Pygame how to change background colour without erasing anything else

I’m making this drawing thing in Python with Pygame. I have functions that draw circles and rectangles, but I want to add a function that lets me change the background colour, while not erasing the other circles and rectangles. Here is the code for the rectangles:

def drawRect(surface, colour, x, y, w, h):
     pygame.draw.rect(surface, colour, (x, y, w, h))

if event.type == pygame.KEYDOWN:
     if event.key == pygame.K_s:
        #print("pressed")
        drawRect(screen, RANDOM_COLOUR, RANDOM_X, RANDOM_Y, RANDOM_SIZE_X, RANDOM_SIZE_Y)
        pygame.display.flip()

A function for it would be best, but any solution would work.

Answer

You can’t. You have to redraw the entire scene in each frame.

Create classes for the shapes. For instance RectShape

class RectShape:
    def __init__(self, colour, x, y, w, h):
        self.colour = colour
        self.rect = pygame.Rect(x, y, w, h)
    def draw(self, surface):
        pygame.draw.rect(surface, self.colour, self.rect)

Add the shapes to a list. Clear the background with the color you want, draw all the shapes in the list, and refresh the display in every frame:

object_list = []

background_color = pygame.Color('gray')

run = True
while run:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_s:
                rect = RectShape(RANDOM_COLOUR, RANDOM_X, RANDOM_Y, RANDOM_SIZE_X, RANDOM_SIZE_Y)
                object_list.append(rect)

    screen.fill(background_color)

    for obj in object_list:
        obj.draw(screen)

    pygame.display.flip()