pygame-core

pygame Core

Build the foundation of a pygame game in Python: the main loop, delta-time movement, drawing with Surface/Rect, input, and Sprite/Group management. Targets pygame-ce 2.5.8 (the actively maintained community fork; same import pygame).

When to use

• Use when starting a pygame game, fixing the loop, frame-rate-dependent speed, input handling, blitting, or sprite/group collision.
• Use when code does import pygame and the project depends on pygame-ce (or pygame).

When not to use: Python language questions unrelated to pygame. 3D rendering (pygame is 2D). For cross-engine save/load use save-systems; for rebindable input architecture see input-systems.

Core workflow

1. Install pygame-ce, not legacy pygame. pip install pygame-ce — it's the maintained fork and imports as pygame. Don't install both in one environment.
2. Init and open a window. pygame.init(), screen = pygame.display.set_mode((w, h)), clock = pygame.time.Clock().
3. Run one loop: events → update → draw → flip. Pump the event queue every frame (for event in pygame.event.get()), update state, redraw, then pygame.display.flip().
4. Make it frame-rate independent. Get dt = clock.tick(60) / 1000 (seconds) and scale all motion by dt. Keep positions as floats; blit at integer rects.
5. Handle input two ways: event-based (KEYDOWN/MOUSEBUTTONDOWN, for discrete actions) and polled (pygame.key.get_pressed(), for held movement).
6. Organise objects with Sprite + Group. Subclass pygame.sprite.Sprite with image/rect; group.update(dt) and group.draw(screen) handle the batch. Run it and watch the window before assuming it works.

Patterns

1. Minimal game loop (the skeleton)

import pygame

pygame.init()
screen = pygame.display.set_mode((800, 600))
pygame.display.set_caption("My Game")
clock = pygame.time.Clock()

running = True
while running:
    dt = clock.tick(60) / 1000          # cap at 60 FPS; dt = seconds since last frame
    for event in pygame.event.get():    # MUST drain the queue or the OS thinks it hung
        if event.type == pygame.QUIT:
            running = False

    # update game state here, scaled by dt ...

    screen.fill((18, 18, 28))           # clear each frame
    # draw everything here ...
    pygame.display.flip()               # present the frame

pygame.quit()

2. Delta-time movement (frame-rate independent)

from pygame.math import Vector2

pos = Vector2(100, 100)        # keep position as floats
speed = 220                    # PIXELS PER SECOND, not per frame

# inside the loop, after computing dt:
keys = pygame.key.get_pressed()
direction = Vector2(
    keys[pygame.K_RIGHT] - keys[pygame.K_LEFT],
    keys[pygame.K_DOWN]  - keys[pygame.K_UP],
)
if direction.length_squared() > 0:
    direction = direction.normalize()      # equal speed on diagonals
pos += direction * speed * dt              # RIGHT: dt-scaled
screen.blit(player_img, (round(pos.x), round(pos.y)))  # blit at integer pixels

3. Input: events vs polling

for event in pygame.event.get():
    if event.type == pygame.QUIT:
        running = False
    elif event.type == pygame.KEYDOWN:        # discrete press: jump, menu, pause
        if event.key == pygame.K_SPACE:
            jump()
        elif event.key == pygame.K_ESCAPE:
            running = False
    elif event.type == pygame.MOUSEBUTTONDOWN:
        shoot_at(event.pos)                   # event.pos = (x, y)

# Polled state (read once per frame) for continuous/held input:
keys = pygame.key.get_pressed()
if keys[pygame.K_a]:
    move_left(dt)

4. A Sprite subclass + a Group

class Player(pygame.sprite.Sprite):
    def __init__(self, x, y):
        super().__init__()
        # convert() once at load makes blits much faster; _alpha keeps transparency.
        self.image = pygame.image.load("player.png").convert_alpha()
        self.rect = self.image.get_rect(center=(x, y))
        self.pos = pygame.math.Vector2(self.rect.center)
        self.speed = 240

    def update(self, dt):                      # Group.update(dt) calls this per sprite
        keys = pygame.key.get_pressed()
        self.pos.x += (keys[pygame.K_RIGHT] - keys[pygame.K_LEFT]) * self.speed * dt
        self.rect.center = (round(self.pos.x), round(self.pos.y))

all_sprites = pygame.sprite.Group()
all_sprites.add(Player(400, 300))

# in the loop:
all_sprites.update(dt)        # calls each sprite's update(dt)
all_sprites.draw(screen)      # blits each sprite at its rect

5. Collision detection

# Sprite vs group: e.g. player picking up coins (True = remove collided coins).
collected = pygame.sprite.spritecollide(player, coins, dokill=True)
score += len(collected)

# Group vs group: bullets vs enemies (kill both on hit).
hits = pygame.sprite.groupcollide(bullets, enemies, True, True)

# Plain rect overlap (no sprites needed):
if player.rect.colliderect(door_rect):
    open_door()

Pitfalls

• Window freezes / "not responding" → you didn't pump the event queue. Call pygame.event.get() (or pygame.event.pump()) every frame.
• Speed differs on faster machines → you moved by a fixed amount per frame. Scale by dt = clock.tick(fps) / 1000 and use pixels-per-second values.
• Sub-pixel movement snaps/jitters → rect coordinates are integers; store the true position as a Vector2 of floats and assign rect.center = round(...) each frame.
• Blits are slow / framerate drops → call .convert() (opaque) or .convert_alpha() (transparent) on loaded images once; un-converted surfaces blit far slower.
• Nothing appears → you forgot pygame.display.flip() (or update()), or you drew before screen.fill(...) so it was cleared away.
• Wrong draw order → pygame uses painter's order; later blits cover earlier ones. Draw background first, sprites last.
• pip install pygame got the old one → for the maintained fork use pip install pygame-ce; having both installed causes import conflicts.
• Diagonal movement is faster → normalise the direction vector before scaling by speed.

References

• For Group variants (GroupSingle, LayeredUpdates for z-order), pixel-perfect collision with mask, slicing a spritesheet, simple animation, sound/music, and text rendering, read references/sprites-and-collision.md.

Related skills

• love2d-core — the same loop concepts in LÖVE/Lua.
• bevy-ecs — a heavier ECS engine when a project outgrows pygame.
• input-systems / save-systems — engine-agnostic input and persistence.
• platformer / roguelike — genre templates that pair with pygame.