Destructors and exceptions

[Nick Jacobson]

You know, I noticed this in the Python Reference Manual, p. 13, and
have been wondering about it.

"...note that catching an exception with a 'try...except' statement
may keep objects alive..."

No explanation is given, and I don't know why that's the case either. 
But at least they're aware of it...HTH

--Nick

dkturner at telkomsa.net (David Turner) wrote in message news:<e251b7ba.0406070651.1c98c09d at posting.google.com>...
> Hi all
> 
> I noticed something interesting while testing some RAII concepts
> ported from C++ in Python.  I haven't managed to find any information
> about it on the web, hence this post.
> 
> The problem is that when an exception is raised, the destruction of
> locals appears to be deferred to program exit.  Am I missing
> something?  Is this behaviour by design?  If so, is there any reason
> for it?  The only rationale I can think of is to speed up exception
> handling; but as this approach breaks many safe programming idioms, I
> see it as a poor trade.
> 
> Here is the code in question:
> 
> ------------------------------------------
> class Foo:
>   def __init__(self):
>     print "--foo %s created" % id(self)
>   def __del__(self):
>     print "--foo %s destroyed" % id(self)
> 
> def normal_exit():
>   print "normal_exit starts"
>   x = Foo()
>   print "normal_exit ends"
> 
> def premature_exit():
>   print "premature_exit starts"
>   x = Foo()
>   return 0
>   print "premature_exit ends"
> 
> def exceptional_exit():
>   print "exceptional_exit starts"
>   x = Foo()
>   raise "oops"
>   print "exceptional_exit ends"
> 
> if __name__ == "__main__":
>   print "main block starts"
>   try:
>     normal_exit()
>     premature_exit()
>     exceptional_exit()
>   except:
>     print "exception"
>   print "main block ends"
> ------------------------------------------
> 
> The output I get is:
> 
> ------------------------------------------
> main block starts
> normal_exit starts
> --foo 141819532 created
> normal_exit ends
> --foo 141819532 destroyed
> premature_exit starts
> --foo 141819532 created
> --foo 141819532 destroyed
> exceptional_exit starts
> --foo 141819532 created
> exception
> main block ends
> --foo 141819532 destroyed
> ------------------------------------------
> 
> ...which indicates to me that the destruction of the local in
> exceptional_exit() only happens when the program exits.  Surely it
> should occur at the point at which the exception is raised?
> 
> Regards
> David Turner