Python - Daemon Threads

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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.

It may be noted that daemon threads execute such non-critical tasks that although 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.

This is a major difference between daemon threads and non-daemon threads. The process will exit if only daemon threads are running, whereas it cannot exit if at least one non-daemon thread is running.

Daemon	Non-daemon
A process will exit if only daemon threads are running (or if no threads are running).	A process will not exit if at least one non-daemon thread is running.

Creating a Daemon Thread

To create a daemon thread, you need to set the daemon property to True.

t1=threading.Thread(daemon=True)

If a thread object is created without any parameter, its daemon property can also be set to True, before calling the start() method.

t1=threading.Thread()
t1.daemon=True

Example

Take a look at the following example −

from time import sleep
from threading import current_thread
from threading import Thread

# function to be executed in a new thread
def run():

   # get the current thread
   thread = current_thread()
   # is it a daemon thread?
   print(f'Daemon thread: {thread.daemon}')

# create a new thread
thread = Thread(target=run, daemon=True)

# start the new thread
thread.start()

# block for a 0.5 sec for daemon thread to run
sleep(0.5)

It will produce the following output −

Daemon thread: True

Daemon threads can perform executing tasks that support non-daemon threads in the program. For example −

• 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.

Example

If a running thread is configured to be daemon, then a RuntimeError is raised. Take a look at the following example −

from time import sleep
from threading import current_thread
from threading import Thread

# function to be executed in a new thread
def 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 thread
thread = Thread(target=run)

# start the new thread
thread.start()

# block for a 0.5 sec for daemon thread to run
sleep(0.5)

It will produce the following output −

Exception in thread Thread-1 (run):
Traceback (most recent call last):
. . . .
. . . .
   thread.daemon = True
   ^^^^^^^^^^^^^
 File "C:\Python311\Lib\threading.py", line 1219, in daemon
  raise RuntimeError("cannot set daemon status of active thread")
RuntimeError: cannot set daemon status of active thread