declare celery backends in a separate module

imagekit/imagecache/__init__.py

@@ -0,0 +1,31 @@
+from django.core.exceptions import ImproperlyConfigured
+from django.utils.importlib import import_module
+
+_default_image_cache_backend = None
+
+
+def get_default_image_cache_backend():
+    """
+    Get the default image cache backend. Uses the same method as
+    django.core.file.storage.get_storage_class
+
+    """
+    global _default_image_cache_backend
+    if not _default_image_cache_backend:
+        from ..settings import DEFAULT_IMAGE_CACHE_BACKEND as import_path
+        try:
+            dot = import_path.rindex('.')
+        except ValueError:
+            raise ImproperlyConfigured("%s isn't an image cache backend module." % \
+                    import_path)
+        module, classname = import_path[:dot], import_path[dot + 1:]
+        try:
+            mod = import_module(module)
+        except ImportError, e:
+            raise ImproperlyConfigured('Error importing image cache backend module %s: "%s"' % (module, e))
+        try:
+            cls = getattr(mod, classname)
+            _default_image_cache_backend = cls()
+        except AttributeError:
+            raise ImproperlyConfigured('Image cache backend module "%s" does not define a "%s" class.' % (module, classname))
+    return _default_image_cache_backend

imagekit/imagecache/base.py

@@ -1,7 +1,3 @@
-from django.core.exceptions import ImproperlyConfigured
-from django.utils.importlib import import_module
-
-
 class PessimisticImageCacheBackend(object):
     """
     A very safe image cache backend. Guarantees that files will always be
@@ -55,33 +51,3 @@ class NonValidatingImageCacheBackend(object):
 
     def clear(self, file):
         file.delete(save=False)
-
-
-_default_image_cache_backend = None
-
-
-def get_default_image_cache_backend():
-    """
-    Get the default image cache backend. Uses the same method as
-    django.core.file.storage.get_storage_class
-
-    """
-    global _default_image_cache_backend
-    if not _default_image_cache_backend:
-        from .settings import DEFAULT_IMAGE_CACHE_BACKEND as import_path
-        try:
-            dot = import_path.rindex('.')
-        except ValueError:
-            raise ImproperlyConfigured("%s isn't an image cache backend module." % \
-                    import_path)
-        module, classname = import_path[:dot], import_path[dot + 1:]
-        try:
-            mod = import_module(module)
-        except ImportError, e:
-            raise ImproperlyConfigured('Error importing image cache backend module %s: "%s"' % (module, e))
-        try:
-            cls = getattr(mod, classname)
-            _default_image_cache_backend = cls()
-        except AttributeError:
-            raise ImproperlyConfigured('Image cache backend module "%s" does not define a "%s" class.' % (module, classname))
-    return _default_image_cache_backend

imagekit/imagecache/celery.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+from celery.task import task
+
+from imagekit.imagecache.base import PessimisticImageCacheBackend
+
+
+@task
+def generate(model, pk, attr):
+    try:
+        instance = model._default_manager.get(pk=pk)
+    except model.DoesNotExist:
+        pass  # The model was deleted since the task was scheduled. NEVER MIND!
+    else:
+        getattr(instance, attr).generate(save=True)
+
+
+class CeleryCacheStateBackend(PessimisticImageCacheBackend):
+    """
+    A pessimistic cache state backend that uses celery to generate its spec
+    images. Like PessimisticCacheStateBackend, this one checks to see if the
+    file exists on validation, so the storage is hit fairly frequently, but an
+    image is guaranteed to exist. However, while validation guarantees the
+    existence of *an* image, it does not necessarily guarantee that you will get
+    the correct image, as the spec may be pending regeneration. In other words,
+    while there are `generate` tasks in the queue, it is possible to get a
+    stale spec image. The tradeoff is that calling `invalidate()` won't block
+    to interact with file storage.
+
+    """
+
+    def invalidate(self, file):
+        generate.delay(file.instance.__class__, file.instance.pk, file.attname)
+
+    def clear(self, file):
+        file.delete(save=False)

imagekit/settings.py

@@ -2,4 +2,4 @@ from django.conf import settings
 
 DEFAULT_IMAGE_CACHE_BACKEND = getattr(settings,
         'IMAGEKIT_DEFAULT_IMAGE_CACHE_BACKEND',
-        'imagekit.imagecache.PessimisticImageCacheBackend')
+        'imagekit.imagecache.base.PessimisticImageCacheBackend')