[Python-Dev] PEP 328

Guido van Rossum guido at python.org
Sat Feb 25 22:31:56 CET 2006

On 2/25/06, Thomas Wouters <thomas at xs4all.net> wrote:
> Since I implemented[*] PEP 328, Aahz suggested I take over editing the PEP,
> too, as there were some minor discussion points to add still. I haven't been
> around for the discussioons, though, and it's been a while for everone else,
> I think, so I'd like to rehash and ask for any other open points.
> The one open point that Aahz forwarded me, and is expressed somewhat in
> http://mail.python.org/pipermail/python-dev/2004-September/048695.html , is
> the case where you have a package that you want to transparently supply a
> particular version of a module for forward/backward compatibility, replacing
> a version elsewhere on sys.path (if any.) I see four distinct situations for
> this:
>  1) Replacing a stdlib module (or a set of them) with a newer version, if the
>     stdlib module is too old, where you want the whole stdlib to use the
>     newer version.
>  2) Same as 1), but private to your package; modules not in your package
>     should get the stdlib version when they import the 'replaced' module.
>  3) Providing a module (or a set of them) that the stdlib might be missing
>     (but which will be a new enough version if it's there)
> 1) and 3) are easy to solve: put the module in a separate directory, insert
> that into sys.path; at the front for 1), at the end for 3). Mailman, IIRC,
> does this, and I think it works fine.
> 2) is easy if it's a single module; include it in your package and import it
> relatively. If it's a package itself, it's again pretty easy; include the
> package and include it relatively. The package itself is hopefully already
> using relative imports to get sibling packages. If the package is using
> absolute imports to get sibling packages, well, crap. I don't think we can
> solve that issue whatever we do: that already breaks.
> The real problem with 2) is when you have tightly coupled modules that are
> not together in a package and not using relative imports, or perhaps when
> you want to *partially* override a package. I would argue that tightly
> coupled modules should always use relative imports, whether they are
> together in a package or not (even though they should probably be in a
> package anyway.) I'd also argue that having different modules import
> different versions of existing modules is a bad idea. It's workable if the
> modules are only used internally, but exposing anything is troublesome. for
> instance, an instance of a class defined in foo (1.0) imported by bar will
> not be an instance of the same class defined in foo (1.1) imported by
> feeble.
> Am I missing anything?
> ([*] incorrectly, to be sure, but I have a 'correct' version ready that I'll
> upload in a second; I was trying to confuse Guido into accepting my version,
> instead.)

One thing you're missing here is that the original assertion about the
impossibility of editing the source code of the third-party package
that's being incorporated into your distribution, is simply wrong.
Systematically modifying all modules in a package to change their
imports to assume a slightly different hierarchy can easily be done

I'd also add that eggs promise to provide a different solution for
most concerns.

I believe we should go ahead and implement PEP 338 faithfully without
revisiting the decisions. If we were wrong (which I doubt) we'll have
the opportunity to take a different direction in 2.6.

--Guido van Rossum (home page: http://www.python.org/~guido/)

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