[Python-Dev] Clarifications for import PEPs (302 and 328)

Phillip J. Eby pje at telecommunity.com
Tue Apr 24 03:21:46 CEST 2007

At 04:23 PM 4/23/2007 -0700, Brett Cannon wrote:
>On 4/23/07, Phillip J. Eby <pje at telecommunity.com> wrote:
> > At 03:16 PM 4/23/2007 -0700, Brett Cannon wrote:
> > >The PEP does not explicitly state how to signal that a loader cannot
> > >load a module it is asked to.  This could happen if someone called a
> > >loader without consulting its respective importer.  I would want to
> > >add something like:
> > >"""
> > >If the loader is unable to load the specified module and a specific
> > >exception is not raised in determining this, ImportError is raised.
> > >This may occur if a loader is called without first consulting an
> > >importer as to if the loader can load the specified module but the
> > >loader is aware of the fact it cannot fulfill the request made.
> > >"""
> >
> > Okay, now I understand what you're trying to say, but I still don't
> > understand what the purpose is.  I mean, what else would you do except
> > raise an error?  I.e., isn't the actual contract "load the specified module
> > or raise an exception of some kind"?
>Not if you read the PEP.  It doesn't explicitly say what should happen.
>I want to standardize on raising ImportError.  The PEP itself has two
>ways of signaling an error; find_module() returns None instead of
>raising an error to signal it can't do something while path hooks
>raise ImportError.  I just want to be as explicit as possible.

Ok, so this should at least lose the passive voice ("ImportError is 
raised") -- it should say that it's the loader's job to raise 
ImportError.  Otherwise, it's not actually removing any ambiguity.

I also don't find the "This may occur" sentence to be helpful; it seems we 
should just say, "load_module() must return a loaded module or raise an 
error, preferably an ImportError unless an existing exception is being 

Meanwhile, if you also mean to say that load_module() is explicitly allowed 
to fail with an ImportError when it's asked for any module other than the 
one it was obtained for (via find_module()), then let's say that, too.

Notice, by the way, that this is a stronger condition than the one you 
proposed, yet it doesn't imply that there is or should be an ongoing 
relationship between the loader and the importer.  It also doesn't imply 
that perhaps it's okay for a loader to not succeed in reloading the *same* 
module!  (i.e., as I understand it, a module can currently invoke 
__loader__.load_module(__name__) as a rough synonym for reload().)

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