Python: variable scope and encapsulation of a list

Question

I have a class Map (simplified):

from Enums import *
import Globals
import Tile

class Map:
    tiles = []  #the actual map, it's a 2D list of Tile objects
    for x in range(Globals.mapWidth):
        for y in range(Globals.mapHeight):
            self.tiles[x][y].addItem(Items.Foliage)

And a class Tile:

class Tile:
    items=[]
    def __init__(self, type):
        self.type = type

    def addItem(self,i):
        self.items.append(i)

My problem is that the items[] array from the class Tile seems to be shared within every instanciation of the class. For example, at the end of the FOR loops, print(len(self.tiles[x][y].items) return 25 for every tile. Why is it so? I should have 25 lists of size 1, but instead printing the list size in the loop increases from 0 to 25. Can someone explan to me what happens here? Thanks a lot for the help :)

John La Rooy · Accepted Answer · 2011-10-28 10:11:57Z

2

Your items is a class attribute, which is why it appears to be shared between all the instances.

class Tile:
    def __init__(self, type):
        self.items = []
        self.type = type

answered Oct 28, 2011 at 10:11

John La Rooy

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4 Comments

gramm Over a year ago

That worked, thanks! I'll accept your answer when the website allows me

gramm Over a year ago

Say while you're here, how can I test if list.index() returns an error? I can't find this on Google.

John La Rooy Over a year ago

@gramm, use a try/except docs.python.org/tutorial/errors.html#handling-exceptions

Sven Marnach Over a year ago

@gramm: try: my_list.index(x); except ValueError: handle_error()

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