[Python-Dev] PEP 0365: Adding the pkg_resources module

[Phillip J. Eby]

I wanted to get this in before the Py3K PEP deadline, since this is a 
Python 2.6 PEP that would presumably impact 3.x as well.  Feedback welcome.


PEP: 365
Title: Adding the pkg_resources module
Version: $Revision: 55032 $
Last-Modified: $Date: 2007-04-30 20:24:48 -0400 (Mon, 30 Apr 2007) $
Author: Phillip J. Eby <pje at telecommunity.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 30-Apr-2007
Post-History: 30-Apr-2007


Abstract
========

This PEP proposes adding an enhanced version of the ``pkg_resources``
module to the standard library.

``pkg_resources`` is a module used to find and manage Python
package/version dependencies and access bundled files and resources,
including those inside of zipped ``.egg`` files.  Currently,
``pkg_resources`` is only available through installing the entire
``setuptools`` distribution, but it does not depend on any other part
of setuptools; in effect, it comprises the entire runtime support
library for Python Eggs, and is independently useful.

In addition, with one feature addition, this module could support
easy bootstrap installation of several Python package management
tools, including ``setuptools``, ``workingenv``, and ``zc.buildout``.


Proposal
========

Rather than proposing to include ``setuptools`` in the standard
library, this PEP proposes only that ``pkg_resources`` be added to the
standard library for Python 2.6 and 3.0.  ``pkg_resources`` is
considerably more stable than the rest of setuptools, with virtually
no new features being added in the last 12 months.

However, this PEP also proposes that a new feature be added to
``pkg_resources``, before being added to the stdlib.  Specifically, it
should be possible to do something like::

     python -m pkg_resources SomePackage==1.2

to request downloading and installation of ``SomePackage`` from PyPI.
This feature would *not* be a replacement for ``easy_install``;
instead, it would rely on ``SomePackage`` having pure-Python ``.egg``
files listed for download via the PyPI XML-RPC API, and the eggs would
be placed in the ``$PYTHONEGGS`` cache, where they would **not** be
importable by default.  (And no scripts would be installed)  However,
if the download egg contains installation bootstrap code, it will be
given a chance to run.

These restrictions would allow the code to be extremely simple, yet
still powerful enough to support users downloading package management
tools such as ``setuptools``, ``workingenv`` and ``zc.buildout``,
simply by supplying the tool's name on the command line.


Rationale
=========

Many users have requested that ``setuptools`` be included in the
standard library, to save users needing to go through the awkward
process of bootstrapping it.  However, most of the bootstrapping
complexity comes from the fact that setuptools-installed code cannot
use the ``pkg_resources`` runtime module unless setuptools is already
installed. Thus, installing setuptools requires (in a sense) that
setuptools already be installed.

Other Python package management tools, such as ``workingenv`` and
``zc.buildout``, have similar bootstrapping issues, since they both
make use of setuptools, but also want to provide users with something
approaching a "one-step install".  The complexity of creating bootstrap
utilities for these and any other such tools that arise in future, is
greatly reduced if ``pkg_resources`` is already present, and is also
able to download pre-packaged eggs from PyPI.

(It would also mean that setuptools would not need to be installed
in order to simply *use* eggs, as opposed to building them.)

Finally, in addition to providing access to eggs built via setuptools
or other packaging tools, it should be noted that since Python 2.5,
the distutils install package metadata (aka ``PKG-INFO``) files that
can be read by ``pkg_resources`` to identify what distributions are
already on ``sys.path``.  In environments where Python packages are
installed using system package tools (like RPM), the ``pkg_resources``
module provides an API for detecting what versions of what packages
are installed, even if those packages were installed via the distutils
instead of setuptools.


Implementation and Documentation
================================

The ``pkg_resources`` implementation is maintained in the Python
SVN repository under ``/sandbox/trunk/setuptools/``; see
``pkg_resources.py`` and ``pkg_resources.txt``.  Documentation for the
egg format(s) supported by ``pkg_resources`` can be found in
``doc/formats.txt``.  HTML versions of these documents are available
at:

* http://peak.telecommunity.com/DevCenter/PkgResources and

* http://peak.telecommunity.com/DevCenter/EggFormats

(These HTML versions are for setuptools 0.6; they may not reflect all
of the changes found in the Subversion trunk's ``.txt`` versions.)


Copyright
=========

This document has been placed in the public domain.