[gentoo-python] Concept for optional Python dependency and USE-flag deps in p-d-ng

["Michał Górny" <mgorny@gentoo.org>]

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Hello,

After some thinking, I think I found an optimal solution to
implementing various kinds of Python dependencies and PYTHON_USE_WITH*.

As a note, all variable names are dirty and not official. Feel free to
suggest better ones.

1. Add a user-defined variable PYTHON_USE. If user needed any specific
flags from a Python dependency, he can set the USE dependency string
there. The string will have to follow the general rules for USE
dependencies and be correct for every Python implementation specified
in PYTHON_COMPAT (this may require USE defaults). For example:

    PYTHON_USE="ncurses,sqlite"

2. Add a eclass-defined variable PYTHON_DEPEND. It will be always
exported, and contain a dependency string created using PYTHON_COMPAT
and PYTHON_USE -- the most simple and most common one, e.g.:

    PYTHON_DEPEND="
      python_targets_pythonX_Y?
        ( dev-python/python:X.Y[ncurses,sqlite] )
    "

3. By default, just append PYTHON_DEPEND to DEPEND and RDEPEND. Provide
an option to disable that. If user wants optional Python, he can
disable it and use ${PYTHON_DEPEND} in his own *DEPEND code. (we could
also provide a quick switch to add 'python? ( )' or '$USEFLAG? ( )').

4. Finally, provide a general, parametrized dependency generator
function which will be useful in all the remaining non-common cases
which can't be handled sanely by any other solution.

For example, the following ::progress string:

  PYTHON_DEPEND="<<[xml]>> database? ( <<[gdbm]>> ) gui? ( <<[tk]>> )"

could be obtained using:

  PYTHON_COMPAT=...
  PYTHON_USE=xml
  inherit python-distutils-ng
  RDEPEND="
    ${PYTHON_DEPEND}
    database? (
      $(python-gen-depend gdbm ${PYTHON_COMPAT})
    )
    gui? (
      $(python-gen-depend tk ${PYTHON_COMPAT})
    )
  "

or even hackier (but faster):

  RDEPEND="
    ${PYTHON_DEPEND}
    database? (
      ${PYTHON_DEPEND//\[xml\]/[gdbm]}
    )
    gui? (
      ${PYTHON_DEPEND//\[xml\]/[tk]}
    )
  "

-- 
Best regards,
Michał Górny

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