@python_2_unicode_compatible, abstract models, working on 1.4 but causing infinite loop on 1.5.

[James Pic]

On Django 1.4.2, I can call unicode(my_city) without any problem. my_city being an instance of City defined here: ​https://github.com/yourlabs/django-cities-light/blob/master/cities_light/models.py

As you can see:

• City inherits from Base abstract model,
• City is decorated by @python_2_unicode_compatible,
• Base is also decorated by @python_2_unicode_compatible,
• Base defines str
• City does not define str

When these conditions are met then Django 1.5 gets stuck in an infinite loop. The solution is to remove @python_2_unicode_compatible decorator from City.

Traceback: ​https://travis-ci.org/yourlabs/django-autocomplete-light/jobs/11953680

I don't know if this a lack of documentation or a bug.

[Aymeric Augustin]

The documentation makes it clear that @python_2_unicode_compatible is only intended for classes that have a __str__ method.

​https://docs.djangoproject.com/en/dev/ref/utils/#django.utils.encoding.python_2_unicode_compatible

Django's historical behavior of building __str__ from __unicode__ and backwards-compatibility concerns make @python_2_unicode_compatible prone to infinite recursion.

This is extremely hard to debug. It's impossible to print the object since unicode, str and repr all go into infinite recursion. I'd like to raise a better exception instead.

[James Pic]

Thanks for the link, also, it seems like the error is the same in both 1.4.2 and 1.5.4, sorry for the false alarm.

Anyway, it does kinds of look like the user should use @python_2_unicode_compatible on any model class that is "Python2-unicode-compatible", even by inheritance.

So maybe it would be cool to raise a specific exception if @python_2_unicode_compatible is applied on a class without an __str__ method indeed. That would avoid the need of checking the documentation for the user :D

[chmodas]

PR - ​https://github.com/django/django/pull/1703

[Aymeric Augustin]

#19362 was an earlier attempt at fixing this problem. The approach suggested here is more robust.

[Aymeric Augustin <aymeric.augustin@…>]

In f0c7649b1692f8441eb9b9b923b2bed8e95f9185:

Fixed #21198 -- Prevented invalid use of @python_2_unicode_compatible.

Thanks jpic for the report and chmodas for working on a patch.

Reverts 2ea80b94. Refs #19362.

Conflicts:

tests/utils_tests/test_encoding.py

[Aymeric Augustin <aymeric.augustin@…>]

In 589dc49e129f63801c54c15e408c944a345b3dfe:

Fixed #21198 -- Prevented invalid use of @python_2_unicode_compatible.

Thanks jpic for the report and chmodas for working on a patch.

Reverts 2ea80b94. Refs #19362.