pygame constant movement with if statement - python-2.7

Question

i am trying to make a simple game but movement doesn't work properly
i want it to constantly move while the buttons are pressed but respect the borders of the screen currently the buttons will work overall but it causes the rectangle to stutter, here is my code,
import pygame
pygame.init()
SD = pygame.display.set_mode((640,480))
x = 16
y = 16
while True:
for event in pygame.event.get():
if event.type == pygame.QUIT:
pygame.quit()
quit()
keys = pygame.key.get_pressed()
if keys[pygame.K_w]:
if y > 0:
y -= 8
if keys[pygame.K_a]:
if x > 0:
x -= 8
if keys[pygame.K_s]:
if y < 448:
y += 8
if keys[pygame.K_d]:
if x < 608:
x += 8
SD.fill((255,255,255))
pygame.draw.rect(SD, (255,0,0), (x,y, 30, 30))
pygame.display.update()

Answer 1

pygame.event.get() will only return an event if a hardware event happened like pressing a key. So your if keys[...] won't be evaluated when nothing happens (a pressed key event won't get repeated)
Move your ifs one level up and it will work without stuttering but you'll have to slow down the movement of your box afterwards (a sleep(0.1) will do for the example but you probably want to move to something more advanced since you don't want to sleep in you drawing loop)

Answer 2

The way key presses are handled in pygame (and most other game engines) is that you only get an event when a key is pressed or released. The reason your character movement is looking so jumpy, is because the key press is being handled like holding a key down in a text editor. If you hold the key down, a letter will show up and after a short while you'll get lots of the letter repeating.
What you really want to do is to have a boolean for each key that you set to True when you get a key press event, and False when you get a key release event (take a very careful look at the process_events function).
I've modified your code to do just that (along with some other changes that I'll explain afterwards):
import pygame
class Game(object):
def __init__(self):
"""
Initialize our game.
"""
# The initial position.
self.x = 16
self.y = 16
# The keyboard state.
self.keys = {
pygame.K_w: False,
pygame.K_a: False,
pygame.K_s: False,
pygame.K_d: False,
}
# Create the screen.
self.SD = pygame.display.set_mode((640,480))
def move_character(self):
"""
Move the character according to the current keyboard state.
"""
# Process vertical movement.
if self.keys[pygame.K_w] and self.y > 0:
self.y -= 1
if self.keys[pygame.K_s] and self.y < 448:
self.y += 1
# Process horizontal movement.
if self.keys[pygame.K_a] and self.x > 0:
self.x -= 1
if self.keys[pygame.K_d] and self.x < 608:
self.x += 1
def process_events(self):
"""
Go through the pending events and process them.
"""
for event in pygame.event.get():
if event.type == pygame.QUIT:
pygame.quit()
quit()
# If the event is a key press or release event, then register it
# with our keyboard state.
elif event.type == pygame.KEYDOWN:
self.keys[event.key] = True
elif event.type == pygame.KEYUP:
self.keys[event.key] = False
def draw(self):
"""
Draw the game character.
"""
self.SD.fill((255,255,255))
pygame.draw.rect(self.SD, (255,0,0), (self.x, self.y, 30, 30))
pygame.display.update()
def run(self):
while True:
self.process_events()
self.move_character()
self.draw()
def main():
pygame.init()
game = Game()
game.run()
# This just means that the main function is called when we call this file
# with python.
if __name__ == '__main__':
main()
The biggest change I've made is to move your game into a class to give you better access to variables from your functions. It also lets you partition your code into different functions that make it easier to read.