Question

I think the bug’s pretty self-explanatory, so I’ll just post the code and a screenie of what happens

'''
Created on Oct 29, 2012

@author: pipsqueaker
'''
import pygame, sys, random
from pygame.locals import *

frapsClock = pygame.time.Clock()

class Alien:
    def __init__(self, arg1, arg2):
        self.x = arg1
        self.y = arg2
    def move(self, craftx, crafty):
        if self.x < craftx:
            self.x += 1
        elif self.x > craftx:
            self.x -= 1
    if self.y < crafty:
        self.y += 1
    elif self.y > crafty:
        self.y -= 1

class PlayShip():
    def __init__(self, ag1, ag2):
        self.x = ag1
        self.y = ag2

class mainLopp():
    Board = pygame.display.set_mode((650, 500))
    pygame.display.set_caption("This")
    pygame.init()

    player = PlayShip(random.randrange(1, 650), random.randrange(1, 500))
    alienList = [Alien(random.randrange(1, 650), random.randrange(1, 500))]

    while True:
        pygame.draw.rect(Board, (0, 0, 255), (player.x, player.y, 12, 12))

        for currAlien in alienList:
            currAlien.move(player.x, player.y)
            pygame.draw.rect(Board, (255, 0, 0), (currAlien.x, currAlien.y, 16, 16))

        for event in pygame.event.get():
            if event.type == QUIT:
                print(alienList)
                pygame.quit()
                sys.exit()
        pygame.display.update()  
        frapsClock.tick(100)

the move() method is supposed to (over time) move the alien (red) to the player (blue). But when that code runs, this happens

It isnt supposed to be continuous like that. Which begs the question, how do I fix it?

Answer 1

A quick fix:

Precreate a color before your main loop:

black = pygame.Color('black')

and in your main loop:

while True:
    Board.fill(black)
    pygame.draw.rect(Board, (0, 0, 255), (player.x, player.y, 12, 12))

This will paint black over the entire screen, effectively erasing everything.
This is OKAY because you re-draw everything again immediately after erasing it.

In more advanced drawing code, instead of filling the screen (which can be slow), you would:

• draw a black rectangle over top of the old sprite
• then update the sprite’s position
• then re-draw the sprite.