Thread Local Storage in Python

Overview:

When a variable is created in thread local storage every thread will have its own copy which it can update.
Thread local storage is not automatic variables.
They are valid for the lifetime of the thread.
They need not be locked for synchronization unless shared among threads.
In Python, the class threading.local incorporates the concept of Thread local storage.
A thread local variable can be added by instantiating a threading.local object and adding attributes to it. Each attribute added to the threading.local object is a thread local variable.

Example:

# Example Python program that uses thread local storage
# to calculate rolling sums in different threads
# and adding them to a global sum
import threading
import time

# Final sum
g_sum = 0

# Calculates rolling sum and update the global sum
def counterThread(numRange, tLock, tIndex):
   # Create thread local storage
   tls = threading.local()

   # Add attribute to the thread local storage
   # Per thread Sum
   tls.tSum = 0
   for i in range(0, numRange):
       # No locking needed
       tls.tSum += i

   # Posting final sum to the global
   tLock.acquire()
   global g_sum
   g_sum += tls.tSum
   tLock.release()

# Create locks
tLock = threading.Lock()

# Create threads that compute individual rolling sums
t1 = threading.Thread(target = counterThread(5, tLock, 1))
t2 = threading.Thread(target = counterThread(2, tLock, 2))
t3 = threading.Thread(target = counterThread(7, tLock, 3))

t1.start()
t2.start()
t3.start()

t1.join()
t2.join()
t3.join()
print("Final sum after adding up numbers from each thread:%d"%g_sum)

Output:

Final sum after adding up numbers from each thread:32