Extract useful backend utils

imagekit/imagecache/__init__.py

@@ -1,34 +1,12 @@
-from django.core.exceptions import ImproperlyConfigured
-from django.utils.importlib import import_module
-
-from imagekit.imagecache.base import InvalidImageCacheBackendError, PessimisticImageCacheBackend, NonValidatingImageCacheBackend
-
-_default_image_cache_backend = None
+from ..utils import get_singleton
+from .base import InvalidImageCacheBackendError, PessimisticImageCacheBackend, NonValidatingImageCacheBackend
 
 
 def get_default_image_cache_backend():
     """
-    Get the default image cache backend. Uses the same method as
-    django.core.file.storage.get_storage_class
+    Get the default validation backend.
 
     """
-    global _default_image_cache_backend
-    if not _default_image_cache_backend:
-        from django.conf import settings
-        import_path = settings.IMAGEKIT_DEFAULT_IMAGE_CACHE_BACKEND
-        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
+    from django.conf import settings
+    return get_singleton(settings.IMAGEKIT_DEFAULT_IMAGE_CACHE_BACKEND,
+            'validation backend')

imagekit/utils.py

@@ -3,10 +3,12 @@ import mimetypes
 import sys
 import types
 
+from django.core.exceptions import ImproperlyConfigured
 from django.core.files.base import ContentFile
 from django.db.models.loading import cache
-from django.utils.functional import wraps
 from django.utils.encoding import smart_str, smart_unicode
+from django.utils.functional import wraps
+from django.utils.importlib import import_module
 
 from .lib import Image, ImageFile, StringIO
 
@@ -371,3 +373,34 @@ def prepare_image(img, format):
         save_kwargs['optimize'] = True
 
     return img, save_kwargs
+
+
+def get_class(path, desc):
+    try:
+        dot = path.rindex('.')
+    except ValueError:
+        raise ImproperlyConfigured("%s isn't a %s module." % (path, desc))
+    module, classname = path[:dot], path[dot + 1:]
+    try:
+        mod = import_module(module)
+    except ImportError, e:
+        raise ImproperlyConfigured('Error importing %s module %s: "%s"' %
+                (desc, module, e))
+    try:
+        cls = getattr(mod, classname)
+        return cls
+    except AttributeError:
+        raise ImproperlyConfigured('%s module "%s" does not define a "%s"'
+                ' class.' % (desc[0].upper() + desc[1:], module, classname))
+
+
+_singletons = {}
+
+
+def get_singleton(class_path, desc):
+    global _singletons
+    cls = get_class(class_path, desc)
+    instance = _singletons.get(cls)
+    if not instance:
+        instance = _singletons[cls] = cls()
+    return instance