[Python-Dev] PEP 302, PEP 338 and imp.getloader (was Re: a Python interface for the AST (WAS: DRAFT: python-dev...)

[Phillip J. Eby]

At 11:51 PM 11/23/2005 +1000, Nick Coghlan wrote:
>The key thing that is missing is the "imp.getloader" functionality discussed
>at the end of PEP 302.

This isn't hard to implement per se; setuptools for example has a 
'get_importer' function, and going from importer to loader is simple:

def get_importer(path_item):
     """Retrieve a PEP 302 "importer" for the given path item

     If there is no importer, this returns a wrapper around the builtin import
     machinery.  The returned importer is only cached if it was created by a
     path hook.
     """
     try:
         importer = sys.path_importer_cache[path_item]
     except KeyError:
         for hook in sys.path_hooks:
             try:
                 importer = hook(path_item)
             except ImportError:
                 pass
             else:
                 break
         else:
             importer = None

     sys.path_importer_cache.setdefault(path_item,importer)
     if importer is None:
         try:
             importer = ImpWrapper(path_item)
         except ImportError:
             pass
     return importer

So with the above function you could do something like:

def get_loader(fullname, path):
     for path_item in path:
         try:
             loader = get_importer(path_item).find_module(fullname)
             if loader is not None:
                 return loader
         except ImportError:
             continue
     else:
         return None

in order to implement the rest.


>** I'm open to suggestions on how to deal with argv[0] and __file__. They
>should be set to whatever __file__ would be set to by the module loader, but
>the Importer Protocol in PEP 302 doesn't seem to expose that information. The
>current proposal is a compromise that matches the existing behaviour of -m
>(which supports scripts like regrtest.py) while still giving a meaningful
>value for scripts which are not part of the normal filesystem.

Ugh.  Those are tricky, no question.  I can think of several simple answers 
for each, all of which are wrong in some way.  :)