a little Python ditty to render animated New Year's wishes - using wasabi2d game framework [https://github.com/lordmauve/wasabi2d]

happy_new_year.py

import math
from wasabi2d import Scene, run, event
from random import uniform as fraction
from random import randint as integer
from random import choice

scene = Scene()
scene.background = 'black'

screen = scene.layers[0]
screen.set_effect('bloom', radius=50, intensity=0.9)

items = []
words = [ '2020', 'Happy', 'New Year' ]

W = scene.width
H = scene.height

@event
def on_mouse_move(pos, rel):
    dx, dy = rel
    r = math.sqrt( (dx**2)+(dy**2) )
    pos=(integer(0, W), integer(0, H))
    
    c = screen.add_circle( radius=r, pos=pos, color=random_color() )
    items.append(c)

    l = screen.add_label(choice(words), align='center', fontsize=integer(0, 144), color=random_color(), pos=(integer(0, W), integer(0, H)) )
    items.append(l)

@event
def update(dt):
    all_items = items[:]
    items[:] = [ i for i in items if i.scale > 0.1 ]
    for i in all_items:
        if i not in items:
            i.delete()
    for item in items:
        item.scale = item.scale * 0.9 # shrink
        item.y = item.y * 1.04 # fall
        item.x = item.x + integer(-3, 3) # jitter
    
def random_color():
    return ( fraction(0,1), fraction(0,1), fraction(0,1), fraction(0,1) )

run()

circles = letters = [] - this assigns the same list to the variables circles and letters - so you're actually only using one list. You're iterating over it twice.

Deleting from the items you're iterating over causes Python to skip visiting the next element (because a for loop almost literally calls items[n] with increasing n until it gets an IndexError). This operation is also O(n ** 2) because deleting a single item at an arbitrary index is O(n). To delete many items from a list, you are best to build a new list.

Thanks for guidance on the deletion from list - it felt a little wrong to be deleting from the list while iterating through it - now I know the side-effects I will avoid it in future.

... and I've hopefully improved code based on feedback

I think I've still got an O(N ** 2) condition in lines 35-37 ... unsure how to approach removing that condition. That said, because I'm filtering out items in line 34, N should never get large enough to be a problem.

Yep. Looks correct though - that's the main thing.

Another tip: math.sqrt( (dx**2)+(dy**2) ) can be replaced with math.hypot(dx, dy).

👍