[Distutils] setuptools special case Pyrex and break Cython

[Stefan Behnel]

Stefan Behnel wrote:
> Phillip J. Eby wrote:
>> The key design question I ask for setuptools features is "who gets the
>> pain?", since nearly all design decisions will cause *somebody* pain. 
>> Ideally, I want that pain to go to somebody who's in a position to do
>> something that improves things for everybody in the long term.
>>
>> In this case, the only additional pain is for the developers of new
>> tools that process .pyx files (such as yourself).
> 
> Wrong. I cannot fix this as a developer of Cython - at least not without
> providing a fake build_ext.py in a fake Pyrex package, which would make
> installing Pyrex and Cython on the same machine near impossible and which
> would break every tool that requires Pyrex instead on Cython.

Ok, admittedly, I could change sys.path to point it to an internal directory
of the installed Cython distribution, so that setuptools would be tricked into
succeeding with a fake Pyrex import - but how ugly is that?

The right way of fixing this is definitely in setuptools, and the quick fix is
to extend the test import of Pyrex.Distutils.build_ext.build_ext with an
additional test for Cython.Distutils.build_ext.build_ext in the failure case -
if you don't feel like taking the cleaner step of checking for a "build_ext"
replacement...

Stefan