Re: a Python bug in processing __del__ method ??

From: Fredrik Lundh (fredrik_at_pythonware.com)
Date: 01/31/05 

To: python-list@python.org
Date: Mon, 31 Jan 2005 08:02:11 +0100

Baoqiu Cui wrote:

> 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?

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>

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