Fix handling of optional guardian module

Currently you cannot import something from rest_framework without
configuring Django, since accessing `settings.INSTALLED_APPS` for the
django-guardian throws an ImproperlyConfigured error:

    from rest_framework.exceptions import ErrorDetail

    ~/Vcs/django-rest-framework/rest_framework/exceptions.py in <module>()
         16
         17 from rest_framework import status
    ---> 18 from rest_framework.compat import unicode_to_repr
         19 from rest_framework.utils.serializer_helpers import ReturnDict, ReturnList
         20

    ~/Vcs/django-rest-framework/rest_framework/compat.py in <module>()
        144 guardian = None
        145 try:
    --> 146     if 'guardian' in settings.INSTALLED_APPS:
        147         import guardian  # noqa
        148 except ImportError:

    ~/Vcs/django-rest-framework/.venv/lib/python3.6/site-packages/django/conf/__init__.py in __getattr__(self, name)
         54         """Return the value of a setting and cache it in self.__dict__."""
         55         if self._wrapped is empty:
    ---> 56             self._setup(name)
         57         val = getattr(self._wrapped, name)
         58         self.__dict__[name] = val

    ~/Vcs/django-rest-framework/.venv/lib/python3.6/site-packages/django/conf/__init__.py in _setup(self, name)
         39                 "You must either define the environment variable %s "
         40                 "or call settings.configure() before accessing settings."
    ---> 41                 % (desc, ENVIRONMENT_VARIABLE))
         42
         43         self._wrapped = Settings(settings_module)

    ImproperlyConfigured: Requested setting INSTALLED_APPS, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

I've thought about just trying to import it always, but that is some
overhead for something optional, and would also regress #1712 - except
for when handling the `ImproperlyConfigured` from there.

Since it only gets used by a single class this patch moves the code
there.

rest_framework/compat.py

@@ -139,16 +139,6 @@ except ImportError:
     requests = None
 
 
-# Django-guardian is optional. Import only if guardian is in INSTALLED_APPS
-# Fixes (#1712). We keep the try/except for the test suite.
-guardian = None
-try:
-    if 'guardian' in settings.INSTALLED_APPS:
-        import guardian  # noqa
-except ImportError:
-    pass
-
-
 # PATCH method is not implemented by Django
 if 'patch' not in View.http_method_names:
     View.http_method_names = View.http_method_names + ['patch']

rest_framework/filters.py

@@ -16,7 +16,7 @@ from django.utils import six
 from django.utils.encoding import force_text
 from django.utils.translation import ugettext_lazy as _
 
-from rest_framework.compat import coreapi, coreschema, distinct, guardian
+from rest_framework.compat import coreapi, coreschema, distinct
 from rest_framework.settings import api_settings
 
 
@@ -282,7 +282,11 @@ class DjangoObjectPermissionsFilter(BaseFilterBackend):
     has read object level permissions.
     """
     def __init__(self):
-        assert guardian, 'Using DjangoObjectPermissionsFilter, but django-guardian is not installed'
+        try:
+            import guardian
+        except ImportError:
+            raise ImportError('Using DjangoObjectPermissionsFilter, but django-guardian is not installed')
+        self.guardian_version = guardian.VERSION
 
     perm_format = '%(app_label)s.view_%(model_name)s'
 
@@ -300,7 +304,7 @@ class DjangoObjectPermissionsFilter(BaseFilterBackend):
             'model_name': model_cls._meta.model_name
         }
         permission = self.perm_format % kwargs
-        if tuple(guardian.VERSION) >= (1, 3):
+        if tuple(self.guardian_version) >= (1, 3):
             # Maintain behavior compatibility with versions prior to 1.3
             extra = {'accept_global_perms': False}
         else:

tests/test_permissions.py

@@ -12,7 +12,10 @@ from rest_framework import (
     HTTP_HEADER_ENCODING, authentication, generics, permissions, serializers,
     status, views
 )
-from rest_framework.compat import guardian
+try:
+    import guardian
+except ImportError:
+    guardian = None
 from rest_framework.filters import DjangoObjectPermissionsFilter
 from rest_framework.routers import DefaultRouter
 from rest_framework.test import APIRequestFactory