In Distutils, every package has some metadata fields, defined in PEP 314.
The setup.py script is the place where you provide them, by calling the setup function, located in distutils.core. Each argument passed to the function can be one of these metadata.
So basically, you can describe your distribution in the setup.py file like this:
from distutils.core import setup
setup(name='MyDistribution', version='0.1', description='cool',
packages=['my_package'], ext_modules=[Extension('foo', 'foo.c')])
Notice that packages and ext_modules in this example are not part of the Metadata fields. They are extra fields used by some commands.
From there, various distutils commands can be called using this script. They will get these options and act upon.
For instance, the sdist command will build a source distribution and create a static PKG-INFO file that contains the metadata fields. It will extract them from the arguments you’ve passed to setup.py.
The install command will install this PKG-INFO file in your Python installation alongside your packages and modules (since Python 2.5) and some tools like Distribute or Setuptools will let you read these information once the distribution is installed.
You can even get the metadata fields values by asking for them directly through setup.py:
$ python setup.py --name
MyDistribution
Another example : the register command can send the metadata or your distribution to PyPI. They will be made available on PyPI website and also through its XML-RPC interface:
>>> import xmlrpclib
>>> server = xmlrpclib.Server('http://pypi.python.org/pypi')
>>> server.release_data('distribute', '0.6')['author']
'The fellowship of the packaging'
>>> server.release_data('distribute', '0.6')['keywords']
'CPAN PyPI distutils eggs package management'
Limitations of metadata
Metadata are pretty handy, but there are some obstructing limitations we bumped into when we started to work on packaging matters during last Pycon.
Platform-dependant metadata
We wanted to extend the Metadata fields list in order to add a “requires” field that can be used to list the requirements (in term others python packages or modules).
For instance, if you want to define that your project depends on simplejson, you could write:
from distutils.core import setup
setup(name='MyDistribution', version='0.1', description='cool',
packages=['my_package'], ext_modules=[Extension('foo', 'foo.c')],
requires=['simplsjon'])
This is not a new proposal. It was proposed in PEP 345, but never really used.
Since then, Setuptools provided a similar field, called “install_requires” together with easy_install script that acts a bit like a package manager. easy_install reads the requirements and install them when you install a distribution.
But the limitation of those requirement fields is that they might be platform-dependant. For example, you don’t need to install simplejson anymore under 2.6 since a json library was included in the standard library. In other cases you might have different dependencies depending if you run under windows or linux, and so on.
So to be able to get the metadata right, you have to work a little bit in your setup.py file:
from distutils.core import setup
import sys
if sys.version_info[0] == 2 and sys.version_info[1] < 6:
requires = ['simplejson']
else:
requires = []
setup(name='MyDistribution', version='0.1', description='cool',
packages=['my_package'], ext_modules=[Extension('foo', 'foo.c')],
requires=requires)
But the metadata will only be available at install time, when the install command will execute the code of setup.py on the target system.
Code-dependant metadata
In other words, once a field like requires is added in the Metadata, you will not know for sure if it’s reliable when you look at the project page at PyPI. That’s because the metadata you will see there will be the one created by the person that called the register command and sent the result. This result is tighted to his environment, not yours.
To be able to get the metadata for your environment you will need to run that code again, by downloading the package, then running a setup.py command.
Let’s try to do it with the lxml source distribution. Let’s try to get the name field :
$ python setup.py --name
Building lxml version 2.2.2.
NOTE: Trying to build without Cython, pre-generated 'src/lxml/lxml.etree.c' needs to be available.
Using build configuration of libxslt 1.1.12
Building against libxml2/libxslt in the following directory: /usr/lib
lxml
What happened here ? Frankly I am not sure. But asking for the name (that appears on the last line) called a bunch of code located in the distribution.
I could probably ask the lxml team to fix this output, and make sure setup.py can still be used to work with the metadata, but this was just to demonstrate a flaw in the way Distutils works : you need to run third party code just to get the metadata of a distribution you’re not even sure you are going to install on your system.
The setup.cfg file
Part of the problem can be resolved by putting the metadata in a static file alongside setup.py. As a matter of fact, the setup.cfg file is already used by distutils to store some options. There’s even a global section that can be used to set the metadata into the Distribution object Distutils creates when you run setup(). Using the global section that way is not documented and probably not intended. What’s intended is to be able to set some global options like “verbose” or such things.
