How to set boundaries in pygame?

How can I set boundaries for pygame that will not let a rectangle ( you can say square ) get out of the screen?
I visited other sites but nothing was useful for me.
Box will move left ( a ) , right ( d ) , up ( w ) , down ( s ).
My code:

import sys
import pygame as pg


def main():
    screen = pg.display.set_mode((640, 480))
    clock = pg.time.Clock()
    rect = pg.Rect(300, 220, 20, 20)
    velocity = (0, 0)
    done = False
    #screen
    while not done:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                done = True

        keys = pg.key.get_pressed()
        #move slow
        if keys[pg.K_a]:
            rect.x -= 4
        if keys[pg.K_d]:
            rect.x += 4
        if keys[pg.K_s]:
            rect.y += 4
        if keys[pg.K_w]:
            rect.y -= 4
        #move fast

        if keys[pg.K_g]:
            rect.x -= 8
        if keys[pg.K_j]:
            rect.x += 8
        if keys[pg.K_h]:
            rect.y += 8
        if keys[pg.K_y]:
            rect.y -= 8

        screen.fill((40, 40, 40))
        pg.draw.rect(screen, (100, 200, 20), rect)
        pg.display.flip()
        clock.tick(30)


if __name__ == '__main__':
    pg.init()
    main()
    pg.quit()
    sys.exit()

On the following code, the boundaries are the screen width and height, so you better define them from the beginning.

A way to handle this is to prevent the x and y coordinates from going below 0 and above the width for x and the height for y, without forgetting to subtract the size of the box.

I suggest a little gameplay improvement to use the booster, rather than pressing other keys, its better to use a modifier like shift or control. The below code use left shift to speed up the movement.

import sys
import pygame as pg

def main():
    width, height = 640, 480
    hbox, vbox = 20, 20
    screen = pg.display.set_mode((width, height))
    clock = pg.time.Clock()
    rect = pg.Rect(300, 220, hbox, vbox)
    velocity = (0, 0)
    done = False

    while not done:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                done = True

        keys = pg.key.get_pressed()

        # booster
        move = 8 if keys[pg.K_LSHIFT] else 4

        if keys[pg.K_a]:  #to move left
            rect.x -= move
        if rect.x < 0 : rect.x = 0

        if keys[pg.K_d]: #to move right
            rect.x += move
        if rect.x > width-hbox : rect.x = width - hbox

        if keys[pg.K_w]:  #to move up
            rect.y -= move
        if rect.y < 0: rect.y = 0

        if keys[pg.K_s]: #to move down
            rect.y += move
        if rect.y > height - hbox: rect.y = height - vbox

        screen.fill((40, 40, 40))
        pg.draw.rect(screen, (150, 200, 20), rect)

        pg.display.flip()
        clock.tick(30)

if __name__ == '__main__':
    pg.init()
    main()
    pg.quit()
    sys.exit()