Refactor the class/function loading code to use common API

[Jannis Leidel]

There are countless code snippets in Django that load a particular callback or backend and each handle the possible exceptions differently.

I think for the most of those examples we can use a common API which would lower the chance of breakage as well be useful to users.

Jonas Obrist implemented this kind of thing in his django-load app (​http://readthedocs.org/docs/django-load/en/latest/).

[Jannis Leidel]

I've updated Jonas' code slightly and added it to the django.utils.importlib module.

​https://github.com/jezdez/django/compare/feature/importutils

If accepted, this would require more work on the actual code parts where imports are done.

[Jannis Leidel]

diff from ​https://github.com/jezdez/django/compare/feature/importutils

[Russell Keith-Magee]

I'll accept this in principle -- the "load a backend" pattern is something that we've reimplemented far too many times. If we can pull this back to a common API without losing all the edge cases, it seems like a no-brainer to me.

From a first inspection, this snippet from Jonas looks like a good approach to me -- I'd just need to see it in use for a couple of actual use cases to be 100% convinced.

[Aymeric Augustin]

Wasn't this fixed by 7c5b2448?

I'm not sure I understand correctly the scope of this ticket.

[Claude Paroz]

I wasn't aware of this ticket when I committed [7c5b2448] (#17061). If the possibility to customize the raised exception is needed somewhere in Django, we can always add it at a later stage.