Add a compatibility layer for Python 3

[Claude Paroz]

For Django to run both on Python 2 and Python 3, we need to add some code which conditionally executes code depending on the Python version at runtime.

This patch is based on Vinay Sajip's branch (​https://bitbucket.org/vinay.sajip/django), minus some code that I'm not sure we will really need (as now we dropped Python 2.5). We can always add some more later.

I've added some docs, but as I'm not the best English writer, this will need careful review.

[Claude Paroz]

Add Python 3 compatibility layer

[Claude Paroz]

Pull request: ​https://github.com/django/django/pull/89

[Claude Paroz <claude@…>]

In [5e6ded2e58597fa324c550bad35a30ee630ce223]:

Fixed #18363 -- Added Python 3 compatibility layer.

Thanks Vinay Sajip for the support of his django3 branch
and Alex Gaynor, kezabelle, YorikSar for the review.

[Claude Paroz]

After some discussion, we will try to replace django.utils.py3 by the six module as django.utils.six.

​http://pypi.python.org/pypi/six

[Vinay Sajip]

Replying to claudep:

After some discussion, we will try to replace django.utils.py3 by the six module as django.utils.six.

​http://pypi.python.org/pypi/six

Did this discussion happen on the django-core mailing list? I didn't see anything about it on django-developers.

I've no very strong views on using six, it's a nice library and I used a few of the ideas, but IMO there are points which go against using it directly; for example, because of how Django uses metaclasses, and how with_metaclass works, we need to distinguish Django's usage from usage in third-party modules. Just grep my sources for "DjangoBase" to see what I mean. I actually use a pretty small subsection of six functionality, and did consider using it in django.utils.six style, but decided against it.

You could of course keep django.utils.py3 and in it just do from .six import * followed by any additions or redefinitions which are needed, like n().

I also think that using io.StringIO on 2.x, as I believe six does, will lead to problems; 2.x io.StringIO expects only Unicode to be written to it, and that's not what e.g. parts of the stdlib (like json) do when writing to streams. Example (using Python 2.6):

>>> from io import StringIO
>>> fp = StringIO()
>>> import json; json.dump({}, fp)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\python26\lib\json\__init__.py", line 181, in dump
    fp.write(chunk)
  File "C:\python26\lib\io.py", line 1515, in write
    s.__class__.__name__)
TypeError: can't write str to text stream
>>>

[Aymeric Augustin <aymeric.augustin@…>]

In [8b0190984116873158ee627c7a033a7bd4c3a199]:

[py3] Bundled six for Python 3 compatibility.

Refs #18363.

[Aymeric Augustin]

It's actually possible to use six.with_metaclass() for ModelBase, see [ae4125ffce6c0a82e9016f67d60d31bc7595887c].