python-peps/pep-0426.txt

PEP: 426
Title: Metadata for Python Software Packages 2.0
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>,
        Daniel Holth <dholth@gmail.com>,
        Donald Stufft <donald@stufft.io>
BDFL-Delegate: Nick Coghlan <ncoghlan@gmail.com>
Discussions-To: Distutils SIG <distutils-sig@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Requires: 440
Created: 30 Aug 2012
Post-History: 14 Nov 2012, 5 Feb 2013, 7 Feb 2013, 9 Feb 2013,
              27 May 2013, 20 Jun 2013, 23 Jun 2013, 14 Jul 2013,
              21 Dec 2013
Replaces: 345


Abstract
========

This PEP describes a mechanism for publishing and exchanging metadata
related to Python distributions. It includes specifics of the field names,
and their semantics and usage.

This document specifies version 2.0 of the metadata format.
Version 1.0 is specified in PEP 241.
Version 1.1 is specified in PEP 314.
Version 1.2 is specified in PEP 345.

Version 2.0 of the metadata format migrates from a custom key-value format
to a JSON-compatible in-memory representation.

This version also adds fields designed to make third-party packaging of
Python software easier, defines a formal extension mechanism, and adds
support for optional dependencies. Finally, this version addresses
several issues with the previous iteration of the standard version
identification scheme.

.. note::

   "I" in this doc refers to Nick Coghlan. Daniel and Donald either wrote or
   contributed to earlier versions, and have been providing feedback as this
   JSON-based rewrite has taken shape. Daniel and Donald have also been
   vetting the proposal as we go to ensure it is practical to implement for
   both clients and index servers.

   Metadata 2.0 represents a major upgrade to the Python packaging ecosystem,
   and attempts to incorporate experience gained over the 15 years(!) since
   distutils was first added to the standard library. Some of that is just
   incorporating existing practices from setuptools/pip/etc, some of it is
   copying from other distribution systems (like Linux distros or other
   development language communities) and some of it is attempting to solve
   problems which haven't yet been well solved by anyone (like supporting
   clean conversion of Python source packages to distro policy compliant
   source packages for at least Debian and Fedora, and perhaps other
   platform specific distribution systems).

   There will eventually be a suite of PEPs covering various aspects of
   the metadata 2.0 format and related systems:

   * this PEP, covering the core metadata format
   * PEP 440, covering the versioning identification and selection scheme
   * PEP 459, covering several standard extensions
   * a yet-to-be-written PEP to define v2.0 of the sdist format
   * an updated wheel PEP (v1.1) to add pydist.json (and possibly convert
     the wheel metadata file from Key:Value to JSON)
   * an updated installation database PEP to add pydist.json
   * a PEP to standardise the expected command line interface for setup.py
     as an interface to an application's build system (rather than requiring
     that the build system support the distutils command system)

   It's going to take a while to work through all of these and make them
   a reality. The main change from our last attempt at this is that we're
   trying to design the different pieces so we can implement them
   independently of each other, without requiring users to switch to
   a whole new tool chain (although they may have to upgrade their existing
   ones to start enjoying the benefits in their own work).

   Many of the inline notes in this version of the PEP are there to aid
   reviewers that are familiar with the old metadata standards. Before this
   version is finalised, most of that content will be moved down to the
   "rationale" section at the end of the document, as it would otherwise be
   an irrelevant distraction for future readers.


Purpose
=======

The purpose of this PEP is to define a common metadata interchange format
for communication between software publication tools and software integration
tools in the Python ecosystem. One key aim is to support full dependency
analysis in that ecosystem without requiring the execution of arbitrary
Python code by those doing the analysis. Another aim is to encourage good
software distribution practices by default, while continuing to support the
current practices of almost all existing users of the Python Package Index
(both publishers and integrators). Finally, the aim is to support an upgrade
path from the existing setuptools defined dependency and entry point
metadata formats that is transparent to end users.

The design draws on the Python community's 15 years of experience with
distutils based software distribution, and incorporates ideas and concepts
from other distribution systems, including Python's setuptools, pip and
other projects, Ruby's gems, Perl's CPAN, Node.js's npm, PHP's composer
and Linux packaging systems such as RPM and APT.

While the specifics of this format are aimed at the Python ecosystem, some
of the ideas may also be useful in the future evolution of other dependency
management ecosystems.


A Note on Time Frames
=====================

There's a lot of work going on in the Python packaging space at the moment.
In the near term (up until the release of Python 3.4), those efforts are
focused on the existing metadata standards, both those defined in Python
Enhancement Proposals, and the de facto standards defined by the setuptools
project.

This PEP is about setting out a longer term goal for the ecosystem that
captures those existing capabilities in a format that is easier to work
with. There are still a number of key open questions (mostly related to
source based distribution), and those won't be able to receive proper
attention from the development community until the other near term
concerns have been resolved.

At this point in time, the PEP is quite possibly still overengineered, as
we're still trying to make sure we have all the use cases covered. The
"transparent upgrade path from setuptools" goal brings in a lot of required
functionality though, and then the aim of supporting automated creation of
policy compliant downstream packages for Linux distributions adds more.
However, we've at least reached the point where we're taking a critical
look at the core metadata, and are pushing as much functionality out to
standard metadata extensions as we can.


Development, Distribution and Deployment of Python Software
===========================================================

The metadata design in this PEP is based on a particular conceptual model
of the software development and distribution process. This model consists of
the following phases:

* Software development: this phase involves working with a source checkout
  for a particular application to add features and fix bugs. It is
  expected that developers in this phase will need to be able to build the
  software, run the software's automated test suite, run project specific
  utility scripts and publish the software.

* Software publication: this phase involves taking the developed software
  and making it available for use by software integrators. This includes
  creating the descriptive metadata defined in this PEP, as well as making
  the software available (typically by uploading it to an index server).

* Software integration: this phase involves taking published software
  components and combining them into a coherent, integrated system. This
  may be done directly using Python specific cross-platform tools, or it may
  be handled through conversion to development language neutral platform
  specific packaging systems.

* Software deployment: this phase involves taking integrated software
  components and deploying them on to the target system where the software
  will actually execute.

The publication and integration phases are collectively referred to as
the distribution phase, and the individual software components distributed
in that phase are formally referred to as "distributions", but are more
colloquially known as "packages" (relying on context to disambiguate them
from the "module with submodules" kind of Python package).

The exact details of these phases will vary greatly for particular use cases.
Deploying a web application to a public Platform-as-a-Service provider,
publishing a new release of a web framework or scientific library,
creating an integrated Linux distribution or upgrading a custom application
running in a secure enclave are all situations this metadata design should
be able to handle.

The complexity of the metadata described in this PEP thus arises directly
from the actual complexities associated with software development,
distribution and deployment in a wide range of scenarios.


Supporting definitions
----------------------

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.

"Projects" are software components that are made available for integration.
Projects include Python libraries, frameworks, scripts, plugins,
applications, collections of data or other resources, and various
combinations thereof. Public Python projects are typically registered on
the `Python Package Index`_.

"Releases" are uniquely identified snapshots of a project.

"Distributions" are the packaged files which are used to publish
and distribute a release.

Depending on context, "package" may refer to either a distribution, or
to an importable Python module that has a ``__path__`` attribute and hence
may also have importable submodules.

"Source archive" and "VCS checkout" both refer to the raw source code for
a release, prior to creation of an sdist or binary archive.

An "sdist" is a publication format providing the distribution metadata and
and any source files that are essential to creating a binary archive for
the distribution. Creating a binary archive from an sdist requires that
the appropriate build tools be available on the system.

"Binary archives" only require that prebuilt files be moved to the correct
location on the target system. As Python is a dynamically bound
cross-platform language, many so-called "binary" archives will contain only
pure Python source code.

"Contributors" are individuals and organizations that work together to
develop a software component.

"Publishers" are individuals and organizations that make software components
available for integration (typically by uploading distributions to an
index server)

"Integrators" are individuals and organizations that incorporate published
distributions as components of an application or larger system.

"Build tools" are automated tools intended to run on development systems,
producing source and binary distribution archives. Build tools may also be
invoked by integration tools in order to build software distributed as
sdists rather than prebuilt binary archives.

"Index servers" are active distribution registries which publish version and
dependency metadata and place constraints on the permitted metadata.

"Public index servers" are index servers which allow distribution uploads
from untrusted third parties. The `Python Package Index`_ is a public index
server.

"Publication tools" are automated tools intended to run on development
systems and upload source and binary distribution archives to index servers.

"Integration tools" are automated tools that consume the metadata and
distribution archives published by an index server or other designated
source, and make use of them in some fashion, such as installing them or
converting them to a platform specific packaging format.

"Installation tools" are integration tools specifically intended to run on
deployment targets, consuming source and binary distribution archives from
an index server or other designated location and deploying them to the target
system.

