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

[Nick Coghlan]

On Wed, May 23, 2012 at 10:40 AM, Eric V. Smith <eric at trueblade.com> wrote:
> On 5/22/2012 2:37 PM, Guido van Rossum wrote:
>> Okay, I've been convinced that keeping the dynamic path feature is a
>> good idea. I am really looking forward to seeing the rationale added
>> to the PEP -- that's pretty much the last thing on my list that made
>> me hesitate. I'll leave the details of exactly how the parent path is
>> referenced up to the implementation team (several good points were
>> made), as long as the restriction that sys.path must be modified in
>> place is lifted.
>
> I've updated the PEP. Let me know how it looks.
>
> I have not updated the implementation yet. I'm not exactly sure how I'm
> going to convert from a path list of unknown origin to ('sys', 'path')
> or ('foo', '__path__'). I'll look at it later tonight to see if it's
> possible. I'm hoping it doesn't require major surgery to
> importlib._bootstrap.

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
defined
- 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.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia