[Distutils] setuptools' "Feature" feature

[P.J. Eby]

At 02:29 PM 7/2/2010 -0400, Tres Seaver wrote:
>I haven't found any docs (outside the code) for the
>setuptools.dist.Feature,

That's because it's essentially deprecated - an experimental thing 
that turned out to be, well, a failed experiment.

The problem is that there really needed to be better ways to store 
the build state with respect to features, not to mention 
distribution-level information about them.  So, they only work 
correctly if you pass them to *every* setup.py command, while never 
changing them.  To really work right, there'd need to be a 
"configure" command, and some sort of option storage.

And, since most of the *other* use cases for the original features 
design were supplanted by "extras" and install_requires, it was 
pretty much left to optional C extensions, which might as well be 
handled by, well, optional C extensions.  ;-)

So, I haven't done anything with features in ages, and didn't 
document them because (AFAIK), they were only used by some ancient 
packages of my own, such as the original PEAK mega-distribution, 
PyProtocols, and RuleDispatch -- mostly to enable or disable various 
C extensions.


>- - Feature instances represent optionally-included bits of the
>   distribution.
>
>- - Normally, features are disabled by default, and can be enabled
>   by passing '--with-<featurename>' to setup.py.
>
>- - Features declared as 'standard' are enabled by default, and can be
>   disabled by passing '--without-<featurename>' to setup.py.
>
>- - Features have an 'available' attribute, which can be set to False
>   in setup.py to disable a feature based on runtime introspection
>   (e.g., using sys.version_info or os.name).
>
>- - It appears that it should be possible to pass an instance with
>   a '__nonzero__' method as the 'available':  the code seems only
>   to use it in boolean tests.
>
>- - Features may depend on (force inclusion of) other features.
>
>- - Enabled features do their actual work by adding **kwargs to
>   the args passed directly to setup.py
>
>- - Distributions created via an invocation to setup.py are not labeled
>   in any way to indicate any '--with-<featurename>' (or '--without')
>   passed to setup.py during their creation.
>
>- - There is no way to enable / disable a feature when installing a
>   distribution via easy_install.
>
>The particular use case which led me to investigate Features was easing
>packaging of projects with optional C extensions, such that they could
>be installable on Jython, IronPython, Windows, or GAE.  It doesn't seem
>possible to figure out whether a compiler actually exists:  jython's
>distutils lies and creates a compiler which raises exceptions, for instance.

Yeah, that's another reason why Features don't even do the optional 
compile thing that well.  Not Jython specifically, but rather, I 
never decided on a good way to handle optional C extensions, that 
doesn't involve trapping compilation errors.  (I hope distutils2 
allows you to declare extensions optional; it's such a common use case.)