Changing a list inside a list of lists is changing all the other ones to the same value

Question

I'm new here and looking for some advice.

I'm learning how to use Python (3.8) by implementing a game I made. Every new topic I see, I restart it more or less from scratch to introduce and practice the new elements I learned. I'm currently trying to get the hang of Classes.

I stumble in this problem with "lists", though: I want a "Class" to create numbered Objects. For this I've created a blank list (players_vector = []) and with a "for" loop I append new objects to that list. So I can access each object by their index on that list: players_vector[i].player_color for example.

The problem I can't understand:

player_vector[i].pawn_coordinates is a list like [[x1,y1], [x2, y2]] # There are two Pawns.

Every time I try to change one of those for one of the players (e.g.: players_vector[2].pawn_coordinates[0] = [new_x, new_y]) all the other players have that same pawn's coordinates changed to the same value too. (Other attributes like player_color work fine)

A piece of code that reproduces the problem:

import pyinputplus as pyip

WIDTH = 5
HEIGHT = 5
players_vector = []

class Player:
    def __init__(
        self,
        player_color=None,
        pawn_position=[-1, -1],
        pawn_coordinates=[[-1, -1], [-1, -1]],
        pawn_previous_position=[-1, -1]
    ):
        self.player_color = player_color
        self.pawn_position = pawn_position
        self.pawn_coordinates = pawn_coordinates
        self.pawn_previous_position = pawn_previous_position
        self.player_score = 0

    def return_position(self, WIDTH, pawn_index):
        return WIDTH*(self.pawn_coordinates[pawn_index][1]-1)+(self.pawn_coordinates[pawn_index][0]-1)

players_vector.append(Player('Brown'))
players_vector.append(Player('Orange'))
players_vector.append(Player('White'))

def place_pawns(WIDTH, players_vector):
        print(
            f"Player {players_vector[1].player_color}, please enter the starting coordinates for your Pawns:"
        )
        x = pyip.inputNum(f"Enter X (1-5) for your 1st Pawn: ", min=1, max=5)
        y = pyip.inputNum(f"Enter Y (1-5) for your 1st Pawn: ", min=1, max=5)
        players_vector[1].pawn_previous_position[0] = players_vector[1].pawn_position[0]
        players_vector[1].pawn_coordinates[0] = [x, y]
        players_vector[1].pawn_position[0] = players_vector[1].return_position(WIDTH, 0)
        x = pyip.inputNum(f"Enter X (1-5) for your 2nd Pawn: ", min=1, max=5)
        y = pyip.inputNum(f"Enter Y (1-5) for your 2nd Pawn: ", min=1, max=5)
        players_vector[1].pawn_previous_position[1] = players_vector[1].pawn_position[1]
        players_vector[1].pawn_coordinates[1] = [x, y]
        players_vector[1].pawn_position[1] = players_vector[1].return_position(WIDTH, 1)

place_pawns(WIDTH, players_vector) 

print(f'Coordinates: {players_vector[0].pawn_coordinates} Color: {players_vector[0].player_color}')
print(f'Coordinates: {players_vector[1].pawn_coordinates} Color: {players_vector[1].player_color}')
print(f'Coordinates: {players_vector[2].pawn_coordinates} Color: {players_vector[2].player_color}')

print('Then trying to change the first [x,y] of White to [3,2] and the Orange color to Green')
players_vector[2].pawn_coordinates[0] = [3,2]
players_vector[1].player_color = 'Green'

print(f'Coordinates: {players_vector[0].pawn_coordinates} Color: {players_vector[0].player_color}')
print(f'Coordinates: {players_vector[1].pawn_coordinates} Color: {players_vector[1].player_color}')
print(f'Coordinates: {players_vector[2].pawn_coordinates} Color: {players_vector[2].player_color}')

Sorry, it's just the mutable default argument, not the "list of lists". — mkrieger1, Oct 26 '20 at 20:42
Default arguments are evaluated *once* at function definition time. Every time you use that default argument, it is *sharing the same object*. — juanpa.arrivillaga, Oct 26 '20 at 20:42
Oh, that is not intuitive at all. But indeed, if I remove the default arguments, it works as intended. I saw there's an answer talking about the "mutable default arguments". I don't understand much, but thank you very much! — Gustavo 'Scary', Oct 26 '20 at 20:54

Changing a list inside a list of lists is changing all the other ones to the same value

0 Answers0