PEP 345 and 386 accepted — Summary of changes
by Tarek Ziadé
Several PEPs were accepted this month by Guido, and among them PEP 345 and PEP 386, which are about Python packaging. I am summarizing in this entry the main changes we’ve made.
PEP 345 – Metadata v1.2
This PEP is about the Metadata that gets added in your project when you build a distribution using Distutils or a Distutils-based tool. Those are fields like “name” or “version” you pass as options in your setup.py file. They eventually land in a PKG-INFO file then at PyPI and in each Python’s site-packages where the project is installed. They are also useful when your project gets repackaged by an OS packager.
PEP 345 adds some new fields :
- Maintainer : a string containing the maintainer’s name
- Maintainer-email : a string containing the maintainer’s e-mail
- Requires-Python : Python version(s) that the distribution is guaranteed to be compatible with.
- Requires-External : Describes some dependencies in the system that the distribution is to be used
- Requires-Dist : String naming some other distutils project required by this distribution.
- Provides-Dist : String naming a Distutils project which is contained within this distribution.
- Obsoletes-Dist : String describing a distutils project’s distribution which this distribution renders obsolete
- Project-URL : String containing a browsable URL for the project and a label for it, separated by a comma.
Maintainer and Maintainer-email were added because people were confused about the maintainer and maintainer_email options they have in Distutils. Before PEP 345, if you used the author and the maintainer fields, one was dropped and one was kept to fill the Author metadata field.
Requires-Python was added so people could list the Python versions their project is compatible with. We do have classifiers already for that in the Trove classifier, but this new field is more than a simple field : the version string that is used supports a syntax that makes it possible to describe any set of Python versions. See the version specifier of the PEP.
Remember the requires.txt metadata Setuptools introduced together with the install_requires option ? Requires-Dist is comparable to this and will let you define dependencies on other Python projects. Distutils had the Requires field, but it was defining dependencies at the module level and this was never really used by the community. So Requires is gone in 1.2. Last, Requires-Dist is using a version comparison scheme described in details in the version specifier section.
Provides-Dist gives you the ability among other things to reorganize your project names or to distribute a subproject in several projects. For instance, the project called ZODB used to include the project transaction that is also distributed as a standalone project now. If ZODB is installed, and if you need transaction, you won’t have to install it again.
Obsoletes-Dist will let you make sure a project that is incompatible with your project is not installed at the same time.
Project-URL is useful to provide a list of URLs for your project like a browsable repository or a tracker. The goal will be to add a small box on PyPI project pages for these URLs, so developers can emphasize them. This will hopefully address the complaints PyPI had in these past months when the comment system was added.
Some small changes were made on existing fields. One important change is on the Description field (long_description in setup.py). Before PEP 345, once this value was written in the metadata file, its empty lines and spaces at the beginning of lines were truncated. In other words, if a tool was reading back this value, its reSTrucured syntax was broken. This is no longer the case.
Once I’ve finished implementing PEP 345, you will be able to read back a project’s metadata usig the DistributionMetadata class. See an example.
When Pip wants to install all the dependencies a project requires, it has to follow these steps:
- download the project from PyPI
- execute a Distutils command on setup.py, like egg_info, to get the metadata. And in particular the list of requirements
This is mandatory because the metadata are not statically defined and the developer might need to run some code in his setup.py to know what dependencies are required, depending on the target platform.
if sys.platform == 'win32': install_requires = ['pywin32'] else: install_requires = 
In other words, there’s no way to get the metadata without running third-party code. Environment markers fix this issue in most cases by providing a micro-language that can be used at the field level. At the end, you can write things like that in the metadata:
Requires-Dist: pywin32 (>1.0); sys.platform == 'win32'
And Distutils will provide a tool to parse and execute this expression, so you know if your target platform have to use this metadata field. Meaning that Pip or other tools will be able to read metadata of a project without running any third party code. For example to get all the dependencies of a project depending on the target platform just by querying PyPI.
See all the details in the section in the PEP.
Throughout all the PEP 345, we had to compare versions. To be able to perform this, we need to have a common standard for versions numbers. That’s what PEP 386 was written for.
The idea is not to force people to version their project using this version scheme, but rather to provide a scheme that is good enough for interoperability and that is human readable.
PEP 386 provides this scheme (in pseudo-format) :
The corresponding regular expression is:
expr = r"""^ (?P<version>\d+\.\d+) # minimum 'N.N' (?P<extraversion>(?:\.\d+)*) # any number of extra '.N' segments (?: (?P<prerel>[abc]|rc) # 'a' = alpha, 'b' = beta # 'c' or 'rc' = release candidate (?P<prerelversion>\d+(?:\.\d+)*) )? (?P<postdev>(\.post(?P<post>\d+))?(\.dev(?P<dev>\d+))?)? $"""
The scheme handles these cases:
- development versions
- final versions
- post-release versions
- development versions of post-release versions
Here are some examples:
>>> from verlib import NormalizedVersion as V >>> (V('1.0a1') ... < V('1.0a2.dev456') ... < V('1.0a2') ... < V('1.0a2.1.dev456') ... < V('1.0a2.1') ... < V('1.0b1.dev456') ... < V('1.0b2') ... < V('1.0b2.post345') ... < V('1.0c1.dev456') ... < V('1.0c1') ... < V('1.0.dev456') ... < V('1.0') ... < V('1.0.post456.dev34') ... < V('1.0.post456')) True
This can look utterly complex to you, but in fact there are good chances that your version scheme is already compatible with this one. We’ve tested in on PyPI, and 88 % of the projects’ distributions were recognized.
The suggest_rational_version function
Let’s face it: there are hundreds of valid versionning schemes that are not compatible with what we’ve done. So we are adding a function called suggest_rational_version that can be used to transform a version that is not “PEP-386” compliant into one that is compliant.
This does a number of simple normalizations to the given string, based on an observation of versions currently in use on PyPI.
Given a dump of those versions on February 10th 2010, the function has given those results out of the 9066 distributions PyPI had:
- 8058 (88.88%) already match NormalizedVersion without any change
- 795 (8.77%) match when using this suggestion method
- 213 (2.35%) don’t match at all.
And here’s an extract of the 2.35% unrecognized scheme:
- working proof of concept
- 1 (first draft)
In other words, they are unusable anyways. If you want to try this on your own versions, grab the code at http://bitbucket.org/tarek/distutilsversion/. And if you version doesn’t match at all and you think its a mistake, let me know so we can work your case.
Conclusion + my rant on packaging
I have started working seriously on packaging issues a year ago. And I kept on hearing complaints on how packaging sucks hard. That was frustrating since some folks and I were working hard to change things (and we still do). I kept seeing the same people ranting about packaging, and most of the time they were not willing to help around. I guess that’s just me being naive, but let me say it one more time 🙂
If you don’t like how packaging works in Python, stop complaining and come in the Distutils mailing list, or in #distutils on freenode and help us !
I am now really glad that these PEPs were accepted by Guido because they are the first milestone we had to reach to improve things for real in packaging. Python 2.7 is going to make a big jump forward in this area !
Next milestone we are trying to reach for 2.7 is to finish PEP 376 (uninstall feature, querying installed packages, etc)