pygame_template.py

import sys, pygame
from pygame import Rect
from pygame.locals import *


size = width, height = 640, 480

pos = (0, 0) #mouse position
inital_drag_pos = (0, 0)
dragging = False
(leftclick, middleclick, rightclick) = (False, False, False)

pygame.init()
screen = pygame.display.set_mode(size)

while True:
    screen.fill((0, 0, 0))

    for event in pygame.event.get():
        # Close the window
        if event.type == pygame.QUIT:
            sys.exit()
        
        # Keyboard input
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_q:
                sys.exit()

        # Mouse input
        elif event.type == MOUSEBUTTONDOWN:
            if not dragging:
                inital_drag_pos = pygame.mouse.get_pos()
                dragging = True

            (leftclick, middleclick, rightclick) = pygame.mouse.get_pressed()
        
        elif event.type == MOUSEBUTTONUP:
            if leftclick:
                pos = pygame.mouse.get_pos()
            elif rightclick:
                pos = pygame.mouse.get_pos()

            (leftclick, middleclick, rightclick) = pygame.mouse.get_pressed()
            dragging = False

    if dragging:
        # continuing to drag
        pos = pygame.mouse.get_pos()
        pygame.draw.line(screen, (0, 0, 255), inital_drag_pos, pos, 4)


    pygame.display.flip()