[issue16242] Pickle and __getattr__

[Joseph Fall]

New submission from Joseph Fall:

This report is to re-raise an old issue from 2009:
http://bugs.python.org/issue5370
Although that issue is marked as "fixed", I disagree that it was ever "fixed", and spent an evil day tracking it down. 

Use Case:
- take two 3rd party packages: django and django-crispy-forms
- django, deep in its inner-workings, uses Pickle
- django-cripsy-forms implements the _getattr__ method in one of it's classes
- when you put the two together such that django pickles a form, the python interpreter goes into a tailspin in very sinister and hard-to-track-down ways (different behaviour in 2.5, 2.6, 2.7, as noted in issue5370, which includes a simple test case)

The "resolution" for issue5370 was to add some documentation to Pickle.  But I would argue this is insufficient - the implementer of django-crispy-forms would have no reason to ever look at the Pickle docs, and so is very likely to implement this bug (which s/he did).

Perhaps, if this issue cannot be resolved in code, as suggested by gagenellina back in 2009, at least some documentation could be added to __getattr__ (and others) warning that these methods could potentially be called before __init__ (similar to the documentation added to Pickle)?  I'd be happy to draft a small bit of documentation for this, although if someone can think of a way to simply resolve this in Pickle so it does not cause the issue in the first place, that seems to me ideal.

A test case is included with issue5370

----------
assignee: docs at python
components: Documentation, Library (Lib)
messages: 172993
nosy: docs at python, powderflask
priority: normal
severity: normal
status: open
title: Pickle and __getattr__
type: behavior
versions: Python 2.6, Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16242>
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