Conditionals in Python: if / elif / else

if/else picks one of two blocks to run. Python checks the condition, then takes whichever branch matches:

score = 100
if score > 0:
    print("On the board!")
else:
    print("No points yet.")

On the board!

score > 0 was true, so the if branch ran and the else was ignored. Exactly one of the two always fires.

elif adds middle branches. Python tests each condition top to bottom and runs the first true one — the rest are skipped:

hit = True
at_bottom = False

if hit:
    print("Direct hit! +100")
elif at_bottom:
    print("Enemy got through")
else:
    print("No collision this frame")

Direct hit! +100

hit was true, so that branch fired and at_bottom was never even checked. elif and else only get a turn once every test above them fails.

Now wire that same ladder to real game objects. laser.hits(enemy) and enemy.at_bottom() each return True or False; with the laser sitting on the enemy, the collision test wins:

python

from game import Stage, Enemy, Laser, HUD

screen = Stage.create()
Stage.clear(screen)

hud = HUD()
enemy = Enemy(240, 300)
laser = Laser(240, 300)

if laser.hits(enemy):
    hud.add_score(100)
    print("Direct hit! +100")
elif enemy.at_bottom():
    hud.lose_life()
    print("Enemy got through")
else:
    print("No collision this frame")

enemy.draw(screen)
hud.draw(screen)

Direct hit! +100

Order matters: because the collision test was true, Python ran that branch and never even checked at_bottom(). elif and else only get a turn when every test above them came out false.