test.py

# mouse handling
import pygame,sys
pygame.init()
global size,color
fps = 60
fpsclock = pygame.time.Clock()
def size():
    global size
    size = int(input('Enter the size of pencil :'))

def color():
    global color
    print('we use RGB color...')
    R = int(input('Enter part R :'))
    G = int(input('Enter part G :'))
    B = int(input('Enter part B :'))
    color = (R,G,B)


# set up color
#          R   G   B
white = (255,255,255)
black = (  0,  0,  0)
blue  = (  0,  0,255)

def draw(): # draw line like the legend paint xp
    # set up window

    pygame.display.set_caption('Pencil_Paint_Xp')
    surf = pygame.display.set_mode((640,480))
    pos1 = (0,0)
    pos2 = (1,1)
    surf.fill(black)
    a = 0
    # setting the size of the pencil and color
    size()
    color()
    print('you can change size and color by pressing SPACE button ')
    while True:

        for event in pygame.event.get():
            if event.type == pygame.MOUSEBUTTONDOWN:
                pos1 = event.pos

                a = event.button
            if a == 1:
                pos2 = pygame.mouse.get_pos()


                pygame.draw.line(surf,color,pos1,pos2,size)
                pos1 = pos2
                if event.type == pygame.MOUSEBUTTONUP:
                    a = 0
            if event.type == pygame.KEYDOWN:
                if event.unicode == ' ':
                    size()
                    color()
            #draw line with pos1 and pos2

            if event.type == pygame.QUIT:
                pygame.quit()
                sys.exit()
        pygame.display.update()


    fpsclock.tick(fps)

draw()