at 18:36

Aidas

114
4

1

That won't fix the problem. – Barmar Apr 05 '22 at 18:36

score 0 · Answer 3 · answered Apr 05 '22 at 18:36

Since random.choice(cards) returns a value from the list cards, that means that if you pass the result into cards.pop(), which expects you to pass in the index of an item, that means that you could potentially pass an index into the pop() which is out of range. For instance, if your list cards has a length of 9 at some point while you're running your script, and random.choice(cards) happens to return 11, then you'll get an error about the pop index.

The solution for how to pop a random item out of a list has already been covered here so I'm not going to repeat it in this answer. What is the most pythonic way to pop a random element from a list?

score 0 · Answer 4 · answered Apr 05 '22 at 19:03

As I mention in a comment, the IndexError is because you're passing and element of the list instead of its index to the pop() method. One way to avoid the problem would be to instead use random.randrange() to pick a valid value to pass it:

import random

cards = [2, 3, 4, 5, 6, 7, 8, 9, 10 ,11, 12, 13, 14,
         2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
         2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
         2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
USER = []
CPU = []
selected = 1
while selected < 26:
    CPU.append(cards.pop(random.randrange(len(cards))))
    USER.append(cards.pop(random.randrange(len(cards))))
    selected += 1

How can I fix an IndexError regarding the random choice method?

4 Answers4