Particles — Explosions & Trails

By the end of this lesson you can:

• Represent a particle as a dictionary with x, y, vx, vy, and life keys.
• Update every particle each frame: move by velocity, reduce life, and remove dead ones.
• Spawn an explosion burst of 20 particles at a point and watch them spread and fade.

Removing dead particles uses the same list filter you used for rocks in PZ-39. Predict the output:

sparks = [{"life": 3}, {"life": 0}, {"life": 1}, {"life": 0}]
sparks[:] = [s for s in sparks if s["life"] > 0]
print(len(sparks))

2

Think of a sparkler. The moment you light it, sparks shoot out in all directions. Each spark moves, slows, dims, and finally disappears. A particle system works the same way: we spawn, update, draw, and remove.

Particle data structure

# one particle = a dict
{
    "x":    100.0,   # current x position
    "y":    200.0,   # current y position
    "vx":   -2.5,    # velocity: pixels per frame, horizontal
    "vy":   -4.0,    # velocity: pixels per frame, vertical
    "life": 30,      # frames remaining before removal
    "colour": "orange",
}

Spawning an explosion

import random
import math

def explode(cx, cy):
    for _ in range(20):
        angle  = random.uniform(0, 2 * math.pi)
        speed  = random.uniform(1.5, 5.0)
        particles.append({
            "x":      float(cx),
            "y":      float(cy),
            "vx":     math.cos(angle) * speed,
            "vy":     math.sin(angle) * speed,
            "life":   random.randint(15, 40),
            "colour": random.choice(["orange", "yellow", "red"]),
        })

Update and remove

def update():
    for p in particles:
        p["x"]    += p["vx"]
        p["y"]    += p["vy"]
        p["vy"]   += 0.1      # gentle gravity pulls sparks down
        p["life"] -= 1
    particles[:] = [p for p in particles if p["life"] > 0]

Drawing — radius shrinks with life

def draw():
    for p in particles:
        radius = max(1, p["life"] // 8)
        screen.draw.filled_circle(
            (int(p["x"]), int(p["y"])), radius, p["colour"]
        )

Anjali wants sparks every time she clicks. Save as particles.py:

Part A — setup and explode

# particles.py — click for explosions
import pgzrun
import random
import math

WIDTH  = 600
HEIGHT = 400
TITLE  = "Particles"

particles = []

def explode(cx, cy):
    for _ in range(25):
        angle  = random.uniform(0, 2 * math.pi)
        speed  = random.uniform(2.0, 6.0)
        particles.append({
            "x":      float(cx),
            "y":      float(cy),
            "vx":     math.cos(angle) * speed,
            "vy":     math.sin(angle) * speed,
            "life":   random.randint(20, 50),
            "colour": random.choice(["orange", "yellow", "red", "white"]),
        })

Part B — draw, update, events

def draw():
    screen.fill("black")
    screen.draw.text(
        "Click anywhere to explode!", topleft=(10, 10),
        fontsize=20, color="gray",
    )
    for p in particles:
        radius = max(1, p["life"] // 10)
        screen.draw.filled_circle(
            (int(p["x"]), int(p["y"])), radius, p["colour"]
        )

def update():
    for p in particles:
        p["x"]    += p["vx"]
        p["y"]    += p["vy"]
        p["vy"]   += 0.15
        p["life"] -= 1
    particles[:] = [p for p in particles if p["life"] > 0]

def on_mouse_down(pos, button):
    explode(pos[0], pos[1])

pgzrun.go()

What you will see

A particle is a small dictionary with position, velocity, and a life counter. Each frame: move by velocity, decrease life by 1, draw a circle whose radius shrinks with life, and filter out dead particles. An explosion spawns many particles at once with random angles and speeds using math.cos and math.sin.