测试pygame精灵碰撞

我一直在浏览一些pygame教程，并根据这些教程开发了自己的代码 . 它如下：

#!/usr/bin/python

import pygame, sys
from pygame.locals import *

size = width, height = 320, 320
clock = pygame.time.Clock()

xDirection = 0
yDirection = 0
xPosition = 32
yPosition = 256

blockAmount = width/32

pygame.init()
screen = pygame.display.set_mode(size)
screen.fill([0, 155, 255])
pygame.display.set_caption("Mario Test")
background = pygame.Surface(screen.get_size())

mainCharacter = pygame.sprite.Sprite()
mainCharacter.image = pygame.image.load("data/character.png").convert()
mainCharacter.rect = mainCharacter.image.get_rect()
mainCharacter.rect.topleft = [xPosition, yPosition]
screen.blit(mainCharacter.image, mainCharacter.rect)

grass = pygame.sprite.Sprite()
grass.image = pygame.image.load("data/grass.png").convert()
grass.rect = grass.image.get_rect()
for i in range(blockAmount):
    blockX = i * 32
    blockY = 288
    grass.rect.topleft = [blockX, blockY]
    screen.blit(grass.image, grass.rect)

grass.rect.topleft = [64, 256]  
screen.blit(grass.image, grass.rect.topleft )

running = False
jumping = False 
falling = False
standing = True

jumpvel = 22
gravity = -1

while True:

    for event in pygame.event.get():
        if event.type == KEYDOWN and event.key == K_ESCAPE:
            pygame.quit()
            sys.exit()
        elif event.type == QUIT:
            pygame.quit()
            sys.exit()
        if event.type == KEYDOWN:
            if event.key == K_LEFT:
                running = True
                xRun = -5
            elif event.key == K_RIGHT:
                running = True
                xRun = 5
            elif event.key == K_UP or event.key == K_SPACE:
                jumping = True
        elif event.type == KEYUP:
            if event.key == K_LEFT or event.key == K_RIGHT:
                running = False


    if running == True:
        xPosition += xRun

    if mainCharacter.rect.right >= width:
        xPosition = xPosition - 10
        print "hit"
        running = False
    elif mainCharacter.rect.left <= 0:
        xPosition = xPosition + 10
        print "hit"
        running = False

    screen.fill([0, 155, 255])

    for i in range(blockAmount):
        blockX = i * 32
        blockY = 288
        grass.rect.topleft = [blockX, blockY]
        screen.blit(grass.image, grass.rect)

    grass.rect.topleft = [64, 64]   
    screen.blit(grass.image, grass.rect.topleft )


    if jumping:
        yPosition -= jumpvel
        print jumpvel
        jumpvel += gravity
        if jumpvel < -22:
            jumping = False
        if mainCharacter.rect.bottom == grass.rect.top:
            jumping = False

    if not jumping:
        jumpvel = 22


    mainCharacter.rect.topleft = [xPosition, yPosition]
    screen.blit(mainCharacter.image,mainCharacter.rect)

    clock.tick(60)
    pygame.display.update()

所以我已经想出了如何让我的家伙跳，但是我已经在天空中放置了一块并试图这样做：

if mainCharacter.rect.bottom == grass.rect.top:
    jumping = False

试图使角色停止跳跃 . 是否有内置函数来测试这个或者可能是一种在我的情况下有效的方法 . 顺便说一句，这不起作用，我似乎无法弄清楚如何做到这一点 . 任何帮助是极大的赞赏 ：）