Refactored the converter backend system

apps/converter/utils.py

@@ -1,6 +1,10 @@
+import os
+
+from django.core.exceptions import ImproperlyConfigured
+from django.utils.importlib import import_module
+
+
 #http://stackoverflow.com/questions/123198/how-do-i-copy-a-file-in-python
-
-
 def copyfile(source, dest, buffer_size=1024 * 1024):
     """
     Copy a file from source to dest. source and dest
@@ -21,3 +25,50 @@ def copyfile(source, dest, buffer_size=1024 * 1024):
 
     source.close()
     dest.close()
+
+
+def _lazy_load(fn):
+    _cached = []
+
+    def _decorated():
+        if not _cached:
+            _cached.append(fn())
+        return _cached[0]
+    return _decorated
+
+    
+@_lazy_load
+def load_backend():
+    from converter.conf.settings import GRAPHICS_BACKEND as backend_name
+
+    try:
+        module = import_module('.base', 'converter.backends.%s' % backend_name)
+        import warnings
+        warnings.warn(
+            "Short names for CONVERTER_BACKEND are deprecated; prepend with 'converter.backends.'",
+            PendingDeprecationWarning
+        )
+        return module
+    except ImportError, e:
+        # Look for a fully qualified converter backend name
+        try:
+            return import_module('.base', backend_name)
+        except ImportError, e_user:
+            # The converter backend wasn't found. Display a helpful error message
+            # listing all possible (built-in) converter backends.
+            backend_dir = os.path.join(os.path.dirname(__file__), 'backends')
+            try:
+                available_backends = [f for f in os.listdir(backend_dir)
+                        if os.path.isdir(os.path.join(backend_dir, f))
+                        and not f.startswith('.')]
+            except EnvironmentError:
+                available_backends = []
+            available_backends.sort()
+            if backend_name not in available_backends:
+                error_msg = ("%r isn't an available converter backend. \n" +
+                    "Try using converter.backends.XXX, where XXX is one of:\n    %s\n" +
+                    "Error was: %s") % \
+                    (backend_name, ", ".join(map(repr, available_backends)), e_user)
+                raise ImproperlyConfigured(error_msg)
+            else:
+                raise # If there's some other error, this must be an error in Mayan itself.