First stage imports app modules. It doesn't catch import errors. This
matches the previous behavior and keeps the code simple.

Second stage import models modules. It catches import errors and retries
them after walking through the entire list once. This matches the
previous behavior and seems useful.

populate_models() is intended to be equivalent to populate(). It isn't
wired yet. That is coming in the next commit.

Since applications that aren't installed no longer have an application
configuration, it is now always True in practice.

Provided an abstraction to temporarily add or remove applications as
several tests messed with app_config.installed to achieve this effect.
For now this API is _-prefixed because it looks dangerous.

It wasn't used anywhere and couldn't be implemented any more since
non-installed apps no longer have a configuration.

Got rid of AppConfig._stub. As a side effect, app_cache.app_configs now
only contains entries for applications that are in INSTALLED_APPS, which
is a good thing and will allow dramatic simplifications (which I will
perform in the next commit). That required adjusting all methods that
iterate on app_configs without checking the "installed" flag, hence the
large changes in get_model[s].

Introduced AppCache.all_models to store models:
- while the app cache is being populated and a suitable app config
  object to register models isn't available yet;
- for applications that aren't in INSTALLED_APPS since they don't have
  an app config any longer.

Replaced get_model(seed_cache=False) by registered_model() which can be
kept simple and safe to call at any time, and removed the seed_cache
argument to get_model[s]. There's no replacement for that private API.

Allowed non-master app caches to go through populate() as it is now
safe to do so. They were introduced in 1.7 so backwards compatibility
isn't a concern as long as the migrations framework keeps working.

Thanks Tim Graham for review.

This reverts commit a25fe3b6.

It didn't do anything. It merely added to every PO file:

msgid "%(name)s"
msgstr "%(name)s"

Thanks Ramiro Morales for the report at:
https://groups.google.com/d/msg/Django-I18N/vc2vQzv80UQ/EuaW38V7X7sJ

Conflicts:
	django/contrib/admin/templates/admin/index.html

Refs #19031.

It's now consistent with RenderContext.get.

Thanks graeme.perrow at sap.com for the original report.

Thanks Bruno Brouard for the report.

* Removed ADMIN_FOR setting and warn warning
* Group view functions by namespace instead of site
* Added a test verifying namespaces are listed

Thanks to Claude Paroz for reviewing and ideas for improvement.

Callable arguments were an untested and undocumented feature.

Using callables as query parameters is undocumented and not working,
so this changes an example from the ForeignKey.limit_choices_to
documentation that uses it.

Thanks Anubhav Joshi for the documentation.

Used the information from the app cache instead of creating a duplicate
based on INSTALLED_APPS.

Model._meta.installed is no longer writable. It was a rather sketchy way
to alter private internals anyway.

Improved Andrew's hack to create temporary app caches to handle
migrations. Now the main app cache has a "master" flag set to True
(which is a non-default keyword argument, thus unlikely to be used by
mistake). Other app cache instances have "master" set to False.

The only sanctioned way to access the app cache is by importing
django.core.apps.app_cache.

If you were instanciating an app cache and relying on the Borg pattern,
you'll have to refactor your code.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

Thanks Marc Tamlyn for the suggestion.

Thanks Florian for catching this mistake.

This commit reverts f44c4a5d and 39bbd165.

django.test.simple will be updated in a separate commit as it requires
invasive changes.

Several parts of Django call get_apps() with a comment along this lines
of "this has the side effect of calling _populate()". I fail to see how
this is better than just calling populate()!

Since it's never called with more than one model at a time the current
signature is needlessly complicated.

It feels more natural for self.available_apps to contain app names (like
INSTALLED_APPS) than app labels, and this is easy to implement now.

Refactored get_apps() to rely on that method.

This commit is fully backwards-compatible.

Refactored get_app() to rely on that method.

get_app() starts by calling _populate(), which goes through
INSTALLED_APPS and, for each app, imports the app module and attempts to
import the models module. At this point, no further imports are
necessary to return the models module for a  given app. Therefore, the
implementation of get_app() can be simplified and the safeguards for
race conditions can be removed.

Besides, the emptyOK parameter isn't used anywhere in Django. It was
introduced in d6c95e93 but not actually used nor documented, and it has
just been carried around since then. Since it's an obscure private API,
it's acceptable to stop supporting it without a deprecation path. This
branch aims at providing first-class support for applications without a
models module eventually.

For backwards-compatibility, get_app() still raises ImproperlyConfigured
when an app isn't found, even though LookupError is technically more
correct. I haven't gone as far as to preserve the exact error messages.
I've adjusted a few tests instead.

Marginally improved creation of AppConfig stubs for non-installed apps.

This commit is a refactoring with no change of functionality, according
to the following invariants:

- An app_label that was in app_configs and app_models stays in
  app_config and has its 'installed' attribute set to True.

- An app_label that was in app_models but not in app_configs is added to
  app_configs and has its 'installed' attribute set to True.

As a consequence, all the code that iterated on app_configs is modified
to check for the 'installed' attribute. Code that iterated on app_models
is rewritten in terms of app_configs.

Many tests that stored and restored the state of the app cache were
updated.

In the long term, we should reconsider the usefulness of allowing
importing models from non-installed applications. This doesn't sound
particularly useful, can be a trap in some circumstances, and causes
significant complexity in sensitive areas of Django.