[Distutils] Are there best practices on creating/using egg extras?

[Dave Peterson]

Phillip J. Eby wrote:
> At 01:50 PM 7/20/2007 -0500, Dave Peterson wrote:
>> Hello,
>>
>> I'm not sure if I've painted Enthought into a corner or not, but I can't
>> figure out if there is a way to help users of a library, delivered as an
>> egg, to know and/or maintain dependencies on extras declared in that
>> egg.  By which I mean that if my library, called X, declares extras 1,
>> 2, and 3, and the source for X includes all the code that implements the
>> features in extras 1, 2, and 3, and the extras document the external
>> dependencies component X has to get those extras to work, how does
>> someone importing from the API in X know whether their dependency on it
>> should be 'X', 'X[1]', X[1,2]', etc. ?   After all, all the API methods
>> are already there even if they didn't install the extras.   So far the
>> only mechanisms I can see are (a) a manual one which depends on people
>> (everyone using X!) knowing the internals of X such that they can tell
>> that if they import symbols a, b, or c, then that means they need extra
>> 1, etc., and (b) trial and error iteration driven by unit / integration
>> tests.
>>
>> Am I just misusing or misunderstanding extras here?
>
> Note that if you have code that imports from your extras, you can 
> always include:
>
>    pkg_resources.require("X[1]")
>
> right before the imports.  Then, instead of an obscure import error, 
> users will get an obscure DistributionNotFound error that at least 
> tells them one of the packages they'll need to install.  ;-)

Right.  Always better to have an obscure error that is more informative 
about how to solve it. :-)

But someone still needs to know when to insert 
pkg_resources.require("X[1]") in their code.  Which means knowing which 
symbols in X's API need a given extra.  So I'm back at my original 
problem, no?

-- Dave