a Python bug in processing __del__ method ??

[Steven Bethard]

Baoqiu Cui wrote:
> Today I was playing with a small Python program using Python 2.4
> on Cygwin (up-to-date version, on Windows XP), but ran into a
> strange error on the following small program (named bug.py):
> 
> -------------------------------
> #!/usr/bin/python
> 
> class Person:
> population = 0
> def __del__(self):
> Person.population -= 1
> 
> peter = Person()
> -------------------------------
> 
> The error returned is this:
> 
> $ python bug.py
> Exception exceptions.AttributeError: "'NoneType' object has no
> attribute 'population'" in <bound method Person.__del__ of
> <__main__.Person instance at 0xa0c9fec>> ignored
> 
> However, if I rename variable name 'peter' to something like 'peter1'
> or 'david', the error is gone.  Looks to me the
> error only happens to variable name 'peter'.
> 
> Does anyone know what is wrong?  Is this a bug only on Cygwin?

I get this error on a normal Python WinXP install too.  Looks like 
Python deletes globals in order when exiting, so the Person class gets 
deleted before the Person instances:

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

<name> = Person()
print list(globals())
----------------------------

I tried this with a few different names.  Here are the results for 
'apple', 'banana' and 'cranberry'.  Only when the name appears after 
'Person' in the globals do I get the error that you get.

[D:\Steve]$ bug.py
['apple', '__builtins__', '__file__', 'Person', '__name__', '__doc__']

[D:\Steve]$ bug.py
['__builtins__', '__file__', 'banana', 'Person', '__name__', '__doc__']

[D:\Steve]$ bug.py
['__builtins__', '__file__', 'Person', 'cranberry', '__name__', '__doc__']
Exception exceptions.AttributeError: "'NoneType' object has no attribute 
'population'" in <bound method Person.__del__ of <__main__.Person object 
at 0x009D1A50>> ignored

I don't know whether this is a bug or a "feature".  ;)

Steve