Condition variables in Python

Overview:

A Condition variable signals a predicate becoming true which wakes up the threads waiting for such states. For example, availability of an item in a list is a predicate which can be signaled to a thread waiting for that state.
A thread can be made to wait for a predicate to become True through the wait() method of the Condition object.
Signaling of a condition, a predicate becoming true is communicated through the notify() method of the Condition object.
Prior to waiting on a condition object, the associated lock must be acquired by the calling thread.
After notify() the lock must be released by the calling thread.

Example:

# Example Python program that uses a condition variable
# to signal from a producer thread to a consumer thread
import threading as th
import time

items = []
count = 0
shouldExit = False

# Function defintion for the producer thread
def produce(cv):
global count
global shouldExit

while(True):
cv.acquire()
count += 1
item = "Item %d"%count
items.append(item)
print("Produced:%s"%item)
if(count >= 10):
shouldExit = True
cv.notify()
cv.release()
break
cv.notify()
cv.release()
time.sleep(2)

# Function defintion for the consumer thread
def consume(cv):
#with cv:
global shouldExit
while(True):
print("Pre-wait")
cv.acquire()
cv.wait()
item = items.pop()
print("Consumed:%s"%item)

if(shouldExit == True):
print("Consumer: This wakeup is for pop + exit")
cv.release()
break

cv.release()

# Create a lock
lock = th.Lock()

# Create a condition variable
cond = th.Condition(lock)

# Create a producer thread
producer = th.Thread(target = produce, args = (cond,))

# Create a consumer thread
consumer = th.Thread(target = consume, args = (cond,))

# Start the consumer and producer threads
consumer.start()
producer.start()

# Wait for the child threads to complete
producer.join()
consumer.join()

print("Producer and consumer threads exited")
print("Main thread exiting")

Output:

Pre-wait
Produced:Item 1
Consumed:Item 1
Pre-wait
Produced:Item 2
Consumed:Item 2
Pre-wait
Produced:Item 3
Consumed:Item 3
Pre-wait
Produced:Item 4
Consumed:Item 4
Pre-wait
Produced:Item 5
Consumed:Item 5
Pre-wait
Produced:Item 6
Consumed:Item 6
Pre-wait
Produced:Item 7
Consumed:Item 7
Pre-wait
Produced:Item 8
Consumed:Item 8
Pre-wait
Produced:Item 9
Consumed:Item 9
Pre-wait
Produced:Item 10
Consumed:Item 10
Consumer: This wakeup is for pop + exit
Producer and consumer threads exited
Main thread exiting