PEP 365 – Adding the pkg_resources module
- Phillip J. Eby <pje at telecommunity.com>
- Standards Track
This PEP proposes adding an enhanced version of the
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
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.
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
instead, it would rely on
SomePackage having pure-Python
files listed for download via the PyPI XML-RPC API, and the eggs would
be placed in the
$PYTHON_EGG_CACHE directory, 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
simply by supplying the tool’s name on the command line.
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
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
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
sys.path. In environments where Python packages are
installed using system package tools (like RPM), the
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
pkg_resources implementation is maintained in the Python
SVN repository under
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
- http://peak.telecommunity.com/DevCenter/PkgResources and
(These HTML versions are for setuptools 0.6; they may not reflect all
of the changes found in the Subversion trunk’s
This document has been placed in the public domain.
Last modified: 2017-11-11 19:28:55 GMT