How to make a character move while key is held dow

Question

I'm trying to create a "player" - black square - which moves when you hold down WASD. I tried looking around here, on google and on youtube on how to make this work, but every solution I've tried has the same problem: instead of moving it while I hold down the key, I have to tap the key constantly to make it move in small bits. I've no clue what I'm doing wrong. Here's the code (using python 3.3 - pygame 1.9):

import pygame
from pygame.locals import *
from pygame.time import *
import sys
pygame.init()

velX = 0
velY = 0


running = True
clock = pygame.time.Clock()

def draw():
    global velX
    global velY

    playerx = 20
    playery = 20

    screen = pygame.display.set_mode((700,300))
    pygame.display.set_caption('something')

    background = pygame.Surface(screen.get_size())
    background = background.convert()
    background.fill((255,255,255))
    screen.blit(background, (0,0))

    playerx = playerx + velX
    playery = playery + velY
    player_filename = 'player.png'
    player = pygame.image.load(player_filename)
    screen.blit(player, (playerx,playery))

    pygame.display.flip()

def main():
    global velX
    global velY
    global running

    while running:

        keys_down = pygame.key.get_pressed()
        pygame.key.set_repeat(1, 50)
        time = 50/1000

        for event in pygame.event.get():
            if event.type == QUIT:
                pygame.quit()
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_ESCAPE:
                    running=False

        if keys_down[K_d]:
            velX += 50*time
        if keys_down[K_w]:
            velY -= 50*time
        if keys_down[K_s]:
            velY += 50*time
        if keys_down[K_a]:
            velX -= 50*time

        clock.tick(50)
        draw()

if __name__ == '__main__':
    main()

I already tried the set repeat command, but it didn't seem to do much anything. I also tried directly copying from a few solutions I found here on stackoverflow, but none of them worked. I suppose there is something else wrong in the code.

Answer 1

There are two problems with your code. First to your question. The reason the player never moves more than one step is that you reset the player position in every call to draw() when you do

playerx = 20
playery = 20

Instead, you should put that code above the draw()function and add

global playerx
global playery

at the top of draw(). Now, the player position is not reset every frame.

The second problem is that you create a new screen in every call to draw() when you do

screen = pygame.display.set_mode((700,300))
pygame.display.set_caption('something')

instead you should move those to lines above draw() and just use the same screen for every draw.

Also, like elyase points out, what you probably want is to set the velocities to fixed values, not increase them. Like so

velX = 0
velY = 0
if keys_down[K_d]:
    velX = 10
if keys_down[K_w]:
    velY = -10
if keys_down[K_s]:
    velY = 10
if keys_down[K_a]:
    velX = -10

This way the player will move around with a constant speed in the direction you steer it.

Hope that clear some stuff up. :)

Answer 2

You are resetting the position inside draw(). Also you should put the code that changes the direction, inside the event.type == pygame.KEYDOWN condition. Something like this:

if event.type == KEYDOWN:
    if event.key == K_d:
        velX += 50*time
     elif event.key == K_w:
        velY -= 50*time
        ...          

 if event.type == KEYUP:
        # set velocity to 0