"Automated tools" is a collective term covering build tools, index servers,
publication tools, integration tools and any other software that produces
or consumes distribution version and dependency metadata.

"Legacy metadata" refers to earlier versions of this metadata specification,
along with the supporting metadata file formats defined by the
``setuptools`` project.

"Distro" is used as the preferred term for Linux distributions, to help
avoid confusion with the Python-specific meaning of the term "distribution".

"Dist" is the preferred abbreviation for "distributions" in the sense defined
in this PEP.

"Qualified name" is a dotted Python identifier. For imported modules and
packages, the qualified name is available as the ``__name__`` attribute,
while for functions and classes it is available as the ``__qualname__``
attribute.

A "fully qualified name" uniquely locates an object in the Python module
namespace. For imported modules and packages, it is the same as the
qualified name. For other Python objects, the fully qualified name consists
of the qualified name of the containing module or package, a colon (``:``)
and the qualified name of the object relative to the containing module or
package.

A "prefixed name" starts with a qualified name, but is not necessarily a
qualified name - it may contain additional dot separated segments which are
not valid identifiers.


Integration and deployment of distributions
-------------------------------------------

The primary purpose of the distribution metadata is to support integration
and deployment of distributions as part of larger applications and systems.

Integration and deployment can in turn be broken down into further substeps.

* Build: the build step is the process of turning a VCS checkout, source
  archive or sdist into a binary archive. Dependencies must be available
  in order to build and create a binary archive of the distribution
  (including any documentation that is installed on target systems).

* Installation: the installation step involves getting the distribution
  and all of its runtime dependencies onto the target system. In this
  step, the distribution may already be on the system (when upgrading or
  reinstalling) or else it may be a completely new installation.

* Runtime: this is normal usage of a distribution after it has been
  installed on the target system.

These three steps may all occur directly on the target system. Alternatively
the build step may be separated out by using binary archives provided by the
publisher of the distribution, or by creating the binary archives on a
separate system prior to deployment. The advantage of the latter approach
is that it minimizes the dependencies that need to be installed on
deployment targets (as the build dependencies will be needed only on the
build systems).

The published metadata for distributions SHOULD allow integrators, with the
aid of build and integration tools, to:

* obtain the original source code that was used to create a distribution
* identify and retrieve the dependencies (if any) required to use a
  distribution
* identify and retrieve the dependencies (if any) required to build a
  distribution from source
* identify and retrieve the dependencies (if any) required to run a
  distribution's test suite
* find resources on using and contributing to the project
* access sufficiently rich metadata to support contacting distribution
  publishers through appropriate channels, as well as finding distributions
  that are relevant to particular problems


Development and publication of distributions
--------------------------------------------

The secondary purpose of the distribution metadata is to support effective
collaboration amongst software contributors and publishers during the
development phase.

The published metadata for distributions SHOULD allow contributors
and publishers, with the aid of build and publication tools, to:

* perform all the same activities needed to effectively integrate and
  deploy the distribution
* identify and retrieve the additional dependencies needed to develop and
  publish the distribution
* specify the dependencies (if any) required to use the distribution
* specify the dependencies (if any) required to build the distribution
  from source
* specify the dependencies (if any) required to run the distribution's
  test suite
* specify the additional dependencies (if any) required to develop and
  publish the distribution


Standard build system
---------------------

.. note::

   The standard build system currently described in the PEP is a draft based
   on existing practices for projects using distutils or setuptools as their
   build system (or other projects, like ``d2to1``, that expose a setup.py
   file for backwards compatibility with existing tools)

   The specification doesn't currently cover expected argument support for
   the commands, which is a limitation that needs to be addressed before the
   PEP can be considered ready for acceptance.

   It is also possible that the "meta build system" will be separated out
   into a distinct PEP in the coming months (similar to the separation of
   the versioning and requirement specification standard out to PEP 440).

   If a `suitable API can be worked out <Metabuild system>`__, then it may
   even be possible to switch to a more declarative API for build system
   specification.

Both development and integration of distributions relies on the ability to
build extension modules and perform other operations in a distribution
independent manner.

The current iteration of the metadata relies on the
``distutils``/``setuptools`` commands system to support these necessary
development and integration activities:

* ``python setup.py dist_info``: generate distribution metadata in place
  given a source archive or VCS checkout
* ``python setup.py sdist``: create an sdist from a source archive
  or VCS checkout
* ``python setup.py build_ext --inplace``: build extension modules in place
  given an sdist, source archive or VCS checkout
* ``python setup.py test``: run the distribution's test suite in place
  given an sdist, source archive or VCS checkout
* ``python setup.py bdist_wheel``: create a binary archive from an sdist,
  source archive or VCS checkout


Metadata format
===============

The format defined in this PEP is an in-memory representation of Python
distribution metadata as a string-keyed dictionary. Permitted values for
individual entries are strings, lists of strings, and additional
nested string-keyed dictionaries.

Except where otherwise noted, dictionary keys in distribution metadata MUST
be valid Python identifiers in order to support attribute based metadata
access APIs.

The individual field descriptions show examples of the key name and value
as they would be serialised as part of a JSON mapping.

The fields identified as core metadata are required. Automated tools MUST
NOT accept distributions with missing core metadata as valid Python
distributions.

All other fields are optional. Automated tools MUST operate correctly
if a distribution does not provide them, except for those operations
which specifically require the omitted fields.

Automated tools MUST NOT insert dummy data for missing fields. If a valid
value is not provided for a required field then the metadata and the
associated distribution MUST be rejected as invalid. If a valid value
is not provided for an optional field, that field MUST be omitted entirely.
Automated tools MAY automatically derive valid values from other
information sources (such as a version control system).

Automated tools, especially public index servers, MAY impose additional
length restrictions on metadata beyond those enumerated in this PEP. Such
limits SHOULD be imposed where necessary to protect the integrity of a
service, based on the available resources and the service provider's
judgment of reasonable metadata capacity requirements.


Metadata files
--------------

The information defined in this PEP is serialised to ``pydist.json``
files for some use cases. These are files containing UTF-8 encoded JSON
metadata.

Each metadata file consists of a single serialised mapping, with fields as
described in this PEP. When serialising metadata, automated tools SHOULD
lexically sort any keys and list elements in order to simplify reviews
of any changes.

There are three standard locations for these metadata files:

* as a ``{distribution}-{version}.dist-info/pydist.json`` file in an
  ``sdist`` source distribution archive
* as a ``{distribution}-{version}.dist-info/pydist.json`` file in a ``wheel``
  binary distribution archive
* as a ``{distribution}-{version}.dist-info/pydist.json`` file in a local
  Python installation database

.. note::

   These locations are to be confirmed, since they depend on the definition
   of sdist 2.0 and the revised installation database standard. There will
   also be a wheel 1.1 format update after this PEP is approved that
   mandates provision of 2.0+ metadata.

Other tools involved in Python distribution may also use this format.

As JSON files are generally awkward to edit by hand, it is RECOMMENDED
that these metadata files be generated by build tools based on other
input formats (such as ``setup.py``) rather than being used directly as
a data input format. Generating the metadata as part of the publication
process also helps to deal with version specific fields (including the
source URL and the version field itself).

For backwards compatibility with older installation tools, metadata 2.0
files MAY be distributed alongside legacy metadata.

Index servers MAY allow distributions to be uploaded and installation tools
MAY allow distributions to be installed with only legacy metadata.

Automated tools MAY attempt to automatically translate legacy metadata to
the format described in this PEP. Advice for doing so effectively is given
in Appendix A.


Metadata validation
-------------------

A `jsonschema <https://pypi.python.org/pypi/jsonschema>`__ description of
the distribution metadata is `available
<http://hg.python.org/peps/file/default/pep-0426/pydist-schema.json>`__.

This schema does NOT currently handle validation of some of the more complex
string fields (instead treating them as opaque strings).

Except where otherwise noted, all URL fields in the metadata MUST comply
with RFC 3986.

.. note::

   The current version of the schema file covers the previous draft of the
   PEP, and has not yet been updated for the split into the essential
   dependency resolution metadata and multiple standard extensions.


Core metadata
=============

This section specifies the core metadata fields that are required for every
Python distribution.

Publication tools MUST ensure at least these fields are present when
publishing a distribution.

Index servers MUST ensure at least these fields are present in the metadata
when distributions are uploaded.

Installation tools MUST refuse to install distributions with one or more
of these fields missing by default, but MAY allow users to force such an
installation to occur.


Metadata version
----------------

Version of the file format; ``"2.0"`` is the only legal value.

Automated tools consuming metadata SHOULD warn if ``metadata_version`` is
greater than the highest version they support, and MUST fail if
``metadata_version`` has a greater major version than the highest
version they support (as described in PEP 440, the major version is the
value before the first dot).

