> I'm attracted to the idea of making a loader lazy by simply doing
> something
> like ``class LazySourceFileLoader(LazyMixin, SourceFileLoader):
> ...``.

But then you have to make a specific Lazy subclass for each delegated
loader implementation. With a proxy class you would simply proxy each
loader instance:

existing_loader = SourceFileLoader(...)
lazy_loader = LazyLoader(existing_loader)
...