[ python-Bugs-1717900 ] Destructor behavior faulty

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Bugs item #1717900, was opened at 2007-05-12 15:41
Message generated for change (Comment added) made by alanmcintyre
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Category: Python Interpreter Core
Group: None
Status: Pending
Resolution: None
Priority: 5
Private: No
Submitted By: Wolf Rogner (wrogner)
Assigned to: Nobody/Anonymous (nobody)
Summary: Destructor behavior faulty

Initial Comment:
I tried example 11.4. from bytesofpython (by C.H. Swaroop).
Example works fine.

Added a new Person instance 'wolf' -> Program terminated with:
Exception exceptions.AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0xb7d48d6c>> ignored

added print list(globals()) ->
['kalam', '__builtins__', '__file__', 'DBGPHideChildren', 'swaroop', 'Person', 'wolf', '__name__', '__doc__']

changed wolf to aaa:

print list(globals()) ->
['aaa', 'kalam', '__builtins__', '__file__', 'DBGPHideChildren', 'swaroop', 'Person', '__name__', '__doc__']

Please note the position of 'aaa' at the beginning of the list, before 'Person'. With 'wolf' being after 'Person'.

If the destructing code removes items in this order, no wonder I get an error.

Person should not get deleted if refcount is still > 0.

Wolf Rogner


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Comment By: Alan McIntyre (alanmcintyre)
Date: 2007-05-14 10:38

Message:
Logged In: YES 
user_id=1115903
Originator: NO

I took the example mentioned from here: 
    http://www.python.org/doc/essays/cleanup/

and added this line to the end:

wolf = Person('wolf')

and it gives the reported error.  Here is a minimal snippet that produces
the same error when executed as the top-level module:

class Person:
        population = 0
        def __del__(self):
                Person.population -= 1

wolf = Person()


This appears to be consistent with the behavior described here:
     http://www.python.org/doc/essays/cleanup/

While I understand that cleaning up a module at exit time is probably not
an easy thing to make arbitrarily smart, this behavior seems a little too
not-smart to me.  It seems like it's not all that hard to get bitten by 
it, and the error makes no sense unless you're familiar with the module
cleanup algorithm.

For what it's worth, I offer to help make module cleanup a little smarter,
although I may not be able to spend much time on it until I finish some
things I'm already committed to do.

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Comment By: Alan McIntyre (alanmcintyre)
Date: 2007-05-12 18:36

Message:
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user_id=1115903
Originator: NO

Could you post the code for your entire script? It makes it a lot easier
to figure out what's going on.

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