Attribute System of Inherited Python class

Question

class Board:
 array = [[" ", " ", " "],
 [" ", " ", " "],
 [" ", " ", " "]]
 def reset(self):
 self.array = [[" ", " ", " "],
 [" ", " ", " "],
 [" ", " ", " "]]
class AI(Board):
 def __init__(self):
 self.array[0][0] = "X"
ai = AI()
board = Board() 
print(ai.array) # [['X', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]
print(board.array) # [['X', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]
ai.reset()
print(ai.array) # [[' ', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]
print(board.array) # [['X', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]

My question is why board.array is changed while ai.array was. If they are connected to each other, why both the attributes are not changed together while the method belongs to AI is run.

Answer 1

This can be understood in the following manner:

• When you get self.array to use or modify it, as in the expression self.array[0][0] = "X", then

  • first the instance is checked to see if it has such an attribute;
  • if it does not, the type of the instance is checked for the attribute.

(this is a simplification of what happens, but its all you need to know for this case)

• When you set an instance, as you do in the expression self.array = [...] you are setting the attribute directly on the instance

So in your example code:

print(ai.array) # ai does not have an array, Board.array is returned
print(board.array) # board does not have an array, Board.array is returned
ai.reset() # this adds an attribute to ai
print(ai.array) # ai **does** have an array, it is returned
print(board.array) # board does not have an array, Board.array is returned