[Distutils] Importable wheels using distlib/distil

[Nick Coghlan]

On Fri, Mar 29, 2013 at 2:02 AM, Vinay Sajip <vinay_sajip at yahoo.co.uk> wrote:
> Thanks for the feedback. How about if I change mount()/unmount() to:
>
>     def mount(self, append=False, destdir=None):
>         """
>         Unzip the wheel's contents to the specified directory, or to
>         a temporary directory if destdir is None. Add this directory to
>         sys.path, either appending or prepending according to whether
>         append is True or False.

No, mutating sys.path for versioned imports is a broken design. You
end up with two possibilities:

* If you append, then you can't override modules that have a default
version available on sys.path. This is not an acceptable restriction,
which is why pkg_resources doesn't do it that way
* If you prepend, then you have the existing pkg_resources failure
mode where it can accidentally shadow more modules than intended. This
is a nightmare to debug when it goes wrong (it took me months to
realise this was why a system install of the main application I work
on was shadowing the version in source checkout when running the test
suite or building the documentation).

The correct way to do it is with a path hook that processes a special
"<versioned-packages>" marker label in sys.path (probably placed after
the standard library but before site-packages by default). Any mounted
directories would be tracked by that path hook, but never included
directly in sys.path itself.

See http://mail.python.org/pipermail/distutils-sig/2013-March/020207.html
for more on how this could be handled (consider mount/unmount as the
lower level API for actually adding new path entries directly to the
dynamic importer).

Cheers,
Nick.

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