python-peps/pep-0390.txt

PEP: 390
Title: Static metadata for Distutils
Version: $Revision: 70705 $
Last-Modified: $Date: 2009-03-30 06:53:39 +0200 (Lun 30 mar 2009) $
Author: Tarek Ziadé <tarek@ziade.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 10-October-2009
Python-Version: 2.7 and 3.2
Post-History:

Abstract
========

This PEP describes a new section and a new format for the ``setup.cfg`` file,
that allows describing the Metadata of a package without using ``setup.py``.

Rationale
=========

Today, if you want to list all the Metadata of a distribution (see PEP 314)
that is not installed, you need to use the ``setup.py`` command line interface.

So, basically, you download it, and run::

   $ python setup.py --name
   Distribute

   $ python setup.py --version
   0.6.4

Where ``name`` and ``version`` are metadata fields. This is working fine but
as soon as the developers add more code in ``setup.py``, this feature might 
break or in worst cases might do unwanted things on the target system.

Moreover, when an OS packager wants to get the metadata of a distribution
he is re-packaging, he might encounter some problems to understand
the ``setup.py`` file he's working with.

So the rationale of this PEP is to provide a way to declare the metadata
in a static configuration file alongside ``setup.py`` that doesn't require
any third party code to run.


Adding a ``metadata`` section in ``setup.cfg``
==============================================

The first thing we want to introduce is a ``[metadata]`` section, in the 
``setup.cfg`` file, that may contain any field from the Metadata::

   [metadata]
   name = Distribute
   version = 0.6.4

The ``setup.cfg`` file is used to avoid adding yet another configuration
file to work with in Distutils.

This file is already read by Distutils when a command is executed, and
if the ``metadata`` section is found, it will be used to fill the metadata
fields. If an option that corresponds to a Metadata field is given to
``setup()``, it will override the value that was possibly present in
``setup.cfg``.

Context-dependant sections
==========================

The ``metadata`` section will also be able to use context-dependant sections.

A context-dependant section is a section with a condition about the execution
environment. Here's some examples::

   [metadata]
   name = Distribute
   version = 0.6.4

   [metadata:sys_platform == 'win32']
   requires = pywin32
   requires = bar > 1.0

   [metadata:os_machine == 'i386']
   requires = foo

   [metadata:python_version == '2.4' or python_version == '2.5']
   requires = bar

   [metadata:'linux' in sys_platform]
   requires = baz

Every ``[metadata:condition]`` section will be used only if the condition
is met when the file is read. The background motivation for these
context-dependant sections is to be able to define requirements that varies
depending on the platform the distribution might be installed on. 
(see PEP 314).

The micro-language behind this is the simplest possible: it compares only
strings, with the ``==`` and ``in`` operators (and their opposites), and 
with the ability to combine expressions. It makes it also easy to understand 
to non-pythoneers.

The pseudo-grammar is ::

    EXPR [in|==|!=|not in] EXPR [or|and] ...

where ``EXPR`` belongs to any of those:

- python_version = '%s.%s' % (sys.version_info[0], sys.version_info[1])
- os_name = os.name
- sys_platform = sys.platform
- os_version = os.uname()[3]
- os_machine = os.uname()[4]
- a free string, like ``2.4``, or ``win32``

Distutils will provide a function that is able to read the metadata
of a distribution, given a ``setup.cfg`` file, for the execution environment::

   >>> from distutils.util import local_metadata
   >>> local_metadata('setup.cfg')
   <DistributionMetadata instance>

This means that a vanilla Python will be able to read the metadata of a
package without running any third party code.

Compatiblity
============

This change is fully backward compatible since it just adds a section in the
``ConfigParser``-compatible ``setup.cfg`` file.

Limitations
===========

We are not providing ``<`` and ``>`` operators at this time, and
``python_version`` is a regular string. This implies using ``or`` operators
when a section needs to be restricted to a couple of Python versions.
Although, if PEP 386 is accepted, ``python_version`` could be changed
internally into something comparable with strings, and 
``<`` and ``>`` operators introduced.

Last, if a distribution is unable to set all metadata fields in ``setup.cfg``,
that's fine, the fields will be set to ``UNKNOWN`` when ``local_metadata`` is
called. Getting ``UNKNOWN`` values will mean that it might be necessary to 
run the ``setup.py`` command line interface to get the whole set of metadata.

Acknowledgments
===============

The Distutils-SIG.


Copyright
=========

This document has been placed in the public domain.


..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8
   End: