I find a weird issue about lists of list in Python

Question

Here is my code, i cannot figure out the difference between these two lists of list:

cow = 1
column = 1
size = 3
board1=[[0,0,0],[0,0,0],[0,0,0]]
print board1
board2=[[0] * size] * size
print board2

if board1==board2: print 'same'

board1[cow][column] =1
board2[1][2] =1
print "Board 1 is :", board1
print "Board 2 is :", board2

Result:

[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
same
Board 1 is : [[0, 0, 0], [0, 1, 0], [0, 0, 0]]
Board 2 is : [[0, 1, 0], [0, 1, 0], [0, 1, 0]]

What is the particular difference that you don't understand? — Ray Toal, Jun 10 '17 at 00:29
If you found my answer satisfactory, could you mark it as accepted please? Cheers. — syntaxError, Jun 10 '17 at 16:03

syntaxError · Answer 1 · 2017-06-10T00:52:52.173

board2=[[0] * size] * size

The inner list gets made and points to a position in memory.

The outer list is made by putting three references to the inner list in a list.

A change to the inner list changes it for all other inner lists as they point to the same data in memory.

To show this put this after you create the lists

print(
    board1[0] is board1[1],    #False
    board2[0] is board2[1],    #True
    )

The is operator returns true if both objects are same object in memory.

I find a weird issue about lists of list in Python

1 Answers1