Session 7: Bug Zapper :: CoderDojo Python & Pygame

Session 7: Coach notes

Mon, 01 Jan 0001 00:00:00 +0000

Goal of this session At the end of the Basic checkpoint a ninja can: Write a class with __init__, attributes and an update() method. Instantiate bugs and keep them in a list. Apply collision detection between bullets and bugs. What is not expected after session 7: subclasses, math.sin movement, DuikBug. Those are Stretch/Expert material. Preparation (30 min before the session) Explain classes in 5 minutes on the projector. This is the most important new concept. Use a simple example: class Hond: with self.naam and a blaf() method. Ask ninjas for analogies. import math is already in the starter, so ninjas don’t have to look that up themselves for Step 3. Check the spawn timer in the starter (by default every 90 frames). Change it if the difficulty isn’t right for your group. Schedule (3 hours) Time Activity 0:00 to 0:10 Welcome, looking back at session 6 0:10 to 0:25 Projector: explaining classes and instances 0:25 to 1:30 Working on your own: steps 1 and 2 1:30 to 2:15 Stretch (SlingerBug) 2:15 to 2:45 Expert (DuikBug) + buffer 2:45 to 3:00 Showcase + cliffhanger Common questions “AttributeError: ‘Bug’ object has no attribute ‘alive’” self.alive = True is missing from __init__. Have the ninja check __init__: is the indentation right? Does alive have self. in front of it?

Session 7: Cheatsheet

Mon, 01 Jan 0001 00:00:00 +0000

Writing a class class Bug: def __init__(self, x, y): self.x = x self.y = y self.alive = True def update(self): self.y += 2 def draw(self, screen): pygame.draw.circle(screen, GREEN, (int(self.x), int(self.y)), 15) Using a class mijn_bug = Bug(200, 0) # create mijn_bug.update() # call a method mijn_bug.draw(screen) Subclass (inheritance) class SlingerBug(Bug): def __init__(self, x, y): super().__init__(x, y) # calls Bug.__init__ self.timer = 0 def update(self): # overrides Bug.update self.timer += 0.05 self.x = self.start_x + math.sin(self.timer) * 60 self.y += 1.5 math.sin for swinging import math self.x = start_x + math.sin(timer) * amplitude # increase the timer every frame: timer += 0.05 Keeping track of bullets bullets = [] # Create: bullets.append(pygame.Rect(x - 3, y - 20, 6, 12)) # Move: for b in bullets[:]: b.y -= 10 if b.y < 0: bullets.remove(b) Collision bullet ↔ bug bug_rect = pygame.Rect(bug.x - 15, bug.y - 15, 30, 30) for bullet in bullets[:]: if bullet.colliderect(bug_rect): bug.alive = False bullets.remove(bullet) score += 1 break Spawn timer with USEREVENT SPAWN = pygame.USEREVENT + 1 pygame.time.set_timer(SPAWN, 1500) # every 1.5 seconds for event in pygame.event.get(): if event.type == SPAWN: bugs.append(Bug(random.randint(30, WIDTH - 30), -20))

Session 7: Demo recording

Mon, 01 Jan 0001 00:00:00 +0000

Session 7 uses pure pygame. Show a GIF of the solution on the session page. How to record Run the solution: cd code/sessions/07-bug-zapper/solution && python main.py. Use a screen recorder: Linux: Peek or SimpleScreenRecorder. Windows: ScreenToGif. macOS: QuickTime → export to GIF via ffmpeg or Kap. Record ~15 seconds: show a normal Bug, a SlingerBug and a DuikBug being shot down. Save as bug-zapper.gif in static/sessions/07-bug-zapper/. Link to it in the worksheet via: ![Demo](/sessions/07-bug-zapper/bug-zapper.gif). Target size: ~800×500 pixels, ≤ 2 MB.