This is the sixth day of my participation in the First Challenge 2022. For details: First Challenge 2022.

Python multithreading

Python implements multithreading primarily through the thread and threading modules. The Thread module of Python is a low-level module. The Threading module of Python encapsulates threads and makes them easier to use.

There are two ways to create a new thread:

  1. throughthreading.Thread()Passed to theThreadObject An executable method (or object)
  2. inheritancethreading.ThreadDefine subclasses and overriderun()methods

threading.Thread()

Main parameters:

  • target: Callable, executable method
  • name: STR, thread name
  • args: Iterable[Any], the argument list of the executable method
  • daemonBool Indicates whether this thread is a daemon thread

The sample code

import random
import threading
from time import sleep


def myPrint(odd: bool) :
    """odd is True and prints odd, otherwise prints even """
    for i in range(10) :if odd and i % 2= =1:
            print(i)
        elif not odd and i % 2= =0:
            print(i)
        sleep(random.random())

t1 = threading.Thread(target= myPrint, args=(True, ))
t2 = threading.Thread(target= myPrint, args=(False, ))
t1.start()
t2.start()
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Run the code to mix odd and even numbers, instead of printing all odd numbers first and then all even numbers.

inheritancethreading.Thread

Create multiple threads by subclassing threading.Thread directly from threading.Thread and overriding the init and run methods. Because of the inheritance from threading.Thread, when the start method is called, the code in its run method is automatically run.

Sample code:

class myCPrint(threading.Thread) :
    def __init__(self , odd) :
        super(myCPrint,self).__init__()
        self.odd = odd

    def run(self) :
        for i in range(10) :if self.odd and i % 2= =1:
                print(i)
            elif not self.odd and i % 2= =0:
                print(i)
            sleep(random.random())

c1 = myCPrint(True)
c2 = myCPrint(False)
c1.start()
c2.start()
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When you run the code, you can also see a mixture of odd and even numbers, instead of printing all odd numbers first and then all even numbers.

Problems with multithreading in Python

Multithreading in Python is false multithreading

Execution of Python code is controlled by the Python virtual machine (interpreter). Python was designed with only one thread executing at a time in the main loop. Access to the Python virtual machine is controlled by the global interpreter lock (GIL), which ensures that only one thread is running at a time. In a multithreaded environment, the Python virtual machine executes as follows:

  1. Set the GIL
  2. Switch to a thread and execute
  3. Thread execution completed or set to sleep (up to 100 bytes)
  4. Unlock the GIL
  5. Repeat 1-4

No matter how many cores there are, cores can only run one thread per unit of time, and then the time slice rotates.

If you want to take full advantage of multiple cores, you can use the multiprocessing library to create multiple processes.