For broader compatibility, build tools MAY choose to produce
distribution metadata using the lowest metadata version that includes
all of the needed fields.

Example::

    "metadata_version": "2.0"


Generator
---------

Name (and optional version) of the program that generated the file,
if any.  A manually produced file would omit this field.

Example::

    "generator": "setuptools (0.9)"


Name
----

The name of the distribution.

As distribution names are used as part of URLs, filenames, command line
parameters and must also interoperate with other packaging systems, the
permitted characters are constrained to:

* ASCII letters (``[a-zA-Z]``)
* ASCII digits (``[0-9]``)
* underscores (``_``)
* hyphens (``-``)
* periods (``.``)

Distribution names MUST start and end with an ASCII letter or digit.

Automated tools MUST reject non-compliant names.

All comparisons of distribution names MUST be case insensitive, and MUST
consider hyphens and underscores to be equivalent.

Index servers MAY consider "confusable" characters (as defined by the
Unicode Consortium in `TR39: Unicode Security Mechanisms <TR39>`_) to be
equivalent.

Index servers that permit arbitrary distribution name registrations from
untrusted sources SHOULD consider confusable characters to be equivalent
when registering new distributions (and hence reject them as duplicates).

Integration tools MUST NOT silently accept a confusable alternate
spelling as matching a requested distribution name.

At time of writing, the characters in the ASCII subset designated as
confusables by the Unicode Consortium are:

* ``1`` (DIGIT ONE), ``l`` (LATIN SMALL LETTER L), and ``I`` (LATIN CAPITAL
  LETTER I)
* ``0`` (DIGIT ZERO), and ``O`` (LATIN CAPITAL LETTER O)


Example::

    "name": "ComfyChair"


Version
-------

The distribution's public version identifier, as defined in PEP 440. Public
versions are designed for consumption by automated tools and support a
variety of flexible version specification mechanisms (see PEP 440 for
details).

Version identifiers MUST comply with the format defined in PEP 440.

Version identifiers MUST be unique within each project.

Example::

    "version": "1.0a2"


Summary
-------

A short summary of what the distribution does.

This field SHOULD contain fewer than 512 characters and MUST contain fewer
than 2048.

This field SHOULD NOT contain any line breaks.

A more complete description SHOULD be included as a separate file in the
sdist for the distribution. Refer to the ``python-details`` extension in
:pep:`459` for more information.

Example::

    "summary": "A module that is more fiendish than soft cushions."


Source code metadata
====================

This section specifies fields that provide identifying details for the
source code used to produce this distribution.

All of these fields are optional. Automated tools MUST operate correctly if
a distribution does not provide them, including failing cleanly when an
operation depending on one of these fields is requested.


Source label
------------

A constrained identifying text string, as defined in PEP 440. Source labels
cannot be used in version specifiers - they are included for information
purposes only.

Source labels MUST meet the character restrictions defined in PEP 440.

Source labels MUST be unique within each project and MUST NOT match any
defined version for the project.

Examples::

    "source_label": "1.0.0-alpha.1"

    "source_label": "1.3.7+build.11.e0f985a"

    "source_label": "v1.8.1.301.ga0df26f"

    "source_label": "2013.02.17.dev123"


Source URL
----------

A string containing a full URL where the source for this specific version of
the distribution can be downloaded.

Source URLs MUST be unique within each project. This means that the URL
can't be something like ``"https://github.com/pypa/pip/archive/master.zip"``,
but instead must be ``"https://github.com/pypa/pip/archive/1.3.1.zip"``.

The source URL MUST reference either a source archive or a tag or specific
commit in an online version control system that permits creation of a
suitable VCS checkout. It is intended primarily for integrators that
wish to recreate the distribution from the original source form.

All source URL references SHOULD specify a secure transport mechanism
(such as ``https``) AND include an expected hash value in the URL for
verification purposes. If a source URL is specified without any hash
information, with hash information that the tool doesn't understand, or
with a selected hash algorithm that the tool considers too weak to trust,
automated tools SHOULD at least emit a warning and MAY refuse to rely on
the URL. If such a source URL also uses an insecure transport, automated
tools SHOULD NOT rely on the URL.

It is RECOMMENDED that only hashes which are unconditionally provided by
the latest version of the standard library's ``hashlib`` module be used
for source archive hashes. At time of writing, that list consists of
``'md5'``, ``'sha1'``, ``'sha224'``, ``'sha256'``, ``'sha384'``, and
``'sha512'``.

For source archive references, an expected hash value may be specified by
including a ``<hash-algorithm>=<expected-hash>`` entry as part of the URL
fragment.

For version control references, the ``VCS+protocol`` scheme SHOULD be
used to identify both the version control system and the secure transport,
and a version control system with hash based commit identifiers SHOULD be
used. Automated tools MAY omit warnings about missing hashes for version
control systems that do not provide hash based commit identifiers.

To handle version control systems that do not support including commit or
tag references directly in the URL, that information may be appended to the
end of the URL using the ``@<commit-hash>`` or the ``@<tag>#<commit-hash>``
notation.

.. note::

   This isn't *quite* the same as the existing VCS reference notation
   supported by pip. Firstly, the distribution name is moved in front rather
   than embedded as part of the URL. Secondly, the commit hash is included
   even when retrieving based on a tag, in order to meet the requirement
   above that *every* link should include a hash to make things harder to
   forge (creating a malicious repo with a particular tag is easy, creating
   one with a specific *hash*, less so).

Example::

    "source_url": "https://github.com/pypa/pip/archive/1.3.1.zip#sha1=da9234ee9982d4bbb3c72346a6de940a148ea686"
    "source_url": "git+https://github.com/pypa/pip.git@1.3.1#7921be1537eac1e97bc40179a57f0349c2aee67d"
    "source_url": "git+https://github.com/pypa/pip.git@7921be1537eac1e97bc40179a57f0349c2aee67d"


Semantic dependencies
=====================

Dependency metadata allows distributions to make use of functionality
provided by other distributions, without needing to bundle copies of those
distributions.

Semantic dependencies allow publishers to indicate not only which other
distributions are needed, but also *why* they're needed. This additional
information allows integrators to install just the dependencies they need
for specific activities, making it easier to minimise installation
footprints in constrained environments (regardless of the reasons for
those constraints).

Distributions may declare five differents kinds of dependency:

* Runtime dependencies: other distributions that are needed to actually use
  this distribution (but are not considered subdistributions).
* "Meta" dependencies: subdistributions that are grouped together into a
  single larger metadistribution for ease of reference and installation.
* Test dependencies: other distributions that are needed to run the
  automated test suite for this distribution (but are not needed just to
  use it).
* Build dependencies: other distributions that are needed to build this
  distribution.
* Development dependencies: other distributions that are needed when
  working on this distribution (but do not fit into one of the other
  dependency categories).

Within each of these categories, distributions may also declare "Extras".
Extras are dependencies that may be needed for some optional functionality,
or which are otherwise complementary to the distribution.

Dependency management is heavily dependent on the version identification
and specification scheme defined in PEP 440.

All of these fields are optional. Automated tools MUST operate correctly if
a distribution does not provide them, by assuming that a missing field
indicates "Not applicable for this distribution".


Dependency specifiers
---------------------

While many dependencies will be needed to use a distribution at all, others
are needed only on particular platforms or only when particular optional
features of the distribution are needed. To handle this, dependency
specifiers are mappings with the following subfields:

* ``requires``: a list of `requirement specifiers
  <Requirement specifiers>`__ needed to satisfy the dependency
* ``extra``: the name of a set of optional dependencies that are requested
  and installed together. See `Extras (optional dependencies)`_ for details.
* ``environment``: an environment marker defining the environment that
  needs these dependencies. See `Environment markers`_ for details.

``requires`` is the only required subfield. When it is the only subfield, the
dependencies are said to be *unconditional*. If ``extra`` or ``environment``
is specified, then the dependencies are *conditional*.

All three fields may be supplied, indicating that the dependencies are
needed only when the named extra is requested in a particular environment.

Automated tools MUST combine related dependency specifiers (those with
common values for ``extra`` and ``environment``) into a single specifier
listing multiple requirements when serialising metadata or
passing it to an install hook.

Despite this required normalisation, the same extra name or environment
marker MAY appear in multiple conditional dependencies. This may happen,
for example, if an extra itself only needs some of its dependencies in
specific environments. It is only the combination of extras and environment
markers that is required to be unique in a list of dependency specifiers.

Any extras referenced from a dependency specifier MUST be named in the
`Extras`_ field for this distribution. This helps avoid typographical
errors and also makes it straightforward to identify the available extras
without scanning the full set of dependencies.


