Pygame how to fix ‘trailing pixels’?

Normally you will do:

def draw():
    # fill screen with solid color.
    # draw, and flip/update screen.

But, you can update just dirty portions of the screen. See pygame.display.update():

pygame.display.update()
Update portions of the screen for software displays
update(rectangle=None) -> None
update(rectangle_list) -> None

This function is like an optimized version of pygame.display.flip() for software displays. It allows only a portion
of the screen to updated, instead of the entire area. If no argument
is passed it updates the entire Surface area like
pygame.display.flip().

You can pass the function a single rectangle, or a sequence of rectangles. It is more efficient to pass many rectangles at once than
to call update multiple times with single or a partial list of
rectangles. If passing a sequence of rectangles it is safe to include
None values in the list, which will be skipped.

It’s best to use it in combination with RenderUpdates, which is a Sprite.Group:

pygame.sprite.RenderUpdates
Group sub-class that tracks dirty updates.
RenderUpdates(*sprites) -> RenderUpdates
This class is derived from pygame.sprite.Group(). It has an extended draw() method that tracks the changed areas of the screen.