PyGame: draw.rect() has invalid parameters

I'm trying to learn mouse events with PyGame, and I'm trying to draw a box wherever the user clicks. I'm setting a variable equal to pygame.mouse.get_pos(), and calling individual tuple members according to the mouse's x, y position. Here's the code:

import pygame, sys

pygame.init()
screen = pygame.display.set_mode((800, 600))

mousepos = pygame.mouse.get_pos()

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()
        if event.type == pygame.MOUSEBUTTONDOWN:
            pygame.draw.rect(mousepos[0], mousepos[1], 20, 20)

The game starts up, but when I click, it crashes, giving this error:

Traceback (most recent call last):
  File "C:\Users\User\Documents\proj\Python\mouse.py", line 13, in <module>
    pygame.draw.rect(mousepos[0], mousepos[1], 20, 20)
TypeError: must be pygame.Surface, not int

I know what I'm doing wrong: my parameters for draw.rect() are of invalid types, but I don't know how to change the parameters so they're appropriate. So how do I fix this?

Lets take a look at the function definition:

pygame.draw.rect(Surface, Color, Rect, Thickness)

Surface is a surface where you want to draw
Color is a tupple with RGB values defining the color to be used
Rect is a tupple in the format: (x,y,width,height)
- x,y are the coordinates of the upper left hand corner
- width, height are the width and height of the rectangle
Thickness is the thickness of the line. If it is zero, the rectangle is filled.

Based on this, you shoud do something like:

redColor = (255,0,0)
pygame.draw.rect(screen, redColor, (mousepos[0], mousepos[1], 20, 20), 1)

Sources:

The official documentation for python.draw can be found here:
http://www.pygame.org/docs/ref/draw.html

Mind the usefull Search examples for <function> button under every function description, which can lead you to multiple real world examples of usage.

Useful tutorials can also be found on the official pages: http://www.pygame.org/wiki/tutorials

Other unofficial tutorials, like this one, can be found with a bit of Googling effort.