Added a draft grepo_base.backend.* loader

apps/grepo_base/backends/__init__.py

@@ -0,0 +1,51 @@
+# -*- coding: utf-8 -*-
+"""
+    grepo_base.backends
+    ~~~~~~~~~~~~~~~~~~~
+
+    Each backend is expected to define **at least** the following
+    functions:
+
+    .. function:: list()
+
+       Returns an iterable, yielding repository data dicts, suitable for
+       passing to :class:`~grepo_base.models.Repository` constructor.
+
+    .. function:: update(repository)
+
+       Updates a given :class:`~grepo_base.models.Repository` object,
+       **without** saving it to the database.
+
+    .. function:: needs_update(repository)
+
+       Returns ``True`` if a given :class:`~grepo_base.models.Repository`
+       instance needs to be updated and ``False`` otherwise.
+"""
+
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+from django.utils.functional import memoize
+from django.utils.importlib import import_module
+
+
+def load_backend(path):
+    try:
+        module = import_module(path)
+    except (ImportError, AttributeError) as e:
+        raise ImproperlyConfigured("Error importing Grepo backend {0}: {1}"
+                                   .format(path, e))
+
+    for func in ["list", "update"]:
+        if not hasattr(module, func):
+            raise ImproperlyConfigured(
+                "Grepo backend {0} is missing {1}() function."
+                .format(module.__name__, func)
+            )
+
+    return module
+
+load_backend = memoize(load_backend, {}, 1)
+
+
+def get_backends():
+    return map(load_backend, settings.GREPO_BACKENDS)