PyGame Local Variables

This is (I assume) a basic question, but I can't seem to figure it out.

Given the following code:

from src.Globals import *
import pygame
# Used to manage how fast the screen updates
clock = pygame.time.Clock()
# This is a list of 'sprites.'
block_list = pygame.sprite.Group()

def update_screen():
    # Loop until the user clicks the close button.
    done = False
    while not done:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                done = True

        # Clear the screen
        screen.fill(WHITE)

        for i in blocks:
            block_list.add(block)

        block_list.draw(screen)

        # Limit to 20 frames per second
        clock.tick(20)

        # Update the screen with what we've drawn.
        pygame.display.flip()

    pygame.quit()

Everything works fine. I can call the function update_screen in a thread and have it work correctly. However, if I move done = False above the function declaration, then I get the error: UnboundLocalError: local variable 'done' referenced before assignment.

My question is: why is it that I can safely have clock, and block_list outside of the function, but not done?