CODEWITHSUNDEEP
pythonpython-3.xlinked-listinstancemember
Nick Anderson
Nick Anderson

Reputation: 13

Why are my different instance variables linking together in python

I am new to python so I did some digging and from what I understand the member variable of a class will update in all instances of that class regardless of which instance you modified it with. However, it doesn't seem that the same should happen with instance variables however when I run this block of code...

class Game(object):

    def__init__(self, active_turn, board):
         self.active_turn = active_turn
         self.board = board


game = Game(1,[1,0,0,0,0,0,0,0,0])

move = 3

print(game.board, "\n")

possible_game = Game(game.active_turn*-1,game.board)

print(game.board)
print(possible_game.board, "\n")

possible_game.board[move] = possible_game.active_turn

print(game.board)
print(possible_game.board, "\n")

game.board[move+1] = game.active_turn

print(game.board)
print(possible_game.board)

I get this output...

[1, 0, 0, 0, 0, 0, 0, 0, 0] 

[1, 0, 0, 0, 0, 0, 0, 0, 0]
[1, 0, 0, 0, 0, 0, 0, 0, 0] 

[1, 0, 0, -1, 0, 0, 0, 0, 0]
[1, 0, 0, -1, 0, 0, 0, 0, 0] 

[1, 0, 0, -1, 1, 0, 0, 0, 0]
[1, 0, 0, -1, 1, 0, 0, 0, 0]

The board variable is updating in each instance of the Game class even though I am only changing it in one of them. Does anyone know why this is happening and what I could do to avoid it?

Thanks, Nick

Upvotes: 1

Views: 78

Answers (2)

laike9m
laike9m

Reputation: 19368

use copy

>>> a = [1,2,3]
>>> from copy import copy
>>> b = copy(a)
>>> b
[1, 2, 3]
>>> b[1] = 5
>>> b
[1, 5, 3]
>>> a
[1, 2, 3]
>>>

EDIT

To truely understand what's under the hood, you should learn about Python DataModel. The official doc is easy to follow.

To be short, list are mutable type in Python, which means that if you assign a list object to multiple variables, they are actually pointing to the same list object, thus changing one of them will affect others.

On the other hand, string, int, tuple, ... are immutable data type.
if you assign a=1 then a=2, 1 and 2 are different int objects, i.e. 1 does not become 2, because int are immutable, they can't be modified.

Upvotes: 3

Nir Alfasi
Nir Alfasi

Reputation: 53535

You're using the same board when you create both instances - so when one of them updates the board - the change is reflected in the other as well.

game = Game(1,[1,0,0,0,0,0,0,0,0]) # first instance - you create the board here
...
possible_game = Game(game.active_turn*-1,game.board) # second instance - you pass the same board (game.board) to the constructor

Upvotes: 3

Related Questions