[Python-Dev] Require loaders set __package__ and __loader__

[Brett Cannon]

An open issue in PEP 302 is whether to require __loader__ attributes on
modules. The claimed worry is memory consumption, but considering importlib
and zipimport are already doing this that seems like a red herring.
Requiring it, though, opens the door to people relying on its existence and
thus starting to do things like loading assets with
``__loader__.get_data(path_to_internal_package_file)`` which allows code to
not care how modules are stored (e.g. zip file, sqlite database, etc.).

What I would like to do is update the PEP to state that loaders are
expected to set __loader__. Now importlib will get updated to do that
implicitly so external code can expect it post-import, but requiring
loaders to set it would mean that code executed during import can rely on
it as well.

As for __package__, PEP 366 states that modules should set it but it isn't
referenced by PEP 302. What I want to do is add a reference and make it
required like __loader__. Importlib already sets it implicitly post-import,
but once again it would be nice to do this pre-import.

To help facilitate both new requirements, I would update the
importlib.util.module_for_loader decorator to set both on a module that
doesn't have them before passing the module down to the decorated method.
That way people already using the decorator don't have to worry about
anything and it is one less detail to have to worry about. I would also
update the docs on importlib.util.set_package and importlib.util.set_loader
to suggest people use importlib.util.module_for_loader and only use the
other two decorators for backwards-compatibility.
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