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In Python 3.0, the interpreter will not shutdown properly after setting
a tracing function and something goes into stdout.
The code attached shows the problem in action: just execute it and
notice how the interpreter will be kept running after the code has been
executed.
There are some more things that need to be considered:
- If the print('here') is not called, it will shutdown
- If BOTH the print('here') and the sys.settrace(None) is not called, it
will NOT shutdown
Note: I've marked the bug as crash because it's halting, but maybe there
could be a better alternative for it...

I haven't investigated, but in py3k print() has currently bits written
in Python, which means it will get (recursively) traced when called from
the trace function. It can of course have all kinds of funny implications!

Yes, this is exactly the problem. The execution never goes beyond print
('here'); if you print frame.f_lineno you'll see it blocks at io.py 
line 1036, waiting for a Lock for the second time.
So the trace function cannot use print, not write to regular files 
(because io.py is written in Python). This is a severe limitation.
As a workaround, you can use the _fileio module (written in C):
import _fileio
f = _fileio._FileIO("output.txt", "w", True)
def tracing_func(frame, event, arg):
 f.write('%s %s %d\n' % (frame.f_code.co_filename, frame.f_code.co_
name, frame.f_lineno))
 return tracing_func
A possible fix would be to use an RLock instead of a Lock object, but 
I haven't investigated it.

Thanks for the investigation! The problem has already been reported in
#3618. Closing this one.