See http://docs.python.org/install/index.html#syntax-of-config-files
But the code is a generic setter, that allows you to pass any field (so the metadata). Call it a bug if you want, but I was pretty excited to see that I could pass my metadata to Distutils through it. Unfortunately these values are not passed to the DistributionMetadata subobject in Distutils, so it doesn’t work exactly like the arguments passed to setup(). Too bad 
I could change this right away in the code, but we have better plans I think.
A new setup section
Instead of working in the global section which should stay specific to running options, let’s create a new section and put the metadata in them.
[setup]
name: MyDistribution
version: 0.1
description: cool
The setup.py script stays, but is now not containing any metadata field, and does only contain what I would call “working arguments”. e.g. argument used by commands that are not part of the Metadata:
from distutils.core import setup
setup(packages=['my_package'], ext_modules=[Extension('foo', 'foo.c')])
What about platform-dependant fields ?
In order not to require any third party code to read the metadata, we need a way to express platform-dependant fields in the setup.cfg file.
The proposed way is to have platform-dependant sections :
[setup]
name: MyDistribution
version: 0.1
description: cool
conditional-sections: py25
[py25]
condition: python_version == '2.5'
requires: simplejson
The py25 section is read only if the expression is true.
Another example:
[setup]
name: MyDistribution
version: 0.1
description: cool
conditional-sections: py25, py26
[py25]
condition: python_version == '2.5' or python_version == '2.4'
requires: simplejson
[py26]
condition: python_version == '2.6' and sys_platform == 'win32'
requires: bar
here, “bar” will be installed under Python 2.6 under Windows, and “simpljson” under Python 2.5 or 2.4 on any platform.
Distutils will provide a new function that is able to interpret the expressions provided in the condition, and calculate the metadata depending on the platform.
That’s still some code we are running here, but:
- We are restricting the execution context to the bare minimum: python_version, sys_platform, os_name, and all values returned by os.uname()
- The function will be vanilla Python: you will be able to extract the metadata without running a third party code, and knowing that the execution is restricted to a few string comparisons.
- The code can be executed at PyPI without any potential security issue, meaning that the XML-RPC functions will be able to send you back the metadata of a packages depending on your environment. In other word, a package manager would be able to list all the dependency of a distribution for the target platform without downloading any of these distribution.
There will always be edge cases
For the 1% of distributions that need more work to calculate the metadata, setup.py will still be present and any option passed as an argument will override a value provided by setup.cfg. They’ll just have to add a flag in the setup.cfg file, indicating that it does not provides all the metadata, and that running setup.py is required:
[setup]
name: MyDistribution
version: 0.1
description: cool
static-metadata: false
If this flag is present, people will now that running setup.py is mandatory to get the full set of metadata.
For example, if the web service provided at PyPI to get the metadata, will be able to return a platform specific set if we provide the target environment. Let’s say we add in distutils an ‘execution_environment‘ that returns the environment used to interpret the setup.cfg file:
>>> import xmlrpclib
>>> server = xmlrpclib.Server('http://pypi.python.org/pypi')
>>> from distutils.util import execution_environment
>>> execution_environment
{'os_version': 'Darwin Kernel Version 9.8.0: Wed Jul 15 16:55:01 PDT 2009; root:xnu-1228.15.4~1/RELEASE_I386',
'os_name': 'posix',
'python_version': '2.6',
'os_release': '9.8.0',
'os_sysname': 'Darwin',
'os_nodename': 'MacZiade',
'os_machine': 'i386',
'sys_platform': 'darwin'}
>>> server.release_data('MyDistribution', '0.6', execution_environment)['requires']
['foo', 'bar']
>>> server.release_data('MyDistribution', '0.6', execution_environment)['static-metadata']
True
PyPI will be able to generate the metadata by interpreting the setup.cfg file with the execution_environment info.
What happens now ?
I won’t write a PEP for this. I don’t think it’s necessary because this feature is backward compatible, and if people don’t use it in Python 2.7 and 3.2, it will just fade away, like other things in Distutils.
But we need to reach a consensus at Distutils-SIG then inform about it at Python-dev. I just hope we will have this consensus real quickly, unlike most topics we are working on for a year
Or maybe I should be a bit of a dictator for this feature and just go ahead and add it ? Because as Brett told me several times, it’s impossible to make everyone happy about everything. And I’d like to see Distutils move on. There’s so much left to do…
What do you think ? How do you like that feature ?
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