Python Note 11 - Condition of Multithreading (Condition Variable)

foreword

When friend a is adding fish balls to the hot pot, this is producer behavior; another friend b is eating fish balls, which is consumer behavior. When the fish balls in the hot pot reach a certain amount and fill up, they can only be eaten. This is a conditional judgment.
This is the Condition (condition variable) to be talked about in this article.

Condition

Condition (condition variable) is usually associated with a lock. When you need to share a lock among multiple Contidions, you can pass a Lock/RLock instance to the constructor, otherwise it will generate an RLock instance by itself.

It can be considered that, in addition to the lock pool with Lock, Condition also contains a waiting pool. The threads in the pool are in the waiting blocking state in the state diagram until another thread calls notify()/notifyAll() notification; after being notified, the thread Enter the lock pool and wait for a lock.

Condition():

• acquire(): thread lock
• release(): release the lock
• wait(timeout): The thread is suspended until it receives a notify notification or timeout (optional, floating point number, in seconds) will be woken up to continue running. wait() must be called only when Lock has been acquired, otherwise a RuntimeError will be triggered.
• notify(n=1): Notify other threads, those suspended threads will start running after receiving this notification. The default is to notify a thread that is waiting for the condition, and at most n waiting threads will be awakened. notify() must be called after Lock has been obtained, otherwise a RuntimeError will be triggered. notify() will not actively release the Lock.
• notifyAll(): If there are many threads in the wait state, the function of notifyAll is to notify all threads

Producer and Consumer

Realization scenario: when classmate a adds fish balls to the Wang hot pot (up to 5, notify b to eat them after filling), inform class b to eat the fish balls (when it reaches 0, inform class a to continue adding)

# coding=utf-8
import threading
import time

con = threading.Condition()

num = 0

#Producer class Producer 
(threading.Thread):

    def __init__(self):
        threading.Thread.__init__(self)

    def run(self):
         #Lock thread global num
        
        con.acquire ()
        while True:
             print  " Start adding!!! " 
            num += 1
             print  " Number of fish balls in the hot pot: %s " % str(num)
            time.sleep( 1 )
             if num >= 5 :
                 print  "The number of fish balls in the hot pot has reached 5 and cannot be added! " 
                #Wake up the waiting thread 
                con.notify() #Wake   up the friends to eat 
                #Wait for notification 
                con.wait()
         #release lock 
        con.release()

#Consumer class Consumers 
(threading.Thread):
     def __init__ (self): 
        threading.Thread.__init__(self)

    def run(self):
        con.acquire ()
        global num
         while True:
             print  " Start eating!!! " 
            num -= 1
             print  " Number of fish balls left in the hot pot: %s " % str(num)
            time.sleep( 2 )
             if num <= 0:
                 print  "The bottom of the pot is out of stock, hurry up and add fish balls! " 
                con.notify() #Wake   up other threads 
                #Waiting for notification 
                con.wait()
        con.release()

p = Producer()
c = Consumers()
p.start()
c.start()

 

Running result:
start adding! ! !
Number of fish balls in the hot pot: 1
Start adding! ! !
Number of fish balls in the hot pot: 2
Start adding! ! !
Number of fish balls in the hot pot: 3
Start adding! ! !
Number of fish balls in the hot pot: 4
and start adding! ! !
The number of fish balls in the hot pot: 5 The number of
fish balls in the hot pot has reached 5 and cannot be added!
Start eating! ! !
Number of fish balls left in the hot pot: 4
Let’s eat! ! !
Number of fish balls left in the hot pot: 3
Let’s start eating! ! !
Number of fish balls left in the hot pot: 2
Let’s start eating! ! !
The number of fish balls left in the hot pot: 1
Let’s start eating! ! !
The number of fish balls left in the hot pot: 0
The bottom of the pot is out of stock, hurry up and add fish balls!
Start adding! ! !
Number of fish balls in the hot pot: 1
Start adding! ! !
Number of fish balls in the hot pot: 2
Start adding! ! !
Number of fish balls in the hot pot: 3
Start adding! ! !
Number of fish balls in the hot pot: 4
and start adding! ! !
Number of fish balls in hot pot: 5