On Tue, May 22, 2012 at 9:58 PM, Nick Coghlan <ncoghlan@gmail.com> wrote:
If you wanted to do this without changing the sys.meta_path hook API,
you'd have to pass an object to find_module() that did the dynamic
lookup of the value in obj.__iter__. Something like:

   class _LazyPath:
       def __init__(self, modname, attribute):
           self.modname = modname
           self.attribute = attribute
       def __iter__(self):
           return iter(getattr(sys.module[self.modname], self.attribute))

A potentially cleaner alternative to consider is tweaking the
find_loader API spec so that it gets used at the meta path level as
well as at the path hooks level and is handed a *callable* that
dynamically retrieves the path rather than a direct reference to the
path itself.

The full signature of find_loader would then become:

   def find_loader(fullname, get_path=None):
       # fullname as for find_module
       # When get_path is None, it means the finder is being called
as a path hook and
       # should use the specific path entry passed to __init__
       # In this case, namespace package portions are returned as
(None, portions)
       # Otherwise, the finder is being called as a meta_path hook
and get_path() will return the relevant path
       # Any namespace packages are then returned as (loader, portions)

There are two major consequences of this latter approach:
- the PEP 302 find_module API would now be a purely legacy interface
for both the meta_path and path_hooks, used only if find_loader is not
- it becomes trivial to tell whether a particular name references a
package or not *without* needing to load it first: find_loader()
returns a non-empty iterable for the list of portions

That second consequence is rather appealing: it means you'd be able to
implement an almost complete walk of a package hierarchy *without*
having to import anything (although you would miss old-style namespace
packages and any other packages that alter their own __path__ in
__init__, so you may still want to load packages to make sure you
found everything. You could definitively answer the "is this a package
or not?" question without running any code, though).

The first consequence is also appealing, since the find_module() name
is more than a little misleading. The "find_module" name strongly
suggests that the method is expected to return a module object, and
that's just wrong -  you actually find a loader, then you use that to
load the module.

While I see no problem with cleaning up the interface, I'm kind of lost as to the point of making a get_path callable, vs. just using the iterable interface you sketched.  Python has iterables, so why add a call to get the iterable, when iter() or a straight "for" loop will do effectively the same thing?