Requirement specifiers
----------------------

Individual requirements are defined as strings containing a distribution
name (as found in the ``name`` field). The distribution name
may be followed by an extras specifier (enclosed in square
brackets) and by a version specifier or direct reference (within
parentheses).

Whitespace is permitted between the distribution name and an opening
square bracket or parenthesis. Whitespace is also permitted between a
closing square bracket and an opening parenthesis.

See `Extras (optional dependencies)`_ for details on extras and PEP 440
for details on version specifiers and direct references.

The distribution names should correspond to names as found on the `Python
Package Index`_; while these names are often the same as the module names
as accessed with ``import x``, this is not always the case (especially
for distributions that provide multiple top level modules or packages).

Example requirement specifiers::

    "Flask"
    "Django"
    "Pyramid"
    "SciPy (0.12)"
    "ComfyChair[warmup]"
    "ComfyChair[warmup] (> 0.1)"


Mapping dependencies to development and distribution activities
---------------------------------------------------------------

The different categories of dependency are based on the various distribution
and development activities identified above, and govern which dependencies
should be installed for the specified activities:

* Implied runtime dependencies:

    * ``run_requires``
    * ``meta_requires``

* Implied build dependencies:

    * ``build_requires``
    * If running the distribution's test suite as part of the build process,
      request the ``:run:``, ``:meta:``, and ``:test:`` extras to also
      install:

      * ``run_requires``
      * ``meta_requires``
      * ``test_requires``

* Implied development and publication dependencies:

    * ``run_requires``
    * ``meta_requires``
    * ``build_requires``
    * ``test_requires``
    * ``dev_requires``

The notation described in `Extras (optional dependencies)`_ SHOULD be used
to determine exactly what gets installed for various operations.

Installation tools SHOULD report an error if dependencies cannot be
satisfied, MUST at least emit a warning, and MAY allow the user to force
the installation to proceed regardless.

See Appendix B for an overview of mapping these dependencies to an RPM
spec file.


Extras
------

A list of optional sets of dependencies that may be used to define
conditional dependencies in dependency fields. See
`Extras (optional dependencies)`_ for details.

The names of extras MUST abide by the same restrictions as those for
distribution names.

Example::

    "extras": ["warmup"]


Run requires
------------

A list of other distributions needed to actually run this distribution.

Automated tools MUST NOT allow strict version matching clauses or direct
references in this field - if permitted at all, such clauses should appear
in ``meta_requires`` instead.

Example::

    "run_requires":
      {
        "requires": ["SciPy", "PasteDeploy", "zope.interface (>3.5.0)"]
      },
      {
        "requires": ["pywin32 (>1.0)"],
        "environment": "sys_platform == 'win32'"
      },
      {
        "requires": ["SoftCushions"],
        "extra": "warmup"
      }
    ]


Meta requires
-------------

An abbreviation of "metadistribution requires". This is a list of
subdistributions that can easily be installed and used together by
depending on this metadistribution.

In this field, automated tools:

* MUST allow strict version matching
* MUST NOT allow more permissive version specifiers.
* MAY allow direct references

Public index servers SHOULD NOT allow the use of direct references in
uploaded distributions. Direct references are intended primarily as a
tool for software integrators rather than publishers.

Distributions that rely on direct references to platform specific binary
archives SHOULD define appropriate constraints in their
``supports_environments`` field.

Example::

    "meta_requires":
      {
        "requires": ["ComfyUpholstery (== 1.0a2)",
                     "ComfySeatCushion (== 1.0a2)"]
      },
      {
        "requires": ["CupOfTeaAtEleven (== 1.0a2)"],
        "environment": "'linux' in sys_platform"
      }
    ]


Test requires
-------------

A list of other distributions needed in order to run the automated tests
for this distribution..

Automated tools MAY disallow strict version matching clauses and direct
references in this field and SHOULD at least emit a warning for such clauses.

Public index servers SHOULD NOT allow strict version matching clauses or
direct references in this field.

Example::

    "test_requires":
      {
        "requires": ["unittest2"]
      },
      {
        "requires": ["pywin32 (>1.0)"],
        "environment": "sys_platform == 'win32'"
      },
      {
        "requires": ["CompressPadding"],
        "extra": "warmup"
      }
    ]


Build requires
--------------

A list of other distributions needed when this distribution is being built
(creating a binary archive from an sdist, source archive or VCS checkout).

Note that while these are build dependencies for the distribution being
built, the installation is a *deployment* scenario for the dependencies.

Automated tools MAY disallow strict version matching clauses and direct
references in this field and SHOULD at least emit a warning for such clauses.

Public index servers SHOULD NOT allow strict version matching clauses or
direct references in this field.

Example::

    "build_requires":
      {
        "requires": ["setuptools (>= 0.7)"]
      },
      {
        "requires": ["pywin32 (>1.0)"],
        "environment": "sys_platform == 'win32'"
      },
      {
        "requires": ["cython"],
        "extra": "c-accelerators"
      }
    ]


Dev requires
------------

A list of any additional distributions needed during development of this
distribution that aren't already covered by the deployment and build
dependencies.

Additional dependencies that may be listed in this field include:

* tools needed to create an sdist from a source archive or VCS checkout
* tools needed to generate project documentation that is published online
  rather than distributed along with the rest of the software

Automated tools MAY disallow strict version matching clauses and direct
references in this field and SHOULD at least emit a warning for such clauses.

Public index servers SHOULD NOT allow strict version matching clauses or
direct references in this field.

Example::

    "dev_requires":
      {
        "requires": ["hgtools", "sphinx (>= 1.0)"]
      },
      {
        "requires": ["pywin32 (>1.0)"],
        "environment": "sys_platform == 'win32'"
      }
    ]


Provides
--------

A list of strings naming additional dependency requirements that are
satisfied by installing this distribution. These strings must be of the
form ``Name`` or ``Name (Version)``, as for the ``requires`` field.

While dependencies are usually resolved based on distribution names and
versions, a distribution may provide additional names explicitly in the
``provides`` field.

For example, this may be used to indicate that multiple projects have
been merged into and replaced by a single distribution or to indicate
that this project is a substitute for another.

For instance, with distribute merged back into setuptools, the merged
project is able to include a ``"provides": ["distribute"]`` entry to
satisfy any projects that require the now obsolete distribution's name.

To avoid malicious hijacking of names, when interpreting metadata retrieved
from a public index server, automated tools MUST NOT pay any attention to
``"provides"`` entries that do not correspond to a published distribution.

However, to appropriately handle project forks and mergers, automated tools
MUST accept ``"provides"`` entries that name other distributions when the
entry is retrieved from a local installation database or when there is a
corresponding ``"obsoleted_by"`` entry in the metadata for the named
distribution.

A distribution may wish to depend on a "virtual" project name, which does
not correspond to any separately distributed project:  such a name
might be used to indicate an abstract capability which could be supplied
by one of multiple projects.  For example, multiple projects might supply
PostgreSQL bindings for use with SQL Alchemy: each project might declare
that it provides ``sqlalchemy-postgresql-bindings``, allowing other
projects to depend only on having at least one of them installed.

To handle this case in a way that doesn't allow for name hijacking, the
authors of the distribution that first defines the virtual dependency should
create a project on the public index server with the corresponding name, and
depend on the specific distribution that should be used if no other provider
is already installed. This also has the benefit of publishing the default
provider in a way that automated tools will understand.

A version declaration may be supplied as part of an entry in the provides
field and must follow the rules described in PEP 440. The distribution's
version identifier will be implied if none is specified.

Example::

    "provides": ["AnotherProject (3.4)", "virtual-package"]


Obsoleted by
------------

A string that indicates that this project is no longer being developed.  The
named project provides a substitute or replacement.

A version declaration may be supplied and must follow the rules described
in PEP 440.

An inactive project may be explicitly indicated by setting this field to
``None`` (which is serialised as ``null`` in JSON as usual).

Automated tools SHOULD report a warning when installing an obsolete project.

Possible uses for this field include handling project name changes and
project mergers.

For instance, with distribute merging back into setuptools, a new version
of distribute may be released that depends on the new version of setuptools,
and also explicitly indicates that distribute itself is now obsolete.

Note that without a corresponding ``provides``, there is no expectation
that the replacement project will be a "drop-in" replacement for the
obsolete project - at the very least, upgrading to the new distribution
is likely to require changes to import statements.

Examples::

    "name": "BadName",
    "obsoleted_by": "AcceptableName"

    "name": "distribute",
    "obsoleted_by": "setuptools (>= 0.7)"


Supports Environments
---------------------

A list of strings specifying the environments that the distribution
explicitly supports. An environment is considered supported if it
matches at least one of the environment markers given.

If this field is not given in the metadata, it is assumed that the
distribution supports any platform supported by Python.

Individual entries are environment markers, as described in
`Environment markers`_.

