Python Advanced Learning Notes (1)

threading module

threading.thread(target = (function)) is responsible for defining the child thread object

threading.enumerate() is responsible for viewing child thread objects

The test code is as follows:

import time
import threading
def sing():
    for i in range(5):
        print('-----Sing-----')
        time.sleep(1)

def dance():
    for i in range(5):
        print('-----dance-----')
        time.sleep(1)

def main():
    t1 = threading.Thread(target=sing)  #Subthread 1
    t2 = threading.Thread(target=dance) #Subthread 2
    t1.start()
    t2.start()
    while True:
        print(threading.enumerate())
        if  len(threading.enumerate()) == 1:
            break
        time.sleep(1)
if __name__ == '__main__':
    main()

Supplement:

Thread scheduling is random, if the main thread GG before the sub-threads, your sub-threads also GG

threading.thread() does not create a thread, it is not created until its instance.start() is called

Create threads from classes

The test code is as follows:

import threading
import time
class Person(threading.Thread):
    def run(self) -> None:
        for i in range(3):
            print('aaa' + self.name)
            time.sleep(1)

if __name__ == '__main__':
    p = Person()
    p.start()
    for i in range(3):
        print('bbb')
        time.sleep(1)

Supplement:

Threading.threading is a class that declares a process by inheriting and override run functions

Multithreaded Sharing Global Variables

The test code is as follows:

import threading
import time
num = 100
def test1():
    global num
    num += 1
    print(num)
def test2():
    print(num)
def main():
    t1 = threading.Thread(target=test1)
    t2 = threading.Thread(target=test2)
    t1.start()
    time.sleep(1)
    test2()
if __name__ == '__main__':
    main()

If creating a thread requires parameters, use args

The test code is as follows:

num = [1,1,2,3]

def test1(args):
    args.append('hhs')
    print(args)

def main():
    t1 = threading.Thread(target=test1,args=(num,))
    t1.start()
    print(num)

if __name__ == '__main__': 
    main()

Plus: args is followed by a tuple, missing commas will cause errors

Problems with multithreaded sharing of global variables

The test code is as follows:

import threading
import time

g_num = 0
def count1(n):
    global g_num
    for i in range(n):
        g_num += 1

def count2(n):
    global g_num
    for i in range(n):
        g_num += 1

def main():
    t1 =threading.Thread(target=count1,args=(100000,))
    t2 = threading.Thread(target=count2, args=(100000,))
    t1.start()
    t2.start()
    time.sleep(2)
    print('normal num For 2000, Actual num by%d' % g_num)

if __name__ == '__main__':
    main()

Supplement: The output is 175586, which is different from the normal result

Resolve the problem:

1.cpu is executed sentence by sentence

2. If you parse g_num += 1 into many sentences, if thread 1 only executes 1-2 sentences and the third sentence has not been assigned back, then thread 2 is called, and the result of the addition is not equal to the expected result