Animating Pygame Rects to move up the Screen - animation

Question

the endeavor is to draw clusters of rects [bubbles] moving up the screen at varying rates of speed. i've hit a roadblock when it comes to rendering the moving rects.
the way i've gone about it thus far is to populate a list with class instances of my Bubble class. from there i've iterated through the list of instances and called their blow_bubble method, effectively drawing each bubble at a different location on the screen and initializing each with its own value for its speed of movement. these bubbles are then appended to a separate list entitled "drawn", to signify that they have indeed been drawn (though not rendered).
the next part is where it gets bumpy.
i have a while loop that accepts the length of the drawn list being greater than zero as its condition for running. as per the form in this article: http://programarcadegames.com/index.php?chapter=introduction_to_animation
the screen surface is set to fill at the beginning of the loop and at the end of it i've updated the screen surface via pygame.display.flip(). in the middle of the while loop i'm iterating through class instances in the drawn list and decrementing the attribute that represents their y value by the instance's attributed rate of movement.
somehow this doesn't work.
i've checked to make sure the y value is actually decrementing; in a print statement the output is what would be expected: the y value descends into the negative numbers. yet the rects remain drawn statically.
as always, any insights are greatly appreciated.
#colors, screen, clock, etc defined above but omitted
pygame.init()
class Bubble():
def __init__(self, screenheight, screenwidth):
self.w = self.h = 0
self.x, self.y = 0, 0
self.moverate = 0
def reset_bubble(self):
self.w = self.h = random.randrange(2, int(screenwidth*1/4))
self.x = random.randrange(0, (screenwidth-self.w))
self.y = screenheight-self.w
self.moverate = 1
def blow_bubble(self):
self.reset_bubble()
pygame.draw.rect(screen, WHITE, (self.x, self.y, self.w, self.h), 10)
bubbles = []
drawn = []
i = 0
for i in range(10): #creates list of bubble objects
bubble = Bubble(screenheight, screenwidth)
bubbles.append(bubble)
for i in range(len(bubbles)): #draws bubbles without rendering them
bubbles[i].blow_bubble()
drawn.append(bubbles[i]) #appends objects to a new list (drawn)
while len(drawn) > 0:
screen.fill((BLACK))
drawn[i].y -= drawn[i].moverate #moves the bubble up the screen
pygame.display.flip() #updates the screen
if i >= 0: #counts up til len(drawn) then counts down [then up again]
i+=1 #to make sure we move every bubble a little each iteration
if i ==len(drawn):
i-= 1
clock.tick(FPS)
pygame.quit()

Answer 1

Unless I'm very much mistaken about how pygame works, you've misunderstood how it works. For oygamr to work it needs a 'render loop', a loop of code in which you repeatedly move the object, draw it with something like pygame.draw.rect, and then flip the display with pygame.display.flip. You keep doing this until you're done animating.
So, you need to make a bunch of changes.
your 'blow bubble' function needs to not reset the position of the bubble, just increment its position and call pygame.draw.rect
you don't need a 'dawn' list, just the first bubble list. Inside your while loop, you need to iterate through the bubble list calling 'blow bubble' on each.
you need to move the 'pygame.display.flip' inside the while loop, but after all the bubbles have been blown.
Your index out of range error is because I is not magically reset after the for loop exits, it leaves I as whatever it was when the for loop finished.