List Methods in Python: append, pop, and managing a wave
A game's wave is never static — enemies spawn and get destroyed. List methods (append, pop, remove) are how you keep the collection in sync with what's on screen.
The big idea
List methods mutate a list in place: append adds to the end, pop(i) removes and returns the item at index i, remove(x) deletes the first matching value.
See it in code
append mutates a list in place, adding to the end. Start with a five-enemy wave (bare x-positions) and spawn a reinforcement on the right:
wave = [55, 129, 203, 277, 351]
wave.append(425)
print("Wave size:", len(wave))
print(wave)Wave size: 6 [55, 129, 203, 277, 351, 425]
No reassignment — append changed wave itself. len confirms six now.
pop(i) removes the item at index i and returns it — that return value is the difference from remove. Here a laser takes out the enemy at index 2:
wave = [55, 129, 203, 277, 351, 425]
destroyed = wave.pop(2)
print("Removed:", destroyed)
print("Wave size now:", len(wave))Removed: 203 Wave size now: 5
pop(2) shrank the list to five and handed back 203. Now the same two moves, on real enemies you can animate.
Same moves, now on Enemy objects. append spawns a reinforcement; pop(2) removes the enemy at index 2 and hands that object back, so you could animate its explosion. Then we draw whatever remains:
from game import Stage, Enemy
screen = Stage.create()
Stage.clear(screen)
wave = [Enemy(55 + i * 74, 90) for i in range(5)]
wave.append(Enemy(425, 90))
destroyed = wave.pop(2)
print("Wave size now:", len(wave))
for enemy in wave:
enemy.draw(screen)Wave size now: 5
append grew the wave to six; pop(2) shrank it to five and returned the removed enemy into destroyed — the same object this time, not just a number. That is the difference from remove: pop gives the item back, so you can keep using it.
In-place list edits run every game loop and data pipeline: append a new price tick, pop a task off a queue, insert a step into a plan, sort a leaderboard. These methods change the list itself — which is powerful, and the source of one classic bug (below).
Try it yourself
Swap pop(2) for remove(wave[2]) and note it deletes by value, not index. Then try wave.sort(key=lambda e: e.x) to order the survivors left-to-right.
The common mistake
Removing items from a list while looping over it. Deleting elements mid-iteration makes the loop skip neighbors, because the indexes shift under it. Build a new list of survivors, or loop over a copy (for e in wave[:]), instead.
What it unlocks
Mutating methods extend lists and list indexing, and set up the update-each-frame pattern behind enemy waves.