Why does my code only sometimes notice collision?

Question

This game is a space shooter and I'm having a weird problem where my lasers only sometimes hit the enemy, and other times pass right through.

for lasers in amtLasers:
    if lasers.y < invaders.y:
        if lasers.x > invaders.x-56 and lasers.x < invaders.x+28:
            amtLasers.pop(amtLasers.index(lasers))
            amtInvaders.pop(amtInvaders.index(invaders))
            score += 20

I have classes for the lasers and invaders and amtLasers is a list that stores the onscreen lasers until they get deleted. I don't have it set to destroy the enemy on contact, so I was able to notice that if I continue shooting an enemy, the same enemy will get hit sometimes and not get hit other times. Why does this happen and how can I fix it?

Here's an edit: The lasers only hit the most recently spawned enemy, and I think I know why

class invader(object):
    def __init__(self):
        self.x = randint(30, 1020)
        self.y = 0
        color = randint(1, 3)
        if color == 1:
            self.col = (225,0,0)
        elif color == 2:
            self.col = (0,225,0)
        elif color == 3:
            self.col = (0,115,255)
        self.vel = randint(1, 2)

When a new enemy is spawned, invaders.x and invaders.y track that specific enemy and none of the other ones because this class has multiple enemies onscreen at once. How would I track each enemy separately?

Not really enough information to respond. try adding more info. you might print your lasers.x and linvaders.x in your loop to see if the values make sense. — LhasaDad, Mar 14 '21 at 01:18
Could it be that your lasers move fast enough to move all the way through an enemy in between updates? — Samwise, Mar 14 '21 at 01:20
ok I found out something important. It passes through any enemy that isn't the highest up onscreen. This means that when a new enemy spawns at the top, the lasers will pass through every enemy except that one. — Cheesy, Mar 14 '21 at 01:25

score 1 · Accepted Answer · answered Mar 14 '21 at 07:52

You will need to test to see if any of the lasers hit any of the enemies. Use 2 nested loops to iterate the enemies and the lasers:

for lasers in amtLasers[:]:
    for invaders in amtInvaders[:]:

        if lasers.y < invaders.y:
            if lasers.x > invaders.x-56 and lasers.x < invaders.x+28:
                amtLasers.pop(amtLasers.index(lasers))
                amtInvaders.pop(amtInvaders.index(invaders))
                score += 20

Note that you need to iterate over shallow copies of the list. See How to remove items from a list while iterating?.

I recommend to use pygame.Rect objects and colliderect for the collision test:

laser_rect = pygame.Rect(laser.x, laser.y, laser_width, laser_height)
invader_rect = pygame.Rect(invaders.x, invaders.y, invaders_width, invaders_height)

if laser_rect .colliderect(invader_rect):
    amtLasers.pop(amtLasers.index(lasers))
    amtInvaders.pop(amtInvaders.index(invaders))
    score += 20

See How do I detect collision in pygame? and How to detect collisions between two rectangular objects or images in pygame.

Why does my code only sometimes notice collision?

1 Answers1