Adjusting a variable then appending to a list in python

Question

I'm making a noughts and crosses AI using negamax and to do this I must run through all the possible moves you could make from any board state. I'm attempting to do this by appending each possible version of the board to a list however it will only append the final state to the list.

Here is my code:

def allPossibleMoves(board, turn):
    l = []
    for i in range(0,3):
        for x in range(0,3):
            if board[i][x] == ' ':
                board[i][x] = turn
                print(board)
                l.append(board)
                board[i][x] = ' '
    return l

You only have a single mutable board in your code -- make copies of it before appending. Also -- your question would be better if it included a minimal reproducible example. In this case, the issue seems clear enough, but whenever code in a question depends upon code defined elsewhere, it is hard to know for certain what is going on. — John Coleman
– John Coleman, Commented Mar 13, 2021 at 13:55

Red · Accepted Answer · 2021-03-13 14:16:44Z

2

The reason is because no matter how many times you append board to the list, the board variable will point at the same object.

Use the deepcopy() method from the built-in copy module in the for loop to make copies of the object:

edited Mar 13, 2021 at 14:16

answered Mar 13, 2021 at 13:54

Red

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Comments

Max Shouman · Accepted Answer · 2021-03-14 17:59:11Z

1

Your trying to append the same variable over over.

If you know for sure that board is a bidimensional array, you can append a list comprehension, like this:

l.append([array[:] for array in board])

If you don't know the shape of board, I suggest you use the same method as per Ann Zen's answer.

edited Mar 14, 2021 at 17:59

answered Mar 13, 2021 at 14:03

Max Shouman

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3 Comments

Red Over a year ago

Nice. But the inner arrays of board will still be the same instances of each others.

Max Shouman Over a year ago

Oof, you are right, they'll all be the same copy of the latest updated board. Thanks for letting me know!

Red Over a year ago

Good idea. You can implement your first suggestion into this answer: [array[:] for array in board].

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