***Python function changing local variables!

Mr_Custom

New Issue!
Here it is!

I am creating a python function that returns all the possible moves that a player can make in one turn in a list of tuples (which represent each board state).
I am running into the issue where the line:

possible[-1][count1] = turn

somehow changes the variable ‘board’, which it shouldn't!
The full function is:

def getPossible(board, turn):
    possible = []
    temp2 = board
    while "-" in temp2:
        count1 = 0
        while temp2[count1] != "-" and count1 < len(temp2)-1:
            count1 += 1
        possible.append(board)
        possible[-1][count1] = turn # << It was that guy
        print(board)
    return possible

An example call is:

list3 = getPossible(["-", "-", "-", "-", "-", "-", "-", "-", "-"], "X")

Last edited by Mr_Custom (Jan. 4, 2020 14:20:32)

Sheep_maker

I think `possible[-1]` points to the same list in memory as `board`, so modifying it by doing `possible[-1][count1] = turn` will also modify `board`. You can prevent it by cloning `board` before appending it to `possible` so that `possible[-1]` points to a fresh new separate list in memory.

Edit: BBCode absorbed my square brackets! I have fixed.

Last edited by Sheep_maker (Dec. 26, 2019 20:52:50)

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Mr_Custom

Sheep_maker wrote:
I think `possible[-1]` points to the same list in memory as `board`, so modifying it by doing `possible[-1][count1] = turn` will also modify `board`. You can prevent it by cloning `board` before appending it to `possible` so that `possible[-1]` points to a fresh new separate list in memory.

I amended it to this (below) with the same results! Sorry if I didn't understand your answer properly!

def getPossible(board, turn):
    possible = []
    temp2 = board
    while "-" in temp2:
        count1 = 0
        while temp2[count1] != "-" and count1 < len(temp2)-1:
            count1 += 1
        temp = board
        possible.append(temp)
        possible[-1][count1] = turn
        print(board)
    return possible

Sheep_maker

temp, board, and possible[-1] all refer to the same list in memory, so changing the items in one will affect the others as well (for example, when you do possible[-1][count] = turn, board[count] will also be turn)

Fortunately, there's a way to prevent this. From the original version of your code, if you changed line 8 to

possible.append(board.copy())

.copy() (as recommended by StackOverflow) stores an exact copy of the list elsewhere in memory so that board and possible[-1] refer to different lists and changing one won't affect the other

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HPD1155

Mr_Custom wrote:
Sheep_maker wrote:
I think `possible[-1]` points to the same list in memory as `board`, so modifying it by doing `possible[-1][count1] = turn` will also modify `board`. You can prevent it by cloning `board` before appending it to `possible` so that `possible[-1]` points to a fresh new separate list in memory.

I amended it to this (below) with the same results! Sorry if I didn't understand your answer properly!
def getPossible(board, turn):
    possible = []
    temp2 = board
    while "-" in temp2:
        count1 = 0
        while temp2[count1] != "-" and count1 < len(temp2)-1:
            count1 += 1
        temp = board
        possible.append(temp)
        possible[-1][count1] = turn
        print(board)
    return possible

you know python too

here's an animation code:

import turtle
turtle.shape('turtle')
turtle.color('green')
turtle.pen('green')
turtle.forward(50)
turtle.left(30*30)
turtle.backward(10)
turtle.right(70)
turtle.forward(500*500)

I give you permission to copy the code.

Last edited by HPD1155 (Jan. 2, 2020 21:00:48)

i luv koenigseggs I is a hackr FYI

console.log(“hi); I’m an ethical hacker FYI

Mr_Custom

Sheep_maker wrote:
possible.append(board.copy())

Thank you for the help! It worked perfectly with a few extra tweaks

Mr_Custom

New Issue!
Here it is!

Last edited by Mr_Custom (Jan. 4, 2020 14:20:18)

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