Dojo Defender Session 2: Coach notes

Goal of this session

After this session a ninja can:

• Create bullets with on_key_down and a cooldown mechanism
• Spawn enemy objects and make them move with a sine oscillation
• Collision detection between bullets and enemies (nested loop)
• Set up a wave system with increasing difficulty

Preparation (30 min before the session)

• Starter ZIPs ready on USB and on the site: dojo-defender-s3-starter.en.zip and dojo-defender-s3-solution.en.zip.
• Run the starter yourself once on the dojo room laptop.
• Ninjas who missed S1: point them to the S1 solution.

Schedule (3 hours)

This session is an integral part of the normal session 3 (Space Blaster). Use the time as follows:

Common questions

"‘on_key_down’ isn’t recognised" This is a Pgzero callback — the function has to be called exactly on_key_down with key as its parameter. No typos.

“My bullets disappear immediately” Bullets do y -= 8 but start at ship.y - 20. Check that bullet.y isn’t immediately below -10. If the ship is too high up, that can happen.

“RuntimeError: list changed during iteration” The ninja uses for enemy in enemies (without [:]) while calling enemies.remove(). Explain that [:] makes a copy.

“The enemies don’t move in a sine” The sine oscillation needs start_x, strafe_amplitude and strafe_speed as fields on the Actor. Newer ninjas can do a simple ping-pong movement first.

“The wave counter is wrong” enemies_destroyed % 10 == 0 triggers every 10 kills. If the ninja doesn’t declare enemies_destroyed as global in the function, the counter doesn’t work.

Hints for Stretch and Expert

Basic: shooting cooldown Explain why a cooldown is needed: without a cooldown the computer fires thousands of bullets per second while SPACE is held down.

Stretch: enemies spawn_interval = 120 is 2 seconds at 60 fps. As the score rises, the interval drops to 60 (1 second).

Expert: collision + waves The nested loop (bullets × enemies) is the hardest concept. Draw it on the board: for every bullet you check all the enemies. break is needed to stop after a hit.

Expert: sine movement math.sin(timer * speed + offset) * amplitude is a wave. timer makes the wave keep going. Without start_x the enemy drifts away from its starting position.

Cliffhanger script

“Today you shot down enemies and survived waves. Next time: POWER-UPS. Shields that protect you, speed boosts that make you faster, and spread shots that fire three bullets at once. Your ship gets stronger the longer you survive.”