How can I make a ball move in pygame when dragged by mouse?

Question

I am making a small game in python where balls are fired by a catapult, and then the balls destroy a structure of boxes and a snake. At the moment I have made the boxes and the snakes however, I am unable to make the balls come from the left direction to destroy the boxes. I want to make a catapult on the left side which can hold a ball and fire it. How can I go about making this? Here is my code

import pygame
from pygame.locals import *
from pygame.color import *
import pymunk as pm
from pymunk import Vec2d
import sys
## ~~~~~~~~~~~~~~~~~~~~~~~~~

class Ball(pygame.sprite.Sprite):
    def __init__(self,screen):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.image.load('ball1.jpg')
        self.original = pygame.image.load('ball1.jpg')
        self.rect = self.image.get_rect()
        self.area = screen.get_rect()

    def add_ball(self, space, offset):#offset
        self.mass = 3
        self.radius = 15
        self.inertia = pm.moment_for_circle(self.mass, 0, self.radius, (0,0))

        self.body = pm.Body(self.mass, self.inertia)
        self.body.position = offset+200, 550
        self.rect.center = to_pygame(self.body.position)

        self.shape = pm.Circle(self.body, self.radius, (0,0))
        space.add(self.body, self.shape)

    def draw(self,screen):
        pygame.draw.circle(screen, (255,255,255), self.rect.center, int(15), 3)

    def update(self):
        print("{0} {1}".format(self.rect.center, self.body.position))
        self.rect.center = to_pygame(self.body.position)


## ~~~~~~~~~~~~~~~~~~~~~~~~~

class Box(pygame.sprite.Sprite):
    def __init__(self,screen):
        pygame.sprite.Sprite.__init__(self)

        self.image = pygame.image.load('box1.jpg')
        self.original = pygame.image.load('box1.jpg')
        #self.image = pygame.image.load('snake.jpg')
        self.rect = self.image.get_rect()
        self.area = screen.get_rect()


    def add_box(self, space, posX, posY):
        global screen
        self.size= 30
        self.points = [(-self.size, -self.size), (-self.size, self.size), (self.size,self.size), (self.size, -self.size)]
        self.mass = 0.3
        self.moment = pm.moment_for_poly(self.mass, self.points, (0,0))

        self.body = pm.Body(self.mass, self.moment)
        self.body.position = Vec2d(posX, posY)
        #print("Box a {0}".format(self.body.position))
        self.rect.center = to_pygame(self.body.position)
        #print("Box b {0}".format(self.rect))
        self.shape = pm.Poly(self.body, self.points, (0,0))
        self.shape.friction = 1
        #self.shape.group = 1

        space.add(self.body, self.shape)


    def draw(self,screen):
        pygame.draw.rect(screen, (255,255,255), self.rect, 2)

    def update(self):
        self.rect.center = to_pygame(self.body.position) #pysics simulations is movin the body

## ~~~~~~~~~~~~~~~~~~~~~~~~~

class Snake(pygame.sprite.Sprite):
    def __init__(self,screen):
        pygame.sprite.Sprite.__init__(self)

        self.image = pygame.image.load('snake.jpg')
        self.original = pygame.image.load('snake.jpg')
        self.rect = self.image.get_rect()
        self.area = screen.get_rect()

    def add_snake(self, space, posX, posY):
        global screen
        self.size= 30
        self.points = [(-self.size, -self.size), (-self.size, self.size), (self.size,self.size), (self.size, -self.size)]
        self.mass = 0.1
        self.moment = pm.moment_for_poly(self.mass, self.points, (0,0))

        self.body = pm.Body(self.mass, self.moment)
        self.body.position = Vec2d(posX, posY)
        #print("Box a {0}".format(self.body.position))
        self.rect.center = to_pygame(self.body.position)
        #print("Box b {0}".format(self.rect))
        self.shape = pm.Poly(self.body, self.points, (0,0))
        self.shape.friction = 1
        #self.shape.group = 1

        space.add(self.body, self.shape)


    def draw(self,screen):
        pygame.draw.rect(screen, (5,5,255), self.rect, 2)

    def update(self):
        self.rect.center = to_pygame(self.body.position) #pysics simulations is movin the body


## ~~~~~~~~~~~~~~~~~~~~~~~~~

def to_pygame(p):
    """Small hack to convert pymunk to pygame coordinates"""
    return int(p[0]), int(-p[1]+600)


def main():
    pygame.init()
    screen = pygame.display.set_mode((600, 600))

    pygame.display.set_caption("Piling boxes")
    clock = pygame.time.Clock()

    space = pm.Space()
    space.gravity = (0.0, -900.0)
    space.damping = 0.5



    ### ground
    body = pm.Body()
    shape = pm.Segment(body, (0,100), (600,100), .0)
    shape.friction = 1.0
    space.add(shape)

    allsprites = pygame.sprite.Group()

    offsetY = 62
    offsetX = 92
    posX = 180
    posY = 130

    for j in range(3):
        for i in range(5):
            box = Box(screen) #Add more boxes
            box.add_box(space, posX, posY)
            allsprites.add(box)
            posY = posY + offsetY
        posY = 130
        posX = posX + offsetX



    snake = Snake(screen)
    snake.add_snake(space, posX, posY)
    allsprites.add(snake)

    selected = None

##
##    posY = 130
##    offset = 92
##    for i in range(5):
##        #offset = offset - 248
##        box = Box(screen) #Add more boxes
##        #box.add_box(space)
##        box.add_box(space, posX, posY)
##        allsprites.add(box)
##        posX = posX + offset
##
##        #offset = offset + 62

    posY =0
    offset = 0
    for i in range(5):
        ball = Ball(screen)
        ball.add_ball(space, offset)
        offset = offset + 16
        allsprites.add(ball)

    #background = pygame.Surface(screen.get_size())
    #background = background.convert()
    #background.fill((0,0,0))

    #screen.blit(background, (0,0))
    test_image = pygame.image.load("AngryCloneBg.jpg")
    #screen.blit(test_image, (0,0)) #coordinates (0,500))

    #pygame.display.flip()


    while 1:
        clock.tick(60)
        space.step(1/90.0)

        for event in pygame.event.get():
            if event.type == QUIT:
                pygame.quit()
                sys.exit(0)
            elif event.type == KEYDOWN:
                if event.key == K_ESCAPE:
                    pygame.quit()
                    sys.exit(0)

        allsprites.update()
        screen.blit(test_image, (0,0))
        #pygame.draw.line(screen, THECOLORS["red"], to_pygame((150,100)), to_pygame((450,100)), 3)
        allsprites.draw(screen)
        for item in allsprites:
            item.draw(screen)
        pygame.display.flip()

## ~~~~~~~~~~~~~~~~~~~~~~~~~

if __name__ == '__main__':
    main()

How can I make the balls to be used with a catapult to fire the balls?

Answer 1

After 2 minutes of looking over the pymunk docs: http://pymunk.googlecode.com/svn/tags/pymunk-2.0.0/docs/api/index.html

To create a cannon like game. Create a ball object, and call apply_force. It's as simple as that.

If you wish to create a catapult, a bit more physics are involved. You need to create line(Segment) with a fixed joint to the ground. Then connect the ball to the other end of the segment. Apply some force to the segment, and when the ball will be high enough, release the joint.

EDIT:

I also found a very helpful tutorial with joints: http://code.google.com/p/pymunk/wiki/SlideAndPinJointsExample