[Distutils] Importable wheels using distlib/distil

[Nick Coghlan]

On Sat, Mar 30, 2013 at 6:42 AM, PJ Eby <pje at telecommunity.com> wrote:
> On Fri, Mar 29, 2013 at 3:55 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> 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.
>
> How is that different from replacing "<versioned-packages>" with the
> path of the versioned package being added?

You don't lose the place where you want the inserts to happen. Without
the marker, you end up having to come up with a heuristic for "make
insertions here" and that gets messy as you modify the path
(particularly since other code may also modify the path without your
involvement). Using a path hook to say "process these additional path
entries here" cleans all that up and lets you precisely control the
relative precedence of the additions as a group, without needing to
care about their contents, or the other contents of sys.path.

Cheers,
Nick.

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