x0, y0 = 250, 250
x1, y1 = 200, 200
x2, y2 = 350, 200
x3, y3 = 150, 300
x4, y4 = 300, 300

d = True

if y1 > 300:
    d = False
elif y1 < 200:
    d = True
if d == False:
    y1 -= 10
    y2 -= 10
    y3 += 10
    y4 += 10
elif d == True:
    y1 += 10
    y2 += 10
    y3 -= 10
    y4 -= 10

pygame.draw.line(screen, (1, 1, 1), (x1, y1), (x2, y2), 2)
pygame.draw.line(screen, (1, 1, 1), (x2, y2), (x4, y4), 2)
pygame.draw.line(screen, (1, 1, 1), (x3, y3), (x4, y4), 2)
pygame.draw.line(screen, (1, 1, 1), (x1, y1), (x3, y3), 2)