Installation tools SHOULD report an error if supported environments are
specified by the distribution and the current platform fails to match
any of them, MUST at least emit a warning, and MAY allow the user to
force the installation to proceed regardless.

The two main uses of this field are to declare which versions of Python
and which underlying operating systems are supported.

Examples indicating supported Python versions::

   # Supports Python 2.6+
   "supports_environments": ["python_version >= '2.6'"]

   # Supports Python 2.6+ (for 2.x) or 3.3+ (for 3.x)
   "supports_environments": ["python_version >= '3.3'",
                             "'3.0' > python_version >= '2.6'"]

Examples indicating supported operating systems::

   # Windows only
   "supports_environments": ["sys_platform == 'win32'"]

   # Anything except Windows
   "supports_environments": ["sys_platform != 'win32'"]

   # Linux or BSD only
   "supports_environments": ["'linux' in sys_platform",
                             "'bsd' in sys_platform"]

Example where the supported Python version varies by platform::

   # The standard library's os module has long supported atomic renaming
   # on POSIX systems, but only gained atomic renaming on Windows in Python
   # 3.3. A distribution that needs atomic renaming support for reliable
   # operation might declare the following supported environments.
   "supports_environments": ["python_version >= '2.6' and sys_platform != 'win32'",
                             "python_version >= '3.3' and sys_platform == 'win32'"]


Metadata Extensions
===================

Extensions to the metadata MAY be present in a mapping under the
``extensions`` key.  The keys MUST be valid prefixed names, while
the values MUST themselves be nested mappings.

The following example shows the ``python.details`` and ``python.commands``
standard extensions from :pep:`459`::

    "extensions" : {
      "python.details": {
        "license": "GPL version 3, excluding DRM provisions",
        "keywords": [
          "comfy", "chair", "cushions", "too silly", "monty python"
        ],
        "classifiers": [
          "Development Status :: 4 - Beta",
          "Environment :: Console (Text Based)",
          "License :: OSI Approved :: GNU General Public License v3 (GPLv3)"
        ],
        "document_names": {
            "description": "README.rst",
            "license": "LICENSE.rst",
            "changelog": "NEWS"
        }
      },
      "python.commands": {
        "wrap_console": [{"chair": "chair:run_cli"}],
        "wrap_gui": [{"chair-gui": "chair:run_gui"}],
        "prebuilt": ["reduniforms"]
      },
    }

Extension names are defined by distributions that will then make use of
the additional published metadata in some way.

To reduce the chance of name conflicts, extension names SHOULD use a
prefix that corresponds to a module name in the distribution that defines
the meaning of the extension. This practice will also make it easier to
find authoritative documentation for metadata extensions.

Extensions MUST be versioned, using the ``extension_version`` key.
However, if this key is omitted, then the implied version is ``1.0``.

Automated tools consuming extension metadata SHOULD warn if
``extension_version`` is greater than the highest version they support,
and MUST fail if ``extension_version`` has a greater major version than
the highest version they support (as described in PEP 440, the major
version is the value before the first dot).

For broader compatibility, build tools MAY choose to produce
extension metadata using the lowest metadata version that includes
all of the needed fields.

Metadata extensions allow development tools to record information in the
metadata that may be useful during later phases of distribution, but is
not essential for dependency resolution or building the software.


Extras (optional dependencies)
==============================

Extras are additional dependencies that enable an optional aspect
of the distribution, often corresponding to a ``try: import
optional_dependency ...`` block in the code.  To support the use of the
distribution with or without the optional dependencies they are listed
separately from the distribution's core dependencies and must be requested
explicitly, either in the dependency specifications of another distribution,
or else when issuing a command to an installation tool.

Note that installation of extras is not tracked directly by installation
tools: extras are merely a convenient way to indicate a set of dependencies
that is needed to provide some optional functionality of the distribution.
If selective *installation* of components is desired, then multiple
distributions must be defined rather than relying on the extras system.

The names of extras MUST abide by the same restrictions as those for
distribution names.

Example of a distribution with optional dependencies::

    "name": "ComfyChair",
    "extras": ["warmup", "c-accelerators"]
    "run_requires": [
      {
        "requires": ["SoftCushions"],
        "extra": "warmup"
      }
    ]
    "build_requires": [
      {
        "requires": ["cython"],
        "extra": "c-accelerators"
      }
    ]

Other distributions require the additional dependencies by placing the
relevant extra names inside square brackets after the distribution name when
specifying the dependency.

Extra specifications MUST allow the following additional syntax:

* Multiple extras can be requested by separating them with a comma within
  the brackets.

* The following special extras request processing of the corresponding
  lists of dependencies:

  * ``:meta:``: ``meta_requires``
  * ``:run:``: ``run_requires``
  * ``:test:``: ``test_requires``
  * ``:build:``: ``build_requires``
  * ``:dev:``: ``dev_requires``
  * ``:*:``: process *all* dependency lists

* The ``*`` character as an extra is a wild card that enables all of the
  entries defined in the distribution's ``extras`` field.

* Extras may be explicitly excluded by prefixing their name with a ``-``
  character (this is useful in conjunction with ``*`` to exclude only
  particular extras that are definitely not wanted, while enabling all
  others).

* The ``-`` character as an extra specification indicates that the
  distribution itself should NOT be installed, and also disables the
  normally implied processing of ``:meta:`` and ``:run:`` dependencies
  (those may still be requested explicitly using the appropriate extra
  specifications).

Command line based installation tools SHOULD support this same syntax to
allow extras to be requested explicitly.

The full set of dependency requirements is then based on the top level
dependencies, along with those of any requested extras.

Dependency examples (showing just the ``requires`` subfield)::

    "requires": ["ComfyChair[warmup]"]
        -> requires ``ComfyChair`` and ``SoftCushions``

    "requires": ["ComfyChair[*]"]
        -> requires ``ComfyChair`` and ``SoftCushions``, but will also
           pick up any new extras defined in later versions

Command line examples::

    pip install ComfyChair
        -> installs ComfyChair with applicable :meta: and :run: dependencies

    pip install ComfyChair[*]
        -> as above, but also installs all extra dependencies

    pip install ComfyChair[-,:build:,*]
        -> installs just the build dependencies with all extras

    pip install ComfyChair[-,:build:,:run:,:meta:,:test:,*]
        -> as above, but also installs dependencies needed to run the tests

    pip install ComfyChair[-,:*:,*]
        -> installs the full set of development dependencies, but avoids
           installing ComfyChair itself


Environment markers
===================

An **environment marker** describes a condition about the current execution
environment. They are used to indicate when certain dependencies are only
required in particular environments, and to indicate supported platforms
for distributions with additional constraints beyond the availability of a
Python runtime.

Here are some examples of such markers::

   "sys_platform == 'win32'"
   "platform_machine == 'i386'"
   "python_version == '2.4' or python_version == '2.5'"
   "'linux' in sys_platform"

And here's an example of some conditional metadata for a distribution that
requires PyWin32 both at runtime and buildtime when using Windows::

    "name": "ComfyChair",
    "run_requires": [
      {
        "requires": ["pywin32 (>1.0)"],
        "environment": "sys.platform == 'win32'"
      }
    ]
    "build_requires": [
      {
        "requires": ["pywin32 (>1.0)"],
        "environment": "sys.platform == 'win32'"
      }
    ]

The micro-language behind this is a simple subset of Python: it compares
only strings, with the ``==`` and ``in`` operators (and their opposites),
and with the ability to combine expressions. Parentheses are supported
for grouping.

The pseudo-grammar is ::

    MARKER: EXPR [(and|or) EXPR]*
    EXPR: ("(" MARKER ")") | (SUBEXPR [CMPOP SUBEXPR])
    CMPOP: (==|!=|<|>|<=|>=|in|not in)

where ``SUBEXPR`` is either a Python string (such as ``'2.4'``, or
``'win32'``) or one of the following marker variables:

* ``python_version``: ``'{0.major}.{0.minor}'.format(sys.version_info)``
* ``python_full_version``: see definition below
* ``os_name````: ``os.name``
* ``sys_platform````: ``sys.platform``
* ``platform_release``: ``platform.release()``
* ``platform_version``: ``platform.version()``
* ``platform_machine``: ``platform.machine()``
* ``platform_python_implementation``: ``platform.python_implementation()``
* ``implementation_name````: ``sys.implementation.name``
* ``implementation_version````: see definition below

If a particular value is not available (such as the ``sys.implementation``
subattributes in versions of Python prior to 3.3), the corresponding marker
variable MUST be considered equivalent to the empty string.

Note that all subexpressions are restricted to strings or one of the
marker variable names (which refer to string values), meaning that it is
not possible to use other sequences like tuples or lists on the right
side of the ``in`` and ``not in`` operators.

Chaining of comparison operations is permitted using the normal Python
semantics of an implied ``and``.

