Detect Interpreter shut down in daemon thread

Question

We were hit by this bug:

http://bugs.python.org/issue1856 Daemon threads segfault during interpreter shut down.

Now I search a way to code around this bug.

At the moment the code looks like this:

while True:
 do_something()
 time.sleep(interval)

Is there a way to check if the interpreter is still usable before do_something()?

Or is it better to not do mythread.setDaemon(True) and the check if the main thread has exited?

• guettli: related: stackoverflow.com/questions/16731115/…

Answer 1

Answer to own question:

I use this pattern now: don't setDaemon(True), don't use sleep(), use parent_thread.join()

while True:
 parent_thread.join(interval)
 if not parent_thread.is_alive():
 break
 do_something()

Related: http://docs.python.org/2/library/threading.html#threading.Thread.join

• freakish: I don't get it. If a child thread is not daemon, then a parent thread won't ever die (before child) and thus is_alive on parent will always be true. I.e. this entire check is pointless. It might work if a child thread is daemon though.
• guettli: @freakish if the parent (main) thread has reached its end, is_alive() will return False, but the child thread is still alive. It works in my tests.
• tktk: @guettli Hi, having similar issue here. Can you please help with explaining how you obtain parent_thread?
• guettli: @Korem I store parent_thread at module level. In my case there is only one parent thread for the whole process/interpreter.
• kollery: I see a similar problem, my curses screen interface is one thread, everthing else is main thread. quit is initiated by child thread ( on pressing 'q' ) - this gave me segfault on linux - going to try this solution - thanks for posting!

Answer 2

This is a code from threading.py module:

import sys as _sys
class Thread(_Verbose):
 def _bootstrap_inner(self):
 # some code
 # If sys.stderr is no more (most likely from interpreter
 # shutdown) use self._stderr. Otherwise still use sys (as in
 # _sys) in case sys.stderr was redefined since the creation of
 # self.
 if _sys:
 _sys.stderr.write("Exception in thread %s:\n%s\n" % 
 (self.name, _format_exc()))
 else:
 # some code

might be helpful. The error you see comes from else statement. So in your case:

import sys as _sys
while True:
 if not _sys:
 break/return/die/whatever
 do_something()
 time.sleep(interval)

I'm not sure if it works though (note that interpreter shutdown may happen inside do_something so you should probably wrap everything with try:except:).

Daemon threads are not necessarily bad, they can definitely speed up development process. You just have to be careful with them.