error with simple module & function

[lewashby]

I'M getting an error in the following program on line 38 to a function call. The function is in the gameobjects module and the there's an error there as well on line 21. Here's the code.

Code:

background_image_filename = 'sushiplate.jpg'
sprite_image_filename = 'fugu.png'

import pygame
from pygame.locals import *
from sys import exit
from gameobjects import Vectory2

pygame.init()

screen = pygame.display.set_mode((640, 480), 0, 32)
background = pygame.image.load(sprite_image_filename).convert_alpha()

clock = pygame.time.Clock()

sprite_pos = Vectory2(200, 150)
sprite_speed = 300.

while True:
    
    for event in pygame.event.get():
        if event.type == QUIT:
            exit()
            
    pressed_key = pygame.key.get_pressed()
    
    key_direction = Vectory2(0, 0)
    
    if pressed_key[K_LEFT]:
        key_direction.x = -1
    elif pressed_key[K_RIGHT]:
        key_direction.x = + 1
    if pressed_key[K_UP]:
        key_direction.y = - 1
    elif pressed_key[K_DOWN]:
        key_direction.y = + 1
        
    key_direction.normalize()
    
    screen.blit(background, (0,0))
    screen.blit(sprite, sprite_pos)
    
    time_passed = clock.tick(30)
    time_passed_seconds = time_passed / 1000.0
    
    sprite_pos += key_direction * sprite_speed * time_passed_seconds
    
    pygame.display.update()

gameobjects.py

import math

class Vectory2(object):
    
    def __init__(self, x = 0.0, y = 0.0):
        self.x = x
        self.y = y
        
    def __str__(self):
        return "(%s, %s)" % (self.x, self.y)
    
    @staticmethod
    def from_points(P1, P2):
        return Vectory2(P2[0] - P1[0], P2[1] - P1[1])
    
    def get_magnitude(self):
        return math.sqrt(self.x**2 + self.y**2)
    
    def normalize(self):
        magnitude = self.get_magnitude()
        self.x /= magnitude
        self.y /= magnitude
        
    # rhs stands for right hand side
    def __add__(self, rhs):
        return Vectory2(self.x + rhs.x, self.y + rhs.y)

Thanks for any and all help.

[Gribouillis]

The only potential error that I see in line 21 in gameobjects.py is division by zero if x and y are both 0 in a Vectory2 instance. Why don't you post the exception traceback ? A bug in your code is that after the sequence with the keys in lines 29-36, the key_direction could very well be zero.

[TrustyTony]

put in print or assert statements to get values of variables at point of error. You did not include image files to run your code.

[lewashby]

ZeroDivisionError: float division
File "/home/developer/Projects/Beginning Game Development/Chapter 4/keymovement.py", line 38, in <module>
key_direction.normalize()
File "/home/developer/Projects/Beginning Game Development/Chapter 4/gameobjects.py", line 21, in normalize
self.x /= magnitude

[Gribouillis]

The problem is that key_direction is (0,0) initially. You can't normalize the null vector. You could start with (1,0), or choose a random angle a in [0, 2*pi] and start with (cos(a), sin(a)).