Pygame: velocity

[ThePythonNoob]

Hi, I have made a square that will move but there are some bugs in the movement. I can't find anything wrong with the logic which is why I need help ;) You have to press left and right to move the square.

The problems with the movement are:
1: when you bounce from the wall, if you keep holding the button that you used to go into it, you will just go the opposite way and not go back into the wall again.
2:when you quickly press left and then right, for a short amount of time it will be simulataneous and the square will just stop.

Here is the code:

import sys
import pygame
from pygame.locals import *
import random
pygame.init()
screen = pygame.display.set_mode((500,500))
pygame.display.set_caption("Square fun.")
pos_x = 300
pos_y = 250
vel_x = 0
vel_y = 0
color_change = False

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            sys.exit()
        if event.type == KEYDOWN:
            if event.key == K_RIGHT:
                if vel_x < 6:
                    vel_x += 1
            elif event.key == K_LEFT:
                if vel_x > -6:
                    vel_x -= 1
        else:
            if not vel_x == 0:
                if vel_x > 0:
                    vel_x -= 1
                if vel_x < 0:
                    vel_x += 1

    screen.fill((0,0,200))

    #move rectangle
    pos_x += vel_x

    #keep rectangle on screen and change colour
    if pos_x > 450 or pos_x < 0:
        vel_x = -vel_x
        color_change = True
        color = random.randint(0,255),random.randint(0,255),random.randint(0,255)
    if pos_y > 450 or pos_y < 0:
        vel_y = -vel_y
        color_change = True
        color = random.randint(0,255),random.randint(0,255),random.randint(0,255)

    #draw rectangle
    if not color_change:
        color = 255,255,0
    width = 0 #solid fill
    pos = pos_x, pos_y, 50, 50
    pygame.draw.rect(screen, color, pos, width)

    pygame.display.update()

I was also wondering why when you run the program in IDLE the sys.exit() will not work? this is the message that I get when I try to close the program.
Traceback (most recent call last):
File "C:\Users\Username\Desktop\draw rectangle.py", line 17, in <module>
sys.exit()
SystemExit
However, it will work if I open the file from the desktop or any other location.

Thanks for any help

[TrustyTony]

I do not understand your error explanations, your code seems working OK. Naturally you must lift the finger from key to push it again and the message you posted is normal SystemExit message (sys.exit() is same as raise SystemExit)

[ThePythonNoob]

ok, hold left, you will go into the wall and bounce off and go in the other direction. However you will keep on going in the other the direction if you keep on holding the key. Ideally you would bounce off the wall and then go back into it again when holding the same key. The second problem, hold the left key and then hold the right key, then let go of the left key, you would of thought the square would then go right but it stays still. Is this suffice? :)