[Distutils] How to implement ‘setup.py’ functionality that itself needs third-party distributions (was: Module from install breaks subsequent install of different distribution)

[Ben Finney]

Tres Seaver <tseaver at palladion.com> writes:

> setuptools itself is extensible by means of entry points. Both entry
> points in your example (as cited by Marius) demonstrate that: one adds
> support for a new keyword argument to 'setup()'[1], and the other
> defines a new "writer" for 'egg-info'[2]. By design, both of those are
> supposed to be loaded / available for any invocation of 'setup()' in a
> Python where the are installed (not merely for packages which
> "mention" them).

What recourse do I have, then?

I'm using entry points because it seems to be the only way I can declare
functionality that resides in a module alongside the ‘setup.py’ which
itself needs third-party packages.

* During the distribution build stage (‘./setup.py build’ or earlier),
  I want to parse a reST document and derive some of the distribution
  metadata from that.

* The ‘setup.py’ is the wrong place for this; it's complex and deserves
  its own module which can be imported for unit testing.

* This also is totally unrelated to the functionality this distribution
  installs, so it doesn't belong in any of the packages to distribute
  for installation.

* So I place it in a separate top-level module, ‘version’, only for use
  within ‘setup.py’.

* That module itself needs a third-party distribution (‘docutils’) from
  PyPI. So I need that distribution to be installed *before* the
  ‘version’ module can be used. Apparently the ‘setup_requires’ option
  is the only way to do that.

* Then, in order to declare how that functionality is to be used for
  commands like ‘./setup.py egg_info’, I have no other way than
  declaring a Setuptools entry point.

* Declaring a Setuptools entry point makes Setuptools think the
  distribution-specific module must be imported by every other
  distribution in the same run. Of course it's not available there so
  those distributions fail to install.

* It even thwarts the installation of ‘docutils’, the very distribution
  that is needed for ending this circular dependency.

What am I missing? How can I implement complex functionality specific to
packaging this distribution, without making an utter mess?

-- 
 \            “The whole area of [treating source code as intellectual |
  `\    property] is almost assuring a customer that you are not going |
_o__)               to do any innovation in the future.” —Gary Barnett |
Ben Finney