获取pygame窗口但没有输出

Question

我正在尝试用python编写RRT路径规划算法。尽管代码执行没有错误，但是结果是一个没有输出的纯pygame窗口。

import sys, random, math, pygame
from math import sqrt,cos,sin,atan2
from pygame.locals import *
pygame.init()

class RRT(object):

X_dimension = 0
Y_dimension = 0
Window_size = 0
EPS         = 0
Max_nodes   = 0 
nodes       = list()
K_ESCAPE    = True
KEYUP       = True
QUIT        = True

def __init__(self,x,y):
    self.x = [10,20,40,50,60,0]
    self.y = [15,30,0,54,75,68]  

#parameters
    self.X_dimension = 1280                                  #length of the window
    self.Y_dimension = 1280                                  #breadth of the window
    self.Window_size = [self.X_dimension, self.Y_dimension]  #Window size
    self.EPS         =  7000                                 #EPSILON or Incremental Distance 
    self.Max_nodes   = 100                                   #maximum number of nodes
    self.QUIT        = QUIT
    self.KEYUP       = KEYUP
    self.K_ESCAPE    = K_ESCAPE


#function for calculating euclidean distance
def Calculate_Distance(self):
    x= self.x
    y=self.y

    return sqrt((x[5]-y[5])*(x[5]-y[5])+(x[4]-y[4])*(x[4]-y[4]))

#Function for calculating all the possible points
def Initiate_Sampling(self):

    self.EPS = 7000 
    y = self.y
    x = self.x

    if self.Calculate_Distance() < 70:
        return y, x
    else:
        theta = atan2(y[1]-x[1], y[0]-x[0])
        return x[0] + self.EPS*cos(theta), x[1] + self.EPS*sin(theta)

#Function for displaying the output
def Start_The_Game(self):

    pygame.init()

    screen = pygame.display.set_mode(self.Window_size)
    caption = pygame.display.set_caption("performing RRT")
    white = 255, 240, 200
    black = 20, 20, 40
    screen.fill(black) 

    return('GAME BEGINS')   


#Main Function
def Node_Generation(self, nodes=True):
    self.nodes      = nodes
    self.QUIT       = QUIT
    self.KEYUP      = KEYUP
    self.K_ESCAPE   = K_ESCAPE

    nodes = [(5.0,5.25),(7.0,7.25),(8,8.25)]

    #nodes.append(X_dimension/2.0, Y_dimension/2.0)        
    nodes.append((0.0,0.0))

for i in range(Max_nodes):
    rand = random.random()*640.0, random.random()*480.0
    nn = self.nodes[0]

for p in nodes:
    if self.Calculate_Distance(p,rand) < self.Calculate_Distance(nn,rand):
        nn = p
        newnode = step_from_to(nn,rand)
        nodes.append(newnode)
        pygame.draw.line(screen,white,nn,newnode)
        pygame.display.update()
        #print (i, "  ", nodes)

for event in pygame.event.get():
    if event.type == QUIT or (event.type == KEYUP and event.key == K_ESCAPE):
        pygame.quit()
        sys.exit("GAME OVER")



path = RRT(0,0)#write the starting nodes
path.Calculate_Distance()
path.Initiate_Sampling()
path.Start_The_Game()
path.Node_Generation()    

上面的结果显示了一个简单的pygame窗口，但没有输出。