Factor out some common pieces of django.conf.LazySettings.

This is in preparation for some reuse elsewhere in the core code.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9945 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/utils/functional.py

@@ -251,3 +251,39 @@ def allow_lazy(func, *resultclasses):
             return func(*args, **kwargs)
         return lazy(func, *resultclasses)(*args, **kwargs)
     return wraps(func)(wrapper)
+
+class LazyObject(object):
+    """
+    A wrapper for another class that can be used to delay instantiation of the
+    wrapped class.
+
+    This is useful, for example, if the wrapped class needs to use Django
+    settings at creation time: we want to permit it to be imported without
+    accessing settings.
+    """
+    def __init__(self):
+        self._wrapped = None
+
+    def __getattr__(self, name):
+        if self._wrapped is None:
+            self._setup()
+        if name == "__members__":
+            # Used to implement dir(obj)
+            return self._wrapped.get_all_members()
+        return getattr(self._wrapped, name)
+
+    def __setattr__(self, name, value):
+        if name == "_wrapped":
+            # Assign to __dict__ to avoid infinite __setattr__ loops.
+            self.__dict__["_wrapped"] = value
+        else:
+            if self._wrapped is None:
+                self._setup()
+            setattr(self._wrapped, name, value)
+
+    def _setup(self):
+        """
+        Must be implemented by subclasses to initialise the wrapped object.
+        """
+        raise NotImplementedError
+