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PEP:390
Title:Static metadata for Distutils
Version:de64d7d6ae6d
Last-Modified:2013-05-18 18:08:49 +1000 (Sat, 18 May 2013)
Author:Tarek Ziadé <tarek at ziade.org>
BDFL-Delegate:Nick Coghlan
Discussions-To:<distutils-sig at python.org>
Status:Rejected
Type:Standards Track
Content-Type:text/x-rst
Created:10-October-2009
Python-Version:2.7 and 3.2
Post-History:
Resolution:http://mail.python.org/pipermail/distutils-sig/2013-April/020597.html

Abstract

This PEP describes a new section and a new format for the setup.cfg file, that allows describing the Metadata of a package without using setup.py.

Rejection Notice

As distutils2 is no longer going to be incorporated into the standard library, this PEP was rejected by Nick Coghlan in late April, 2013.

A replacement PEP based on PEP 426 (metadata 2.0) will be created that defines the minimum amount of information needed to generate an sdist archive given a source tarball or VCS checkout.

Rationale

Today, if you want to list all the Metadata of a distribution (see PEP 314) that is not installed, you need to use the setup.py command line interface.

So, basically, you download it, and run:

$ python setup.py --name
Distribute

$ python setup.py --version
0.6.4

Where name and version are metadata fields. This is working fine but as soon as the developers add more code in setup.py, this feature might break or in worst cases might do unwanted things on the target system.

Moreover, when an OS packager wants to get the metadata of a distribution he is re-packaging, he might encounter some problems to understand the setup.py file he's working with.

So the rationale of this PEP is to provide a way to declare the metadata in a static configuration file alongside setup.py that doesn't require any third party code to run.

Adding a metadata section in setup.cfg

The first thing we want to introduce is a [metadata] section, in the setup.cfg file, that may contain any field from the Metadata:

[metadata]
name = Distribute
version = 0.6.4

The setup.cfg file is used to avoid adding yet another configuration file to work with in Distutils.

This file is already read by Distutils when a command is executed, and if the metadata section is found, it will be used to fill the metadata fields. If an option that corresponds to a Metadata field is given to setup(), it will override the value that was possibly present in setup.cfg.

Notice that setup.py is still used and can be required to define some options that are not part of the Metadata fields. For instance, the sdist command can use options like packages or scripts.

Multi-lines values

Some Metadata fields can have multiple values. To keep setup.cfg compatible with ConfigParser and the RFC 822 LONG HEADER FIELDS (see section 3.1.1), these are expressed with ,-separated values:

requires = pywin32, bar > 1.0, foo

When this variable is read, the values are parsed and transformed into a list: ['pywin32', 'bar > 1.0', 'foo'].

Context-dependant sections

The metadata section will also be able to use context-dependant sections.

A context-dependant section is a section with a condition about the execution environment. Here's some examples:

[metadata]
name = Distribute
version = 0.6.4

[metadata:sys_platform == 'win32']
requires = pywin32, bar > 1.0
obsoletes = pywin31

[metadata:os_machine == 'i386']
requires = foo

[metadata:python_version == '2.4' or python_version == '2.5']
requires = bar

[metadata:'linux' in sys_platform]
requires = baz

Every [metadata:condition] section will be used only if the condition is met when the file is read. The background motivation for these context-dependant sections is to be able to define requirements that varies depending on the platform the distribution might be installed on. (see PEP 314).

The micro-language behind this is the simplest possible: it compares only strings, with the == and in operators (and their opposites), and with the ability to combine expressions. It makes it also easy to understand to non-pythoneers.

The pseudo-grammar is

EXPR [in|==|!=|not in] EXPR [or|and] ...

where EXPR belongs to any of those:

  • python_version = '%s.%s' % (sys.version_info[0], sys.version_info[1])
  • os_name = os.name
  • sys_platform = sys.platform
  • platform_version = platform.version()
  • platform_machine = platform.machine()
  • a free string, like 2.4, or win32

Notice that in is restricted to strings, meaning that it is not possible to use other sequences like tuples or lists on the right side.

Distutils will provide a function that is able to generate the metadata of a distribution, given a setup.cfg file, for the execution environment:

>>> from distutils.util import local_metadata
>>> local_metadata('setup.cfg')
<DistributionMetadata instance>

This means that a vanilla Python will be able to read the metadata of a package without running any third party code.

Notice that this feature is not restricted to the metadata namespace. Consequently, any other section can be extended with such context-dependant sections.

Impact on PKG-INFO generation and PEP 314

When PKG-INFO is generated by Distutils, every field that relies on a condition will have that condition written at the end of the line, after a ; separator:

Metadata-Version: 1.2
Name: distribute
Version: 0.6.4
...
Requires: pywin32, bar > 1.0; sys_platform == 'win32'
Requires: foo; os_machine == 'i386'
Requires: bar; python_version == '2.4' or python_version == '2.5'
Requires: baz; 'linux' in sys_platform
Obsoletes = pywin31; sys_platform == 'win32'
...
Classifier: Development Status :: 5 - Production/Stable
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: Python Software Foundation License

Notice that this file can be opened with the DistributionMetadata class. This class will be able to use the micro-language using the execution environment.

Let's run in on a Python 2.5 i386 Linux:

>>> from distutils.dist import DistributionMetadata
>>> metadata = DistributionMetadata('PKG_INFO')
>>> metadata.get_requires()
['foo', 'bar', 'baz']

The execution environment can be overriden in case we want to get the meyadata for another environment:

>>> env = {'python_version': '2.4',
...        'os_name': 'nt',
...        'sys_platform': 'win32',
...        'platform_version': 'MVCC++ 6.0'
...        'platform_machine': 'i386'}
...
>>> metadata = DistributionMetadata('PKG_INFO', environment=env)
>>> metadata.get_requires()
['bar > 1.0', 'foo', 'bar']

PEP 314 is changed accordingly, meaning that each field will be able to have that extra condition marker.

Compatiblity

This change is is based on a new metadata 1.2 format meaning that Distutils will be able to distinguish old PKG-INFO files from new ones.

The setup.cfg file change will stay ConfigParser-compatible and will not break existing setup.cfg files.

Limitations

We are not providing < and > operators at this time, and python_version is a regular string. This implies using or operators when a section needs to be restricted to a couple of Python versions. Although, if PEP 386 is accepted, python_version could be changed internally into something comparable with strings, and < and > operators introduced.

Last, if a distribution is unable to set all metadata fields in setup.cfg, that's fine, the fields will be set to UNKNOWN when local_metadata is called. Getting UNKNOWN values will mean that it might be necessary to run the setup.py command line interface to get the whole set of metadata.

Acknowledgments

The Distutils-SIG.