Daemon Threads Explanation

Question

In the Python documentation it says      A thread can be flagged as a  daemon thread   The significance of this   flag is that the entire Python program exits when only daemon threads   are left  The initial value is inherited from the creating thread    Does anyone have a clearer explanation of what that means or a practical example showing where you would set threads as daemonic   Clarify it for me  so the only situation you wouldn t set threads as daemonic  is when you want them to continue running after the main thread exits

User · Accepted Answer

Some threads do background tasks  like sending keepalive packets  or performing periodic garbage collection  or whatever  These are only useful when the main program is running  and it s okay to kill them off once the other  non-daemon  threads have exited   Without daemon threads  you d have to keep track of them  and tell them to exit  before your program can completely quit  By setting them as daemon threads  you can let them run and forget about them  and when your program quits  any daemon threads are killed automatically

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Other posters gave some examples for situations in which you d use daemon threads   My recommendation  however  is never to use them   It s not because they re not useful  but because there are some bad side effects you can experience if you use them   Daemon threads can still execute after the Python runtime starts tearing down things in the main thread  causing some pretty bizarre exceptions   More info here   https   joeshaw org python-daemon-threads-considered-harmful   https   mail python org pipermail python-list 2005-February 343697 html  Strictly speaking you never need them  it just makes implementation easier in some cases

User · Answer

Let s say you re making some kind of dashboard widget   As part of this  you want it to display the unread message count in your email box   So you make a little thread that will    Connect to the mail server and ask how many unread messages you have  Signal the GUI with the updated count  Sleep for a little while    When your widget starts up  it would create this thread  designate it a daemon  and start it   Because it s a daemon  you don t have to think about it  when your widget exits  the thread will stop automatically

User · Answer

A simpler way to think about it  perhaps  when main returns  your process will not exit if there are non-daemon threads still running   A bit of advice  Clean shutdown is easy to get wrong when threads and synchronization are involved - if you can avoid it  do so  Use daemon threads whenever possible

User · Answer

A simpler way to think about it  perhaps  when main returns  your process will not exit if there are non-daemon threads still running   A bit of advice  Clean shutdown is easy to get wrong when threads and synchronization are involved - if you can avoid it  do so  Use daemon threads whenever possible

User · Answer

When your second thread is non-Daemon  your application s primary main thread cannot quit because its exit criteria is being tied to the exit also of non-Daemon thread s   Threads cannot be forcibly killed in python  therefore your app will have to really wait for the non-Daemon thread s  to exit  If this behavior is not what you want  then set your second thread as daemon so that it won t hold back your application from exiting

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Chris already explained what daemon threads are  so let s talk about practical usage  Many thread pool implementations use daemon threads for task workers  Workers are threads which execute tasks from task queue    Worker needs to keep waiting for tasks in task queue indefinitely as they don t know when new task will appear  Thread which assigns tasks  say main thread  only knows when tasks are over  Main thread waits on task queue to get empty and then exits  If workers are user threads i e  non-daemon  program won t terminate  It will keep waiting for these indefinitely running workers  even though workers aren t doing anything useful  Mark workers daemon threads  and main thread will take care of killing them as soon as it s done handling tasks

User · Answer

Chris already explained what daemon threads are  so let s talk about practical usage  Many thread pool implementations use daemon threads for task workers  Workers are threads which execute tasks from task queue    Worker needs to keep waiting for tasks in task queue indefinitely as they don t know when new task will appear  Thread which assigns tasks  say main thread  only knows when tasks are over  Main thread waits on task queue to get empty and then exits  If workers are user threads i e  non-daemon  program won t terminate  It will keep waiting for these indefinitely running workers  even though workers aren t doing anything useful  Mark workers daemon threads  and main thread will take care of killing them as soon as it s done handling tasks

User · Answer

When your second thread is non-Daemon  your application s primary main thread cannot quit because its exit criteria is being tied to the exit also of non-Daemon thread s   Threads cannot be forcibly killed in python  therefore your app will have to really wait for the non-Daemon thread s  to exit  If this behavior is not what you want  then set your second thread as daemon so that it won t hold back your application from exiting

User · Answer

A simpler way to think about it  perhaps  when main returns  your process will not exit if there are non-daemon threads still running   A bit of advice  Clean shutdown is easy to get wrong when threads and synchronization are involved - if you can avoid it  do so  Use daemon threads whenever possible

User · Answer

Let s say you re making some kind of dashboard widget   As part of this  you want it to display the unread message count in your email box   So you make a little thread that will    Connect to the mail server and ask how many unread messages you have  Signal the GUI with the updated count  Sleep for a little while    When your widget starts up  it would create this thread  designate it a daemon  and start it   Because it s a daemon  you don t have to think about it  when your widget exits  the thread will stop automatically

User · Answer

Quoting Chris       when your program quits  any daemon threads are killed automatically    I think that sums it up  You should be careful when you use them as they abruptly terminate when main program executes to completion

User · Answer

Quoting Chris       when your program quits  any daemon threads are killed automatically    I think that sums it up  You should be careful when you use them as they abruptly terminate when main program executes to completion

User · Answer

Other posters gave some examples for situations in which you d use daemon threads   My recommendation  however  is never to use them   It s not because they re not useful  but because there are some bad side effects you can experience if you use them   Daemon threads can still execute after the Python runtime starts tearing down things in the main thread  causing some pretty bizarre exceptions   More info here   https   joeshaw org python-daemon-threads-considered-harmful   https   mail python org pipermail python-list 2005-February 343697 html  Strictly speaking you never need them  it just makes implementation easier in some cases

User · Answer

A simpler way to think about it  perhaps  when main returns  your process will not exit if there are non-daemon threads still running   A bit of advice  Clean shutdown is easy to get wrong when threads and synchronization are involved - if you can avoid it  do so  Use daemon threads whenever possible

User · Answer

Let s say you re making some kind of dashboard widget   As part of this  you want it to display the unread message count in your email box   So you make a little thread that will    Connect to the mail server and ask how many unread messages you have  Signal the GUI with the updated count  Sleep for a little while    When your widget starts up  it would create this thread  designate it a daemon  and start it   Because it s a daemon  you don t have to think about it  when your widget exits  the thread will stop automatically

User · Answer

Let s say you re making some kind of dashboard widget   As part of this  you want it to display the unread message count in your email box   So you make a little thread that will    Connect to the mail server and ask how many unread messages you have  Signal the GUI with the updated count  Sleep for a little while    When your widget starts up  it would create this thread  designate it a daemon  and start it   Because it s a daemon  you don t have to think about it  when your widget exits  the thread will stop automatically

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