The ``python_full_version`` and ``implementation_version`` marker variables
are derived from ``sys.version_info()`` and ``sys.implementation.version``
respectively, in accordance with the following algorithm::

    def format_full_version(info):
        version = '{0.major}.{0.minor}.{0.micro}'.format(info)
        kind = info.releaselevel
        if kind != 'final':
            version += kind[0] + str(info.serial)
        return version

    python_full_version = format_full_version(sys.version_info)
    implementation_version = format_full_version(sys.implementation.version)

``python_full_version`` will typically correspond to the leading segment
of ``sys.version()``.


Updating the metadata specification
===================================

The metadata specification may be updated with clarifications without
requiring a new PEP or a change to the metadata version.

Changing the meaning of existing fields or adding new features (other than
through the extension mechanism) requires a new metadata version defined in
a new PEP.


Appendix A: Conversion notes for legacy metadata
================================================

The reference implementations for converting from legacy metadata to
metadata 2.0 are:

* the `wheel project <https://bitbucket.org/dholth/wheel/overview>`__, which
  adds the ``bdist_wheel`` command to ``setuptools``
* the `Warehouse project <https://github.com/dstufft/warehouse>`__, which
  will eventually be migrated to the Python Packaging Authority as the next
  generation Python Package Index implementation
* the `distlib project <https://bitbucket.org/pypa/distlib/>`__ which is
  derived from the core packaging infrastructure created for the
  ``distutils2`` project

.. note::

   These tools have yet to be updated for the switch to standard extensions
   for several fields.

While it is expected that there may be some edge cases where manual
intervention is needed for clean conversion, the specification has been
designed to allow fully automated conversion of almost all projects on
PyPI.

Metadata conversion (especially on the part of the index server) is a
necessary step to allow installation and analysis tools to start
benefiting from the new metadata format, without having to wait for
developers to upgrade to newer build systems.


Appendix B: Mapping dependency declarations to an RPM SPEC file
===============================================================

As an example of mapping this PEP to Linux distro packages, assume an
example project without any extras defined is split into 2 RPMs
in a SPEC file: ``example`` and ``example-devel``.

The ``meta_requires`` and ``run_requires`` dependencies would be mapped
to the Requires dependencies for the "example" RPM (a mapping from
environment markers relevant to Linux to SPEC file conditions would
also allow those to be handled correctly)

The ``build_requires`` dependencies would be mapped to the BuildRequires
dependencies for the "example" RPM.

All defined dependencies relevant to Linux, including those in
``dev_requires`` and ``test_requires`` would become Requires dependencies
for the "example-devel" RPM.

A documentation toolchain dependency like Sphinx would either go in
``build_requires`` (for example, if man pages were included in the
built distribution) or in ``dev_requires`` (for example, if the
documentation is published solely through ReadTheDocs or the
project website). This would be enough to allow an automated converter
to map it to an appropriate dependency in the spec file.

If the project did define any extras, those could be mapped to additional
virtual RPMs with appropriate BuildRequires and Requires entries based on
the details of the dependency specifications. Alternatively, they could
be mapped to other system package manager features (such as package lists
in ``yum``).

Other system package managers may have other options for dealing with
extras (Debian packagers, for example, would have the option to map them
to "Recommended" or "Suggested" package entries).

The metadata extension format should also allow distribution specific hints
to be included in the upstream project metadata without needing to manually
duplicate any of the upstream metadata in a distribution specific format.


Appendix C: Summary of differences from \PEP 345
=================================================

* Metadata-Version is now 2.0, with semantics specified for handling
  version changes

* The increasingly complex ad hoc "Key: Value" format has been replaced by
  a more structured JSON compatible format that is easily represented as
  Python dictionaries, strings, lists.

* Most fields are now optional and filling in dummy data for omitted fields
  is explicitly disallowed

* Explicit permission for in-place clarifications without releasing a new
  version of the specification

* The PEP now attempts to provide more of an explanation of *why* the fields
  exist and how they are intended to be used, rather than being a simple
  description of the permitted contents

* Changed the version scheme to be based on PEP 440 rather than PEP 386

* Added the source label mechanism as described in PEP 440

* Support for different kinds of dependencies

* The "Extras" optional dependency mechanism

* A well-defined metadata extension mechanism, and migration of any fields
  not needed for dependency resolution to standard extensions.

* Clarify and simplify various aspects of environment markers:

  * allow use of parentheses for grouping in the pseudo-grammar
  * consistently use underscores instead of periods in the variable names
  * allow ordered string comparisons and chained comparisons

* New system for defining supported environments

* Updated obsolescence mechanism

* Metadata hook system

* More flexible system for defining contact points and contributors

* Defined a recommended set of project URLs

* Identification of supporting documents in the ``dist-info`` directory:

  * Allows markup formats to be indicated through file extensions
  * Standardises the common practice of taking the description from README
  * Also supports inclusion of license files and changelogs

* With all due respect to Charles Schulz and Peanuts, many of the examples
  have been updated to be more `thematically appropriate`_ for Python ;)

The rationale for major changes is given in the following sections.


Metadata-Version semantics
--------------------------

The semantics of major and minor version increments are now specified,
and follow the same model as the format version semantics specified for
the wheel format in PEP 427: minor version increments must behave
reasonably when processed by a tool that only understand earlier metadata
versions with the same major version, while major version increments
may include changes that are not compatible with existing tools.

The major version number of the specification has been incremented
accordingly, as interpreting PEP 426 metadata obviously cannot be
interpreted in accordance with earlier metadata specifications.

Whenever the major version number of the specification is incremented, it
is expected that deployment will take some time, as either metadata
consuming tools must be updated before other tools can safely start
producing the new format, or else the sdist and wheel formats, along with
the installation database definition, will need to be updated to support
provision of multiple versions of the metadata in parallel.

Existing tools won't abide by this guideline until they're updated to
support the new metadata standard, so the new semantics will first take
effect for a hypothetical 2.x -> 3.0 transition. For the 1.x -> 2.0
transition, we will use the approach where tools continue to produce the
existing supplementary files (such as ``entry_points.txt``) in addition
to any equivalents specified using the new features of the standard
metadata format (including the formal extension mechanism).


Switching to a JSON compatible format
-------------------------------------

The old "Key:Value" format was becoming increasingly limiting, with various
complexities like parsers needing to know which fields were permitted to
occur more than once, which fields supported the environment marker
syntax (with an optional ``";"`` to separate the value from the marker) and
eventually even the option to embed arbitrary JSON inside particular
subfields.

The old serialisation format also wasn't amenable to easy conversion to
standard Python data structures for use in the new install hook APIs, or
in future extensions to the importer APIs to allow them to provide
information for inclusion in the installation database.

Accordingly, we've taken the step of switching to a JSON-compatible metadata
format. This works better for APIs and is much easier for tools to parse and
generate correctly. Changing the name of the metadata file also makes it
easy to distribute 1.x and 2.x metadata in parallel, greatly simplifying
several aspects of the migration to the new metadata format.


Changing the version scheme
---------------------------

See PEP 440 for a detailed rationale for the various changes made to the
versioning scheme.


Source labels
-------------

See PEP 440 for the rationale behind the addition of this field.


Support for different kinds of dependencies
-------------------------------------------

The separation of the five different kinds of dependency allows a
distribution to indicate whether a dependency is needed specifically to
develop, build, test or use the distribution.

To allow for metadistributions like PyObjC, while still actively
discouraging overly strict dependency specifications, the separate
``meta`` dependency fields are used to separate out those dependencies
where exact version specifications are appropriate.

The advantage of having these distinctions supported in the upstream Python
specific metadata is that even if a project doesn't care about these
distinction themselves, they may be more amenable to patches from
downstream redistributors that separate the fields appropriately. Over time,
this should allow much greater control over where and when particular
dependencies end up being installed.

The names for the dependency fields have been deliberately chosen to avoid
conflicting with the existing terminology in setuptools and previous
versions of the metadata standard. Specifically, the names ``requires``,
``install_requires`` and ``setup_requires`` are not used, which will
hopefully reduce confusion when converting legacy metadata to the new
standard.


Support for optional dependencies for distributions
---------------------------------------------------

The new extras system allows distributions to declare optional
behaviour, and to use the dependency fields to indicate when
particular dependencies are needed only to support that behaviour. It is
derived from the equivalent system that is already in widespread use as
part of ``setuptools`` and allows that aspect of the legacy ``setuptools``
metadata to be accurately represented in the new metadata format.

The additions to the extras syntax relative to setuptools are defined to
make it easier to express the various possible combinations of dependencies,
in particular those associated with build systems (with optional support
for running the test suite) and development systems.


Support for metadata extensions
-------------------------------

The new extension effectively allows sections of the metadata
namespace to be delegated to other distributions, while preserving a
standard overal format metadata format for easy of processing by
distribution tools that do not support a particular extension.

It also works well in combination with the new ``build_requires`` field
to allow a distribution to depend on tools which *do* know how to handle
the chosen extension, and the new extras mechanism, allowing support for
particular extensions to be provided as optional features.

Possible future uses for extensions include declaration of plugins for
other distributions, hints for automatic conversion to Linux system
packages, and inclusion of CVE references to mark security releases.


Support for metadata hooks
---------------------------

This feature is provided by the ``python.metadata_hooks`` extension in
:pep:`459`.

The new metadata hook system is designed to allow the wheel format to fully
replace direct installation on deployment targets, by allowing projects to
explicitly define code that should be executed following installation from
a wheel file.

This may be something relatively simple, like the `two line
refresh <https://twistedmatrix.com/documents/current/core/howto/plugin.html#auto3>`__
of the Twisted plugin caches that the Twisted developers recommend for
any project that provides Twisted plugins, to more complex platform
dependent behaviour, potentially in conjunction with appropriate
metadata extensions and ``supports_environments`` entries.

For example, upstream declaration of external dependencies for various
Linux distributions in a distribution neutral format may be supported by
defining an appropriate metadata extension that is read by a postinstall
hook and converted into an appropriate invocation of the system package
manager. Other operations (such as registering COM DLLs on Windows,
registering services for automatic startup on any platform, or altering
firewall settings) may need to be undertaken with elevated privileges,
meaning they cannot be deferred to implicit execution on first use of the
distribution.

The metadata hook and metadata extension systems allow support for such
activities to be pursued independently by the individual platform
communities, while still interoperating with the cross-platform Python
tools.

Legacy packages that expect to able to run code on target systems using
``setup.py install`` will no longer work correctly. Such packages will
already break when pip 1.4+ is configured to use a wheel cache directory.


Changes to environment markers
------------------------------

There are three substantive changes to environment markers in this version:

* ``platform_release`` was added, as it provides more useful information
  than ``platform_version`` on at least Linux and Mac OS X (specifically,
  it provides details of the running kernel version)
* ordered comparison of strings is allowed, as this is more useful for
  setting minimum and maximum versions where conditional dependencies
  are needed or where a platform is supported
* comparison chaining is explicitly allowed, as this becomes useful in the
  presence of ordered comparisons

The other changes to environment markers are just clarifications and
simplifications to make them easier to use.

The arbitrariness of the choice of ``.`` and ``_`` in the different
variables was addressed by standardising on ``_`` (as these are all
predefined variables rather than live references into the Python module
namespace)

The use of parentheses for grouping was explicitly noted to address some
underspecified behaviour in the previous version of the specification.


Updated contact information
---------------------------

This feature is provided by the ``python.project`` and
``python.integrator`` extensions in :pep:`459`.

The switch to JSON made it possible to provide a more flexible
system for defining multiple contact points for a project, as well as
listing other contributors.

The ``type`` concept allows for preservation of the distinction between
the original author of a project, and a lead maintainer that takes over
at a later date.


Changes to project URLs
-----------------------

This feature is provided by the ``python.project`` and
``python.integrator`` extensions in :pep:`459`.

In addition to allow arbitrary strings as project URL labels, the new
metadata standard also defines a recommend set of four URL labels for
a distribution's home page, documentation, source control and issue tracker.


Changes to platform support
---------------------------

The new environment marker system makes it possible to define supported
platforms in a way that is actually amenable to automated processing. This
has been used to replace several older fields with poorly defined semantics.

For the moment, the old ``Requires-External`` field has been removed
entirely. The combination of explicit support for post install hooks and the
metadata extension mechanism will hopefully prove to be a more useful
replacement.


Updated obsolescence mechanism
------------------------------

The marker to indicate when a project is obsolete and should be replaced
has been moved to the obsolete project (the new ``obsoleted_by`` field),
replacing the previous marker on the replacement project (the removed
``Obsoletes-Dist`` field).

This should allow distribution tools to more easily warn users of
obsolete projects and their suggested replacements.

The ``Obsoletes-Dist`` header is removed rather than deprecated as it
is not widely supported, and so removing it does not present any significant
barrier to tools and projects adopting the new metadata format.


Included text documents
-----------------------

This feature is provided by the ``python.details`` extension in :pep:`459`.

Currently, PyPI attempts to determine the description's markup format by
rendering it as reStructuredText, and if that fails, treating it as plain
text.

Furthermore, many projects simply read their long description in from an
existing README file in ``setup.py``. The popularity of this practice is
only expected to increase, as many online version control systems
(including both GitHub and BitBucket) automatically display such files
on the landing page for the project.

Standardising on the inclusion of the long description as a separate
file in the ``dist-info`` directory allows this to be simplified:

* An existing file can just be copied into the ``dist-info`` directory as
  part of creating the sdist
* The expected markup format can be determined by inspecting the file
  extension of the specified path

Allowing the intended format to be stated explicitly in the path allows
the format guessing to be removed and more informative error reports to be
provided to users when a rendering error occurs.

This is especially helpful since PyPI applies additional restrictions to
the rendering process for security reasons, thus a description that renders
correctly on a developer's system may still fail to render on the server.

The document naming system used to achieve this then makes it relatively
straightforward to allow declaration of alternative markup formats like
HTML, Markdown and AsciiDoc through the use of appropriate file
extensions, as well as to define similar included documents for the
project's license and changelog.

Grouping the included document names into a single top level field gives
automated tools the option of treating them as arbitrary documents without
worrying about their contents.

Requiring that the included documents be added to the ``dist-info`` metadata
directory means that the complete metadata for the distribution can be
extracted from an sdist or binary archive simply by extracting that
directory, without needing to check for references to other files in the
sdist.


Appendix D: Deferred features
=============================

Several potentially useful features have been deliberately deferred in
order to better prioritise our efforts in migrating to the new metadata
standard. These all reflect information that may be nice to have in the
new metadata, but which can be readily added in metadata 2.1 without
breaking any use cases already supported by metadata 2.0.

Once the ``pypi``, ``setuptools``, ``pip``, ``wheel`` and ``distlib``
projects support creation and consumption of metadata 2.0, then we may
revisit the creation of metadata 2.1 with some or all of these additional
features.


MIME type registration
----------------------

At some point after acceptance of the PEP, I will likely submit the
following MIME type registration requests to IANA:

* Full metadata: ``application/vnd.python.pydist+json``
* Essential dependency resolution metadata:
  ``application/vnd.python.pydist-dependencies+json``

It's even possible we may be able to just register the ``vnd.python``
namespace under the banner of the PSF rather than having to register
the individual subformats.


String methods in environment markers
-------------------------------------

Supporting at least ".startswith" and ".endswith" string methods in
environment markers would allow some conditions to be written more
naturally. For example, ``"sys.platform.startswith('win')"`` is a
somewhat more intuitive way to mark Windows specific dependencies,
since ``"'win' in sys.platform"`` is incorrect thanks to ``cygwin``
and the  fact that 64-bit Windows still shows up as ``win32`` is more
than a little strange.


Additional install hooks
------------------------

In addition to the postinstall and postuninstall hooks currently described
in :pep:`459`, other distribution systems (like RPM) include the notion of
preinstall and postuninstall hooks. These hooks would run with the runtime
dependencies installed, but without the distribution itself. These have
been deliberately omitted for the time being.


Metabuild system
----------------

This version of the metadata specification continues to use ``setup.py``
and the distutils command syntax to invoke build and test related
operations on a source archive or VCS checkout.

It may be desirable to replace these in the future with tool independent
entry points that support:

* Generating the metadata file on a development system
* Generating an sdist on a development system
* Generating a binary archive on a build system
* Running the test suite on a built (but not installed) distribution

Metadata 2.0 deliberately focuses on wheel based installation, leaving
sdist, source archive, and VCS checkout based installation to use the
existing ``setup.py`` based ``distutils`` command interface.

In the meantime, the above operations will be handled through the
``distutils``/``setuptools`` command system:

* ``python setup.py dist_info``
* ``python setup.py sdist``
* ``python setup.py build_ext --inplace``
* ``python setup.py test``
* ``python setup.py bdist_wheel``

The following metabuild hooks may be defined in metadata 2.1 to
cover these operations without relying on ``setup.py``:

* ``make_dist_info``: generate the sdist's dist_info directory
* ``make_sdist``: create the contents of an sdist
* ``build_dist``: create the contents of a binary wheel archive from an
  unpacked sdist
* ``test_built_dist``: run the test suite for a built distribution

Tentative signatures have been designed for those hooks, but in order to
better focus initial development efforts on the integration and installation
use cases, they will not be pursued further until metadata 2.1::

    def make_dist_info(source_dir, info_dir):
        """Generate the contents of dist_info for an sdist archive

        *source_dir* points to a source checkout or unpacked tarball
        *info_dir* is the destination where the sdist metadata files should
        be written

        Returns the distribution metadata as a dictionary.
        """

    def make_sdist(source_dir, contents_dir, info_dir):
        """Generate the contents of an sdist archive

        *source_dir* points to a source checkout or unpacked tarball
        *contents_dir* is the destination where the sdist contents should be
        written (note that archiving the contents is the responsibility of
        the metabuild tool rather than the hook function)
        *info_dir* is the destination where the sdist metadata files should
        be written

        Returns the distribution metadata as a dictionary.
        """

    def build_dist(sdist_dir, built_dir, info_dir, compatibility=None):
        """Generate the contents of a binary wheel archive

        *sdist_dir* points to an unpacked sdist
        *built_dir* is the destination where the wheel contents should be
        written (note that archiving the contents is the responsibility of
        the metabuild tool rather than the hook function)
        *info_dir* is the destination where the wheel metadata files should
        be written
        *compatibility* is an optional PEP 425 compatibility tag indicating
        the desired target compatibility for the build. If the tag cannot
        be satisfied, the hook should throw ``ValueError``.

        Returns the actual compatibility tag for the build
        """

    def test_built_dist(sdist_dir, built_dir, info_dir):
        """Check a built (but not installed) distribution works as expected

        *sdist_dir* points to an unpacked sdist
        *built_dir* points to a platform appropriate unpacked wheel archive
        (which may be missing the wheel metadata directory)
        *info_dir* points to the appropriate wheel metadata directory

        Requires that the distribution's test dependencies be installed
        (indicated by the ``:test:`` extra).

        Returns ``True`` if the check passes, ``False`` otherwise.
        """

As with the existing install hooks, checking for extras would be done
using the same import based checks as are used for runtime extras. That
way it doesn't matter if the additional dependencies were requested
explicitly or just happen to be available on the system.

There are still a number of open questions with this design, such as whether
a single build hook is sufficient to cover both "build for testing" and
"prep for deployment", as well as various complexities like support for
cross-compilation of binaries, specification of target platforms and
Python versions when creating wheel files, etc.

Opting to retain the status quo for now allows us to make progress on
improved metadata publication and binary installation support, rather than
having to delay that awaiting the creation of a viable metabuild framework.


Appendix E: Rejected features
=============================

The following features have been explicitly considered and rejected as
introducing too much additional complexity for too small a gain in
expressiveness.


Separate lists for conditional and unconditional dependencies
-------------------------------------------------------------

Earlier versions of this PEP used separate lists for conditional and
unconditional dependencies. This turned out to be annoying to handle in
automated tools and removing it also made the PEP and metadata schema
substantially shorter, suggesting it was actually harder to explain as well.


Disallowing underscores in distribution names
---------------------------------------------

Debian doesn't actually permit underscores in names, but that seems
unduly restrictive for this spec given the common practice of using
valid Python identifiers as Python distribution names. A Debian side
policy of converting underscores to hyphens seems easy enough to
implement (and the requirement to consider hyphens and underscores as
equivalent ensures that doing so won't introduce any conflicts).


Allowing the use of Unicode in distribution names
-------------------------------------------------

This PEP deliberately avoids following Python 3 down the path of arbitrary
Unicode identifiers, as the security implications of doing so are
substantially worse in the software distribution use case (it opens
up far more interesting attack vectors than mere code obfuscation).

In addition, the existing tools really only work properly if you restrict
names to ASCII and changing that would require a *lot* of work for all
the automated tools in the chain.

It may be reasonable to revisit this question at some point in the (distant)
future, but setting up a more reliable software distribution system is
challenging enough without adding more general Unicode identifier support
into the mix.


Single list for conditional and unconditional dependencies
----------------------------------------------------------

It's technically possible to store the conditional and unconditional
dependencies of each kind in a single list and switch the handling based on
the entry type (string or mapping).

However, the current ``*requires`` vs ``*may-require`` two list design seems
easier to understand and work with, since it's only the conditional
dependencies that need to be checked against the requested extras list and
the target installation environment.


Depending on source labels
--------------------------

There is no mechanism to express a dependency on a source label - they
are included in the metadata for internal project reference only. Instead,
dependencies must be expressed in terms of either public versions or else
direct URL references.


Alternative dependencies
------------------------

An earlier draft of this PEP considered allowing lists in place of the
usual strings in dependency specifications to indicate that there aren
multiple ways to satisfy a dependency.

If at least one of the individual dependencies was already available, then
the entire dependency would be considered satisfied, otherwise the first
entry would be added to the dependency set.

Alternative dependency specification example::

   ["Pillow", "PIL"]
   ["mysql", "psycopg2 (>= 4)", "sqlite3"]

However, neither of the given examples is particularly compelling,
since Pillow/PIL style forks aren't common, and the database driver use
case would arguably be better served by an SQL Alchemy defined "supported
database driver" metadata extension where a project depends on SQL Alchemy,
and then declares in the extension which database drivers are checked for
compatibility by the upstream project (similar to the advisory
``supports_environments`` field in the main metadata).

We're also getting better support for "virtual provides" in this version of
the metadata standard, so this may end up being an installer and index
server problem to better track and publish those.


Compatible release comparisons in environment markers
-----------------------------------------------------

PEP 440 defines a rich syntax for version comparisons that could
potentially be useful with ``python_version`` and ``python_full_version``
in environment markers. However, allowing the full syntax would mean
environment markers are no longer a Python subset, while allowing
only some of the comparisons would introduce yet another special case
to handle.

Given that environment markers are only used in cases where a higher level
"or" is implied by the metadata structure, it seems easier to require the
use of multiple comparisons against specific Python versions for the rare
cases where this would be useful.


Conditional provides
--------------------

Under the revised metadata design, conditional "provides" based on runtime
features or the environment would go in a separate "may_provide" field.
However, it isn't clear there's any use case for doing that, so the idea
is rejected unless someone can present a compelling use case (and even then
the idea won't be reconsidered until metadata 2.1 at the earliest).


A hook to run tests against installed distributions
---------------------------------------------------

Earlier drafts of this PEP defined a hook for running automated
tests against an *installed* distribution. This isn't actually what you
generally want - you want the ability to test a *built* distribution,
potentially relying on files which won't be included in the binary archives.

RPM's "check" step also runs between the build step and the install step,
rather than after the install step.

Accordingly, the ``test_installed_dist`` hook has been removed, and the
``test_built_dist`` metabuild hook has been tentatively defined. However,
along with the rest of the metabuild hooks, further consideration has been
deferred until metadata 2.1 at the earliest.


Extensible signatures for the install hooks
-------------------------------------------

The install hooks have been deliberately designed to NOT accept arbitary
keyword arguments that the hook implementation is then expected to ignore.

The argument in favour of that API design technique is to allow the addition
of new optional arguments in the future, without requiring the definition
of a new install hook, or migration to version 3.0 of the metadata
specification. It is a technique very commonly seen in function wrappers
which merely pass arguments along to the inner function rather than
processing them directly.

However, the install hooks are already designed to have access to the full
metadata for the distribution (including all metadata extensions and
the previous/next version when appropriate), as well as to the full target
deployment environment.

This means there are two candidates for additional information that
could be passed as arbitrary keyword arguments:

* installer dependent settings
* user provided installation options

The first of those runs explicitly counter to one of the core goals of the
metadata 2.0 specification: decoupling the software developer's choice of
development and publication tools from the software integrator's choice of
integration and deployment tools.

The second is a complex problem that has a readily available workaround in
the form of operating system level environment variables (this is also
one way to interoperate with platform specific installation tools).

Alternatively, installer developers may either implicitly inject an
additional metadata extension when invoking the install hook, or else
define an alternate hook signature as a distinct metadata extension to be
provided by the distribution. Either of these approaches makes the
reliance on installer-dependent behaviour suitably explicit in either
the install hook implementation or the distribution metadata.


References
==========

This document specifies version 2.0 of the metadata format.
Version 1.0 is specified in PEP 241.
Version 1.1 is specified in PEP 314.
Version 1.2 is specified in PEP 345.

The initial attempt at a standardised version scheme, along with the
justifications for needing such a standard can be found in PEP 386.

.. [1] reStructuredText markup:
   http://docutils.sourceforge.net/

.. _Python Package Index: http://pypi.python.org/pypi/

.. [2] PEP 301:
   http://www.python.org/dev/peps/pep-0301/

.. _thematically appropriate: https://www.youtube.com/watch?v=CSe38dzJYkY

.. _TR39: http://www.unicode.org/reports/tr39/tr39-1.html#Confusable_Detection


Copyright
=========

This document has been placed in the public domain.


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