Local variable referenced before assignment, using multi-threading

Question

Code to add and delete values in a list are operations performed in different threads.

using these global variables in multi-threading:

from threading import Thread
import time

a=[]
i = 0
j = 0

function for thread1:

def val_in():
    while 1:
        a.append(raw_input())
        print "%s value at %d: %d added" % ( time.ctime(time.time()), i ,int(a[i])) // line 14
        i+=1

function for thread 2:

def val_out():
    while 1:
        time.sleep(5)
        try:
            print "%s value at %d: %d deleted" % (time.ctime(time.time()), j, int(a.pop(j)))
            i-=1
        except:
            print"no values lefts"
        time.sleep(2)

defining and starting threads:

t = Thread(target = val_in)
t1 = Thread(target= val_out)
t.start()
t1.start()

Now when input is given as 1, it should be added to the list a, but there is an error:

Error: Exception in thread Thread-1:
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 810, in __bootstrap_inner
   self.run()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 763, in run
   self.__target(*self.__args, **self.__kwargs)
  File "/Users/dhiraj.agarwal/Documents/workspace/try3/multithread.py", line 14, in val_in
UnboundLocalError: local variable 'i' referenced before assignment

Answer 1

You should tell python that i is global:

def val_in():
    global i
    ...

def val_out():
    global i
    ...