Pygame - How to stop an image from leaving the edge of the screen?

Question

A section of jetfighterx leaves the screen when the mouse hovers over the edge of the window, this causes tarantula to explode from time to time as soon as it respawns to the top of the window, how can I stop this from happening (without the use of classes)?

Code:

import pygame, sys, pygame.mixer
from pygame.locals import *
import random 
pygame.init()

bif = "space.jpg"
jf = "spacefightersprite.png"
enemy = "TarantulaSpaceFighter.png"

laser = pygame.mixer.Sound("LaserBlast.wav")
explosionsound = pygame.mixer.Sound("Explosion.wav") 
screen = pygame.display.set_mode((1000,900),0,32)
caption = pygame.display.set_caption("Jet Fighter X") 
background = pygame.image.load(bif).convert()

jetfighterx = pygame.image.load(jf)
jetfighterx = pygame.transform.scale(jetfighterx, (400,400)) 
tarantula = pygame.image.load(enemy)
tarantula = pygame.transform.scale(tarantula, (100,100)) 
laserblast = pygame.image.load("C:\Python27\laser.png")
explosion=pygame.image.load("C:\Python27\explosion.png")
explosion=pygame.transform.scale(explosion, (150,150))

ex,ey = 450,0
movex,movey = 0,0
clock = pygame.time.Clock()
speed = 300
shoot_y = 0
laser_fired = False
collision = False
alive = True 
explo_timer = 25

while True:
    pygame.mouse.set_visible(False) 
    mx,my = pygame.mouse.get_pos() 
    jetfighterx_rect = jetfighterx.get_rect(center=(mx, my))
    jetfighterx_rect = jetfighterx_rect.inflate(-200,-200) 
    tarantula_rect = tarantula.get_rect(center=(ex, ey))
    tarantula_rect = tarantula_rect.inflate(-180,-200) 
    # Check for player inputs
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
        if event.type == KEYDOWN:
            if event.key == K_ESCAPE or event.key == K_q:
                sys.exit() 
        if event.type == MOUSEBUTTONDOWN:
            laser_fired = True
            laser.play()
            shoot_y = my-200
            shoot_x = mx-16

    # Update Game 
    milli = clock.tick()
    seconds = milli/1000. 
    dmy = seconds * speed
    ey += dmy

    if ey > 900:
        explo_timer = 25 
        collision = False
        alive = True 
        ey = 0
        ex = random.randint(50,900)
    if laser_fired:
        shoot_y -= 10
        if shoot_y < 0:
            laser_fired = False
        else:
            laserblast_rect = laserblast.get_rect(center=(shoot_x, shoot_y)) 
            if laserblast_rect.colliderect(tarantula_rect):
                explosionsound.play()
                collision = True
                alive = False 

    if jetfighterx_rect.colliderect(tarantula_rect) and alive:
        explosionsound.play()
        collision = True
        alive = False 

  # Draw on screen
    screen.blit(background, (0,0))
    screen.blit(jetfighterx,(mx-200,my-200))
    if not collision:
        screen.blit(tarantula, (ex, ey))
    elif collision:
        explo_timer-=2
        if explo_timer > 0 and alive == False:
            screen.blit(explosion, (ex, ey-50))
    if laser_fired:
        screen.blit(laserblast, (shoot_x, shoot_y))

    pygame.display.update()

Answer 1

Just add a limit that does not allow the fighter to move within x pixels of the border.

Assuming that the x,y coordinates of the centre of your fighter are jetfighter_x, jetfighter_y (You will need to change the variable names to whatever your code has) then write something like this:

LBuffer = 16
RBuffer = 1000 - 16
TBuffer = 900 - 16
BBuffer = 16

if jetfighter_x > RBuffer:
    jetfighter_x = RBuffer

if jetfighter_x < LBuffer:
    jetfighter_x = LBuffer

if jetfighter_y > TBuffer:
    jetfighter_y = TBuffer

if jetfighter_y < BBuffer:
    jetfighter_y = BBuffer

This should prevent the center of the ship from getting closer than 16 pixels from the edge. Obviously you will need to tweak this to accommodate the size of your ship. (The buffer for the sides would be the width of the image/2 .Respectively the buffer for the top and bottom would be the height of the image/2).