UnboundLocalError: local variable 'L' referenced before assignment Python [duplicate]

Question

when trying to compile the code below I get this error

UnboundLocalError: local variable 'L' referenced before assignment

Can someone explain why ? Isn't a global variable assigned before anything else?

My Python version is 2.7.3

#!/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()  

Answer 1

The minimal code to reproduce your bug is

x = 1
def foo():
    x += 1
foo()

This is happening for a number of reasons

1. First - because in python we have mutable and immutable classes. Ints are immutable, that is when you write x+=1 you actually create another object (which is not true for certain ints due to optimisations CPython does). What actually happens is x = x + 1.
2. Second - because python compiler checks every assignment made inside a scope and makes every variable assigned inside that scope local to it.
3. So as you see when you try to increment x compiler has to access a variable that's local to that scope, but was never assigned a value before.

If you're using python2 - you only have the option to declare variable global. But this way you would be unable to get a variable from an in-between function like

x = 0
def foo():
  x = 1
  def bar():
    global x
    print x  # prints 0
  bar()
foo()    

In python3 you have nonlocal keyword to address this situation.

Also I would advise you to avoid using globals. Also there is a collection.Counter class that might be useful to you.

Further reading: python docs