Distutils and Distribute status (part #1)

Someone told me on IRC that it's currently hard to follow what's going on in the packaging front. The truth is that it's almost impossible if you don't read all mails posted in Distutils-SIG.

So here's a quick wrap-up that can save you some time if you are not reading Distutils-SIG.

PEP 345 - Metadata 1.2

We are almost done with the update of PEP 345. This PEP is describing the Metadata fields for a distribution, that get added in the file named PKG-INFO. This file is inserted in all your distribution and also published at PyPI. It's the ID card of your project.

We are adding these fields in the metadata:
- Maintainer : the maintainer's name - Maintainer-email : the maintainer's email - Requires-Python : What Python versions are compatible with this distribution - Requires-External : A list of external dependencies, like "libpng", "libxslt" - Requires-Dist : A list of Python dependencies, from the names founded at PyPI. like "zope.interface" - Provides-Dist : A list of additional distribution names this distribution provides (as a complement to the one provided in "Name") - Obsoletes-Dist : A list of Python dependencies that are incompatible with the current distribution

Another important change is environment markers. An environment marker is a marker that can be added at the end of a field after a semi-colon (';'), to add a condition about the execution environment.

Requires-Dist: pywin32, bar > 1.0; sys.platform == 'win32'

Obsoletes-Dist: pywin31; sys.platform == 'win32'

Requires-Dist: foo; os.machine == 'i386'

Requires-Dist: bar; python_version == '2.4' or python_version == '2.5'

Requires-External: libxslt; 'linux' in sys.platform

This will allow developers to define different conditions depending on the target platform. Moreover, this will allow tools like Pip to get a list of all dependencies for a given project and a given platform just by querying PyPI, and with no downloads or build required !

Last, for all the fields that manipulates versions, PEP 345 will use the version scheme described in PEP 386.

PEP 386 - Version scheme

We've designed in PEP 386 a version scheme that works with most Python software we know about. This version scheme comes with a new version comparison algorithm that will be provided by Distutils.

The scheme is in pseudo-regexpr (read the PEP for more details):

Don't be afraid ! It looks complex but it's not. The apparent complexity is due to the fact that we need to be able to work with development versions and post-release versions.

There are good chances that your project already works with this version scheme. If you want to give it a shot, there's a prototype you can play with in an external repo here: http://bitbucket.org/tarek/distutilsversion/

PEP 376 - Installation standard

PEP 376 is quite completed now. We have our "standard" for site-packages, we know how to query installed projects, and how to remove them.

The discussions are now focusing on the "data" problem. Which is : how to describe in Distutils, in a more elegant way, the data files you are using, such as images, man files etc.

This is required to provide to developers more control on how their data files are installed on the target system, and to the packagers more tools to re-package a Python distribution.

Wolodja Wentland has been doing a lot of work in this area and leads this "data" effort. You can follow the discussion on this work in the Python wiki, starting at: http://wiki.python.org/moin/Distutils/DiscussionOverview.

PEP 382 -Namespaces packages

Distribute comes with a namespace package system, that allows you to have packages under the same namespace, spread into several distributions.

That's what Plone and Zope use to be able to release all those plone.* and zope.* distributions.

Martin von Loewis proposed to implement it in Python, and this is described in PEP 382.

We are now waiting for Martin to implement it, and are ready to drop in Distribute 0.7.x the namespace feature in favor of supporting the PEP 382 one.

Distutils redesign discussions

One thing that makes Distutils a bit hard to work with, is how commands are designed. David Cournapeau (from the Numpy project) gave us an example of a use case that makes it hard. He basically needs to run the "build" command knowing the finalized options from the "install" command.

In other words, when you call something like :
$ python setup.py install --prefix=/some/place

The install command will use the prefix option to cook some other options. The build command that needs all the options needs in that case to look over the install command to get the values.

This is not optimal because it means that a build command depends on an install command to run. It also makes options redundants from one command to the other.

The solution we are going to try is to create a new command, called configure, that will be in charge of building a file with all the options that are required by the build command and the install command.

This is not new. It has been implemented years ago in 4suite, and it's the philosophy behind tools like scons, etc: a configure/make/make install principle applied to a Python project.

This redesign is going to occur in Distribute 0.7. Once it's ready, if the community has tried it and gave us positive feedback, I'll push it in Distutils.

It might happen before Python 2.7 is out, it might not.

Other topics

There are many other topics, like PyPI mirroring (PEP 381) etc. I'll write a blog entry later for these.

Comments !