[Python-Dev] difficulty of implementing phase 2 of PEP 302 in Python source

Brett Cannon brett at python.org
Thu Sep 28 20:25:25 CEST 2006

On 9/27/06, Phillip J. Eby <pje at telecommunity.com> wrote:
> At 05:26 PM 9/27/2006 -0700, Brett Cannon wrote:
> >Ah, OK.  So for importing 'email', the zipimporter would call the .pyc
> >importer and it would ask the zipimporter, "can you get me email.pyc?"
> and
> >if it said no it would move on to asking the .py importer for email.py,
> etc.
> Yes, exactly.
> >That's fine.  Just thinking about how the current situation sucks for NFS
> >but how caching just isn't done.  But obvoiusly this could change.
> Well, with this design, you can have a CachingFilesystemImporter as your
> storage mechanism to speed things up.
> >> >>Of course, to fully support .pyc timestamp checking and writeback,
> you'd
> >> >>need some sort of "stat" or "getmtime" feature on the parent
> importer, as
> >> >>well as perhaps an optional "save_data" method.  These would be
> extensions
> >> >>to PEP 302, but welcome ones.
> >> >
> >> >Could pass the string representing the location of where the string
> came
> >> >from.  That would allow for the required stat calls for .pyc files as
> >> >needed without having to implement methods just for this one use case.
> >>
> >>Huh?  In order to know if a .pyc is up to date, you need the st_mtime of
> >>the .py file.  That can't be done in the parent importer without giving
> it
> >>format knowledge, which goes against the point of the exercise.
> >
> >Sorry, thought .pyc files based whether they needed to be recompiled
> based
> >on the stat info on the .py and .pyc file, not on data stored from within
> >the .pyc .
> It's not just that (although I believe it's also the case that there is a
> timestamp inside .pyc), it's that to do the check in the parent importer,
> the parent importer would have to know that there is such a thing as
> .py-and-.pyc.  The whole point of this design is that the parent importer
> doesn't have to know *anything* about filename extensions OR how those
> files are formatted internally.  In this scheme, adding more child
> importers is sufficient to add all the special handling needed for
> .py/.pyc-style schemes.
> Of course, for maximum flexibility, you might want get_stream() and
> get_file() methods optionally available, since a .so loader really needs a
> file, and .pyc might want to read in two stages.  But the child importers
> can be defensively coded so as to be able to live with only a
> parent.get_data(), if necessary, and do the enhanced behaviors only if
> stat() or get_stream() or write_data() etc. attributes are available on
> the
> parent.

Yeah, how to get the proper information to the data importers is going to be
the trick.

If we get some standards for these additional attributes, we can document
> them as standard PEP 302 extensions.
> The format importer mechanism might want to have something like
> 'sys.import_formats' as a list of importer classes (or factories).  Parent
> (storage) importer classes would then create instances to use.
> If you add a new format importer to sys.import_formats, you would of
> course
> need to clear sys.path_importer_cache, so that the individual importers
> are
> rebuilt on the next import, and thus they will create new child importer
> chains.
> Yeah, that pretty much ought to do it.

I will think about it, but I am still trying to get the original question of
how bad the C code is compared to rewriting import in Python from people.

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