Fixed #17304 -- Allow single-path and configured-path namespace packages as apps.

Also document the conditions under which a namespace package may or may not be
a Django app, and raise a clearer error message in those cases where it may not
be.

Thanks Aymeric for review and consultation.

django/apps/base.py

@@ -39,9 +39,18 @@ class AppConfig(object):
         # egg. Otherwise it's a unicode on Python 2 and a str on Python 3.
         if not hasattr(self, 'path'):
             try:
-                self.path = upath(app_module.__path__[0])
+                paths = app_module.__path__
             except AttributeError:
                 self.path = None
+            else:
+                # Convert paths to list because Python 3.3 _NamespacePath does
+                # not support indexing.
+                paths = list(paths)
+                if len(paths) > 1:
+                    raise ImproperlyConfigured(
+                        "The namespace package app %r has multiple locations, "
+                        "which is not supported: %r" % (app_name, paths))
+                self.path = upath(paths[0])
 
         # Module containing models eg. <module 'django.contrib.admin.models'
         # from 'django/contrib/admin/models.pyc'>. Set by import_models().

docs/ref/applications.txt

@@ -160,17 +160,23 @@ Configurable attributes
 
     This attribute defaults to ``label.title()``.
 
-Read-only attributes
---------------------
-
 .. attribute:: AppConfig.path
 
     Filesystem path to the application directory, e.g.
     ``'/usr/lib/python2.7/dist-packages/django/contrib/admin'``.
 
+    In most cases, Django can automatically detect and set this, but you can
+    also provide an explicit override as a class attribute on your
+    :class:`~django.apps.AppConfig` subclass. In a few situations this is
+    required; for instance if the app package is a `namespace package`_ with
+    multiple paths.
+
     It may be ``None`` if the application isn't stored in a directory, for
     instance if it's loaded from an egg.
 
+Read-only attributes
+--------------------
+
 .. attribute:: AppConfig.module
 
     Root module for the application, e.g. ``<module 'django.contrib.admin' from
@@ -209,6 +215,32 @@ Methods
         def ready(self):
             MyModel = self.get_model('MyModel')
 
+.. _namespace package:
+
+Namespace packages as apps (Python 3.3+)
+----------------------------------------
+
+Python versions 3.3 and later support Python packages without an
+``__init__.py`` file. These packages are known as "namespace packages" and may
+be spread across multiple directories at different locations on ``sys.path``
+(see :pep:`420`).
+
+Django applications require a single base filesystem path where Django
+(depending on configuration) will search for templates, static assets,
+etc. Thus, namespace packages may only be Django applications if one of the
+following is true:
+
+1. The namespace package actually has only a single location (i.e. is not
+   spread across more than one directory.)
+
+2. The :class:`~django.apps.AppConfig` class used to configure the application
+   has a :attr:`~django.apps.AppConfig.path` class attribute, which is the
+   absolute directory path Django will use as the single base path for the
+   application.
+
+If neither of these conditions is met, Django will raise
+:exc:`~django.core.exceptions.ImproperlyConfigured`.
+
 Application registry
 ====================
 

tests/apps/namespace_package_base/nsapp/apps.py

@@ -0,0 +1,8 @@
+import os
+
+from django.apps import AppConfig
+from django.utils._os import upath
+
+class NSAppConfig(AppConfig):
+    name = 'nsapp'
+    path = upath(os.path.dirname(__file__))

tests/apps/tests.py

@@ -1,10 +1,16 @@
 from __future__ import absolute_import, unicode_literals
 
+from contextlib import contextmanager
+import os
+import sys
+from unittest import skipUnless
+
 from django.apps import apps
 from django.apps.registry import Apps
 from django.core.exceptions import ImproperlyConfigured
 from django.db import models
 from django.test import TestCase, override_settings
+from django.utils._os import upath
 from django.utils import six
 
 from .default_config_app.apps import CustomConfig
@@ -28,6 +34,8 @@ SOME_INSTALLED_APPS_NAMES = [
     'django.contrib.auth',
 ] + SOME_INSTALLED_APPS[2:]
 
+HERE = os.path.dirname(__file__)
+
 
 class AppsTests(TestCase):
 
@@ -166,3 +174,62 @@ class AppsTests(TestCase):
         with self.assertRaises(LookupError):
             apps.get_model("apps", "SouthPonies")
         self.assertEqual(new_apps.get_model("apps", "SouthPonies"), temp_model)
+
+
+
+@skipUnless(
+    sys.version_info > (3, 3, 0),
+    "Namespace packages sans __init__.py were added in Python 3.3")
+class NamespacePackageAppTests(TestCase):
+    # We need nsapp to be top-level so our multiple-paths tests can add another
+    # location for it (if its inside a normal package with an __init__.py that
+    # isn't possible). In order to avoid cluttering the already-full tests/ dir
+    # (which is on sys.path), we add these new entries to sys.path temporarily.
+    base_location = os.path.join(HERE, 'namespace_package_base')
+    other_location = os.path.join(HERE, 'namespace_package_other_base')
+    app_path = os.path.join(base_location, 'nsapp')
+
+    @contextmanager
+    def add_to_path(self, *paths):
+        """Context manager to temporarily add paths to sys.path."""
+        _orig_sys_path = sys.path[:]
+        sys.path.extend(paths)
+        try:
+            yield
+        finally:
+            sys.path = _orig_sys_path
+
+    def test_single_path(self):
+        """
+        A Py3.3+ namespace package can be an app if it has only one path.
+        """
+        with self.add_to_path(self.base_location):
+            with self.settings(INSTALLED_APPS=['nsapp']):
+                app_config = apps.get_app_config('nsapp')
+                self.assertEqual(app_config.path, upath(self.app_path))
+
+    def test_multiple_paths(self):
+        """
+        A Py3.3+ namespace package with multiple locations cannot be an app.
+
+        (Because then we wouldn't know where to load its templates, static
+        assets, etc from.)
+
+        """
+        # Temporarily add two directories to sys.path that both contain
+        # components of the "nsapp" package.
+        with self.add_to_path(self.base_location, self.other_location):
+            with self.assertRaises(ImproperlyConfigured):
+                with self.settings(INSTALLED_APPS=['nsapp']):
+                    pass
+
+    def test_multiple_paths_explicit_path(self):
+        """
+        Multiple locations are ok only if app-config has explicit path.
+        """
+        # Temporarily add two directories to sys.path that both contain
+        # components of the "nsapp" package.
+        with self.add_to_path(self.base_location, self.other_location):
+            with self.settings(INSTALLED_APPS=['nsapp.apps.NSAppConfig']):
+                app_config = apps.get_app_config('nsapp')
+                self.assertEqual(app_config.path, upath(self.app_path))