strange results

[Fig]

I am having a problem with a graphics program that I created.
Actually, I don't know if it is the program or a bug in Python. The
code for the program is as follows;

from gasp import *

begin_graphics(width=640, height=480, title='Houses at Night',
background=color.BLACK)

def draw_house(x, y):
    a = (x, y  + 100)
    b = (x +50, y +140)
    c = (x +100, y + 100)
    Box((x, y), 100, 100, filled=True, color=color.BLUE)        # the
house
    Box((x + 35, y), 30, 50, filled=True, color=color.GREEN)        #
the door
    Circle((x + 62, y + 16), 1, filled=True, color=color.GOLD)
# the door knob
    Box((x + 20, y + 60), 20, 20, filled=True,
color=color.YELLOW)        # the left window
    Line((x + 20, y + 71), (x + 40, y + 71),
color=color.ORANGE)        # horizontal line (left window)
    Line((x + 29, y + 60), (x + 29, y + 80),
color=color.ORANGE)        # vertical line (left window)
    Box((x + 60, y + 60), 20, 20, filled=True,
color=color.YELLOW)        # the right window
    Line((x + 60, y + 71), (x + 80, y + 71),
color=color.ORANGE)        # horizontal line (right window)
    Line((x + 69, y + 60), (x + 69, y + 80),
color=color.ORANGE)        # vertical line (right window)
    Polygon([a, b, c], filled=True, color=color.RED)        # the roof

draw_house(20, 20)
draw_house(270, 20)
draw_house(520, 20)
draw_house(145, 180)
draw_house(395, 180)
draw_house(270, 340)

update_when('key_pressed')
end_graphics()
ends here, but it


When I run the program, I do not get any error messages but there are
parts of the 'draw_house' function missing on the canvas. Each call to
'draw_house has different things missing, they are not all the exact
same. There are some parts that show in one call to 'draw_house' that
do not show in others and visa versa. Does anyone have any idea what
may be going on?