a Python bug in processing __del__ method ??

[Baoqiu Cui]

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.populati on -= 1

peter = Person()
-------------------------------

The error returned is this:

$ python bug.py
Exception exceptions.Attr ibuteError: "'NoneType' object has no
attribute 'population'" in <bound method Person.__del__ of
<__main__.Perso n 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?
- Baoqiu

[Fredrik Lundh]

Re: a Python bug in processing __del__ method ??

Baoqiu Cui wrote:
[color=blue]
> The error returned is this:
>
> $ python bug.py
> Exception exceptions.Attr ibuteError: "'NoneType' object has no
> attribute 'population'" in <bound method Person.__del__ of
> <__main__.Perso n 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?[/color]

it is not a bug, and the documentation has the answer:

language reference -> index -> __del__

http://docs.python.org/ref/customization.html#l2h-175

Warning: Due to the precarious circumstances under which __del__()
methods are invoked, exceptions that occur during their execution are
ignored, and a warning is printed to sys.stderr instead. Also, when
__del__() is invoked in response to a module being deleted (e.g.,
when execution of the program is done), other globals referenced by
the __del__() method may already have been deleted. For this reason,
__del__() methods should do the absolute minimum needed to
maintain external invariants.

if you absolutely need to have reliable access to globals, you need to make
sure they're available as instance variables, or are bound in some other way.

</F>

[Steven Bethard]

Re: a Python bug in processing __del__ method ??

Baoqiu Cui wrote:[color=blue]
> 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.populati on -= 1
>
> peter = Person()
> -------------------------------
>
> The error returned is this:
>
> $ python bug.py
> Exception exceptions.Attr ibuteError: "'NoneType' object has no
> attribute 'population'" in <bound method Person.__del__ of
> <__main__.Perso n 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?[/color]

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.populati on -= 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.Attr ibuteError: "'NoneType' object has no attribute
'population'" in <bound method Person.__del__ of <__main__.Perso n object
at 0x009D1A50>> ignored

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

Steve

[Steven Bethard]

Re: a Python bug in processing __del__ method ??

Fredrik Lundh wrote:[color=blue]
> Baoqiu Cui wrote:
>[color=green]
>>The error returned is this:
>>
>>$ python bug.py
>>Exception exceptions.Attr ibuteError: "'NoneType' object has no
>>attribute 'population'" in <bound method Person.__del__ of
>><__main__.Per son 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?[/color]
>
> it is not a bug, and the documentation has the answer:
>
> language reference -> index -> __del__
>
> http://docs.python.org/ref/customization.html#l2h-175
>
> Warning: Due to the precarious circumstances under which __del__()
> methods are invoked, exceptions that occur during their execution are
> ignored, and a warning is printed to sys.stderr instead. Also, when
> __del__() is invoked in response to a module being deleted (e.g.,
> when execution of the program is done), other globals referenced by
> the __del__() method may already have been deleted. For this reason,
> __del__() methods should do the absolute minimum needed to
> maintain external invariants.
>
> if you absolutely need to have reliable access to globals, you need to make
> sure they're available as instance variables, or are bound in some other way.[/color]

Along these lines, I think changing your code to:

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

peter = Person()

solves the problem. (At least it did for me.)

Steve

[Baoqiu Cui]

Re: a Python bug in processing __del__ method ??

Fredrik and Steve,

Thank you so much for the help. Now I know more about Python. :-)

Steve's test does explain why 'peter1' is OK while 'peter' is not:
'peter1' appears
before 'Person' in the globals while 'peter' is after.

The error message is just a little confusing to a Python newbie, I
think.

Thanks again!

- Baoqiu