Python - Daemon Threads

Daemon threads in Python are useful for running background tasks that are not critical to the program's operation. They allow you to run tasks in the background without worrying about keeping track of them.

Python provides two types of threads: non-daemon and daemon threads. By default, threads are non-daemon threads. This tutorial provides a detailed explanation with relevant examples about daemon threads in Python programming.

Overview of Daemon Threads

Sometimes, it is necessary to execute a task in the background. A special type of thread is used for background tasks, called a daemon thread. In other words, daemon threads execute tasks in the background. These threads handle non-critical tasks that may be useful to the application but do not hamper it if they fail or are canceled mid-operation.

Also, a daemon thread will not have control over when it is terminated. The program will terminate once all non-daemon threads finish, even if there are daemon threads still running at that point of time.

Difference Between Daemon & Non-Daemon Threads

Daemon threads can perform tasks such as −

• Create a file that stores Log information in the background.

• Perform web scraping in the background.

• Save the data automatically into a database in the background.

Creating a Daemon Thread in Python

To create a daemon thread, you need to set thedaemon property to True of theThread constructor.

t1=threading.Thread(daemon=True)

By default thedaemon property is set to None, If you change it to not None, daemon explicitly sets whether the thread is daemonic.

Example

Take a look at the following example to create a daemon thread and check whether the thread daemonic or not using thedaemon attribute.

import threading from time import sleep# function to be executed in a new threaddef run():   # get the current thread   thread = threading.current_thread()   # is it a daemon thread?   print(f'Daemon thread: {thread.daemon}')# Create a new thread and set it as daemonthread = threading.Thread(target=run, daemon=True)# start the threadthread.start()print('Is Main Thread is Daemon thread:', threading.current_thread().daemon)# Block for a short time to allow the daemon thread to runsleep(0.5)

It will produce the followingoutput −

Daemon thread: TrueIs Main Thread is Daemon thread: False

If a thread object is created in the main thread without any parameters, then the created thread will be a non-daemon thread because the main thread is not a daemon thread. Therefore, all threads created in the main thread default to non-daemon. However, we can change thedaemon property toTrue by using theThread.daemon attribute beforestarting the thread, nothing but before calling thestart() method.

Example

Here is an example −

import threading from time import sleep# function to be executed in a new threaddef run():   # get the current thread   thread = threading.current_thread()   # is it a daemon thread?   print(f'Daemon thread: {thread.daemon}')# Create a new thread  thread = threading.Thread(target=run)# Using the daemon property set the thread as daemon before starting the threadthread.daemon = True# start the threadthread.start()print('Is Main Thread is Daemon thread:', threading.current_thread().daemon)# Block for a short time to allow the daemon thread to runsleep(0.5)

On executing the above program, we will get the following output −

Daemon thread: TrueIs Main Thread is Daemon thread: False

Managing the Daemon Thread Attribute

If you attempt to set the daemon status of a thread after starting it, then a RuntimeError will be raised.

Example

Here is another example that demonstrates the getting the RuntimeError when you try to set the daemon status of a thread after starting it.

from time import sleepfrom threading import current_threadfrom threading import Thread# function to be executed in a new threaddef run():   # get the current thread   thread = current_thread()   # is it a daemon thread?   print(f'Daemon thread: {thread.daemon}')   thread.daemon = True   # create a new threadthread = Thread(target=run)# start the new threadthread.start()# block for a 0.5 sec for daemon thread to runsleep(0.5)

It will produce the followingoutput −

Daemon thread: FalseException in thread Thread-1 (run):Traceback (most recent call last):   . . . .   . . . .    thread.daemon = True  File "/usr/lib/python3.10/threading.py", line 1203, in daemon    raise RuntimeError("cannot set daemon status of active thread")RuntimeError: cannot set daemon status of active thread