UnboundLocalError:在赋值python之前引用了局部变量”l”

UnboundLocalError: local variable 'L' referenced before assignment Python

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当试图编译下面的代码时，我得到了这个错误

1	UnboundLocalError: local variable 'L' referenced before assignment

有人能解释为什么吗？全局变量是否比其他变量先赋值？

我的python版本是2.7.3

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#!/usr/bin/env python

import pygame
from pygame.locals import *
from sys import exit
import random
import math

R = int(8) # promien planety
N = 5 # liczba planet
G = 2 # stala"grawitacyjna"
L = 1

def compute_dv(p1,p2):
dx = p2[0]-p1[0]
dy = p2[1]-p1[1]
r = math.hypot(dx,dy)
dx /= r*r
dy /= r*r
if(L>1000):
print"r=", r,"dx=", dx,"dy=", dy,"dx/ r*r =", dx,"dy/ r*r =", dy
L+=1
return G*dx,G*dy

def rand_color():
r = 32*random.randint(0,7)
g = 32*random.randint(0,7)
b = 22*random.randint(0,7)
return (r,g,b)

pygame.init()
screen = pygame.display.set_mode((640, 480), 0, 32)

points = []
vs = []
colors = []

for i in range(N):
points.append( [random.randint(0,639), random.randint(0,480)] )
vs.append( [0,0] )
colors.append( rand_color() )

clock = pygame.time.Clock()

screen.fill( (255,255,255))

while True:
clock.tick(30)

for event in pygame.event.get():
if event.type == QUIT:
exit()

for i in range(len(points)):
for j in range(len(points)):
if points[i]!=points[j]:
dvx,dvy = compute_dv( points[i],points[j])
vs[i][0] += dvx
vs[i][1] += dvy

for i in range(len(points)):
points[i][0] += vs[i][0]
points[i][1] += vs[i][1]

screen.fill( (255,255,255))

for i in range(len(points)):
L = []
for w in points[i]:
print int(round(w))
L.append(int(round(w)))
points[i] = L
print points[i],"stop"
#x = raw_input()

pygame.draw.circle(screen, colors[i], points[i], R)

pygame.display.update()