[Python-Dev] PEP 420 - dynamic path computation is missing rationale

[PJ Eby]

On May 23, 2012 9:02 AM, "Nick Coghlan" <ncoghlan at gmail.com> wrote:
>
> On Wed, May 23, 2012 at 10:31 PM, Eric V. Smith <eric at trueblade.com>
wrote:
> > On 05/22/2012 09:49 PM, PJ Eby wrote:
> >> It shouldn't - all you should need is to use
> >> getattr(sys.modules[self.modname], self.attr) instead of referencing a
> >> parent path object directly.
> >
> > The problem isn't the lookup, it's coming up with self.modname and
> > self.attr. As it currently stands, PathFinder.find_module is given the
> > parent path, not the module name and attribute name used to look up the
> > parent path using sys.modules and getattr.
>
> Right, that's what PJE and I were discussing. Instead of passing in
> the path object directly, you can instead pass an object that *lazily*
> retrieves the path object in its __iter__ method:
>
>    class LazyIterable:
>        """On iteration, retrieves a reference to a named iterable and
> returns an iterator over that iterable"""
>        def __init__(self, modname, attribute):
>            self.modname = modname
>            self.attribute = attribute
>        def __iter__(self):
>            mod = import_module(self.modname) # Will almost always get
> a hit directly in sys.modules
>            return iter(getattr(mod, self.attribute)
>
> Where importlib currently passes None or sys.path as the path argument
> to find_module(), instead pass "LazyIterable('sys', 'path')" and where
> it currently passes package.__path__, instead pass
> "LazyIterable(package.__name__, '__path__')".
>
> The existing for loop iteration and tuple() calls should then take
> care of the lazy lookup automatically.
>
> That way, the only code that needs to know the values of modname and
> attribute is the code that already has access to those values.

Perhaps calling it a ModulePath instead of a LazyIterable would be better?

Also, this is technically a change from PEP 302, which says the actual
sys.path or __path__ are passed to find_module().  I'm not sure whether any
find_module() code ever written actually *cares* about this, though.
(Especially if, as I believe I understand in this context, we're only
talking about meta-importers.)
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