Instances calling method seem to reference same list

Question

I am working through Cravens pygame tutorial and need some clarification. Here is the code that is causing the issue: CODE1:

import pygame
import random

# Define some colors
BLACK = ( 0, 0, 0)
WHITE = ( 255, 255, 255)
GREEN = ( 0, 255, 0)
RED = ( 255, 0, 0)

pygame.init()

size = (700, 500)
screen = pygame.display.set_mode(size)

pygame.display.set_caption("'Classy' Snow Globe")

class Snowstorm():
    snow_list = []
    snow_num = 0
    snow_size = 0
    snow_speed = 0


    def get_snow(self):

            for i in range(self.snow_num):
                    x = random.randrange(0, 700)
                    y = random.randrange(0, 500)
                    self.snow_list.append([x,y])

    def draw_snow(self,screen):
            for i in range(len(self.snow_list)):
                    pygame.draw.circle(screen,WHITE,self.snow_list[i],2)
                    self.snow_list[i][1] = self.snow_list[i][1] + self.snow_speed

                    if self.snow_list[i][1] > 500: 
                            y = random.randrange(-50,-10)
                            self.snow_list[i][1] = y
                            x = random.randrange(0,700)
                            self.snow_list[i][0] = x

slow_storm = Snowstorm()
slow_storm.snow_num = 1
slow_storm.snow_size = 20
slow_storm.snow_speed = 1
slow_storm.get_snow()

med_storm = Snowstorm()
med_storm.snow_num = 2
med_storm.snow_size = 20
med_storm.snow_speed = 3
med_storm.get_snow()

done = False

clock = pygame.time.Clock()

while not done:
for event in pygame.event.get():
    if event.type == pygame.QUIT:
        done = True

screen.fill(BLACK)


slow_storm.draw_snow(screen)
med_storm.draw_snow(screen)


pygame.display.flip()

clock.tick(60)

pygame.quit()

The get_snow method seems to be creating the same list for both instances of my Snowstorm class. Example: slow_storm calls get_snow method, the list is appended with [123,454] med_storm calls get_snow method, and gets the same list as slow_storm [123,454]

I changed the code by adding self.snow_list = [], and was able to produce the desired output. CODE2:

 def get_snow(self):
            self.snow_list = []
            for i in range(self.snow_num):
                    x = random.randrange(0, 700)
                    y = random.randrange(0, 500)
                    self.snow_list.append([x,y])

I was also able to produce the correct output by using another variation of code. CODE3:

I added this before the def get_snow(self):

 def __init__(self,list):
            self.snow_list=list

Then modified the instances to:

slow_storm = Snowstorm([])
med_storm.get_snow([])

Question:
1. Why do the instances in the original code (CODE1) seem to be referencing the same list even though they are using self.snow_list? 2. What are the differences that enable CODE2 and CODE3 to work properly?

Thank you.

Answer 1

You put the snow_list = [] variable in the beginning of the class. This makes it a class variable which keeps it the same across all instances of the class. To make the lists separate, move snow_list = [] to be under def __init__(): part of the class.