Why does this 'for' loop stop randomly in Python

Question

I have a for loop in a section of code I have been working on where for every item in a list it loops a process of eventually removing one item and goes back around again.

When I run the code it stops at 10 items left in the list out of 30 items at the start.

cards_blue = ["b1","b2","b3","b4","b5","b6","b7","b8","b9","b10"]
cards_red = ["r1","r2","r3","r4","r5","r6","r7","r8","r9","r10"]
cards_yellow = ["y1","y2","y3","y4","y5","y6","y7","y8","y9","y10"]
deck = cards_blue + cards_red + cards_yellow
random.shuffle(deck)
for card in deck:

Then I have at the end of the card game process:

    if not deck:
        if card_score1 > card_score2:
            print("Player 1 is the final winner, they have the most cards")
        elif car_score2 > card_score1:
            print("Player 2 is the final winner, they have the most cards")
        print("Deck is empty")

At the end of all 30 loops of the game it will decide the winner using a scoring system but it stops before it can finish. I am unsure about why this is, could anyone help explain why this happens?

The code you have posted runs for 30 cards. Can you post the code inside the for loop? — Nico Griffioen, Sep 19 '19 at 11:05
What is this `removing one item` that you speak of. This may be incompatible with your loop: `for card in deck`. — quamrana, Sep 19 '19 at 11:05
When I say removing one item it means after the loop process the first item in the list is removed,and the code loops for every remaining item until there is none left. — ben.m04, Sep 19 '19 at 11:06
You shouldn't be removing items from a list while you are iterating over it — roganjosh, Sep 19 '19 at 11:07
I put "(runs the card game process)" there because there is a lot of code that goes in that loop that I am not sure could be posted without a 'good' answer help link. — ben.m04, Sep 19 '19 at 11:07
You could loop over a *copy* of the deck: `for card in deck[:]:` — quamrana, Sep 19 '19 at 11:08
You are mutating the list while iterating https://stackoverflow.com/questions/1207406/how-to-remove-items-from-a-list-while-iterating — James, Sep 19 '19 at 11:08
I am just confused about why it works until the last 7 items are left. — ben.m04, Sep 19 '19 at 11:08

score 1 · Accepted Answer · answered Sep 19 '19 at 11:23

1

You should consider switching to a while loop if you will edit the list, for example:

while deck:
    card = deck.pop(0)
    ...

answered Sep 19 '19 at 11:23

qmeeus

2,341
2
12
21

thanks, this solved it. – ben.m04 Sep 19 '19 at 19:09

Why does this 'for' loop stop randomly in Python

1 Answers1