[issue13077] Unclear behavior of daemon threads on main thread exit

[Eric Snow]

Change by Eric Snow :


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nosy: +eric.snow

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[issue13077] Unclear behavior of daemon threads on main thread exit

[Vandana Rao]

Vandana Rao added the comment:

Since the program depends on receiving a raw ^C == 0x03 on stdin, it will never 
be running under Idle because the Idle process tk gui normally keeps control of 
keyboard input and the Idle process code intercepts ^C and turns it into 
KeyboardInterrupt raised in the user process.
This is not a bug anymore.It is working fine.

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[issue13077] Unclear behavior of daemon threads on main thread exit

[R. David Murray]

R. David Murray added the comment:

etuardu: I think you rewording is indeed clearer.

Vandana: Please open a new issue with a reproducer.

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stage: patch review - needs patch
versions: +Python 3.4, Python 3.5 -Python 3.2, Python 3.3

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[issue13077] Unclear behavior of daemon threads on main thread exit

[Vandana Rao]

Vandana Rao added the comment:

On Windows8.1,this is not the situation.Irrespective of the thread being daemon 
or non-daemon,the process continues. The program doesn't terminate when daemon 
thread is being used.

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[issue13077] Unclear behavior of daemon threads on main thread exit

[etuardu]

etuardu edo...@gmail.com added the comment:

Let me put it this way: the definition of daemon thread describes the behaviour 
of the Python program running it (its exit condition in particular) instead of 
going straight to the point describing the behaviour of the daemon thread 
itself first, and finally add other considerations.

Specifically, I think a situation like the following is not quite clear from 
the given definition:
- I have a main thread and a secondary thread, both printing to stdout.
- At some point, I press Ctrl+c raising an unhandled KeyboardInterrupt 
exception in the main thread, which kills it.

This is what I get using a daemon thread:

etuardu@subranu:~/Desktop$ python foo.py # other = daemon
other thread
main thread
other thread
main thread
^C
Traceback [...]
KeyboardInterrupt
etuardu@subranu:~/Desktop$ # process terminates

This is what I get using a non-daemon thread:

etuardu@subranu:~/Desktop$ python foo.py # other = non-daemon
other thread
main thread
other thread
main thread
^C
Traceback [...]
KeyboardInterrupt
other thread
other thread
other thread
... (process still running)

So, to explain the significance of the daemon flag, I'd say something like:

A daemon thread is shut down when the main thread and all others non-daemon 
threads end.
This means a Python program runs as long as non-daemon threads, such as the 
main thread, are running.
When only daemon threads are left, the Python program exits.

Of course this can be understood from the current definition («the entire 
Python program exits when only daemon threads are left»), still it looks a bit 
sybilline to me.

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Added file: http://bugs.python.org/file23283/foo.py

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[issue13077] Unclear behavior of daemon threads on main thread exit

[etuardu]

New submission from etuardu edo...@gmail.com:

The definition of daemon thread in the current documentation reads:

«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. [...]»
(http://docs.python.org/library/threading.html#thread-objects)

I think it's not very clear from this that daemon threads themselves terminate 
when the program (main thread plus other non-daemon threads) terminates. I 
think this have to be said more explicitly. I'd propose a change with something 
like:

«A thread can be flagged as a daemon thread, which means it will get shut 
down when the overall program terminates. The entire Python program exits when 
only daemon threads are left.»

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assignee: docs@python
components: Documentation
messages: 144691
nosy: docs@python, etuardu
priority: normal
severity: normal
status: open
title: Unclear behavior of daemon threads on main thread exit
type: behavior
versions: Python 2.6, Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4

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[issue13077] Unclear behavior of daemon threads on main thread exit

[R. David Murray]

R. David Murray rdmur...@bitdance.com added the comment:

It seems clear enough to me that when a process terminates (the entire Python 
program exits) then all of its threads must terminate.  That's part of the 
definition of threads, to my understanding.

I think the confusion arises from the use of the word deamon, which has been 
discussed as a bad thing elsewhere in this tracker.  A unix user would expect a 
daemon to keep running in the background, whereas here it is exactly the 
opposite.  See issue 5906 for example, where in the context of Multiprocessing 
it becomes even more confusing.  Perhaps a similar note that 'daemon' does not 
mean what it does in unix would be a good idea.

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nosy: +r.david.murray
stage:  - patch review
versions:  -Python 2.6, Python 3.1, Python 3.4

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