skip to navigation
skip to content

Python Wiki

Python Insider Blog

Python 2 or 3?

Help Fund Python

[Python resources in languages other than English]

Non-English Resources

Add an event to this calendar.

Times are shown in UTC/GMT.

Add an event to this calendar.

PEP:426
Title:Metadata for Python Software Packages 2.0
Version:471651c1fe20
Last-Modified:2014-07-02 22:55:34 -0700 (Wed, 02 Jul 2014)
Author:Nick Coghlan <ncoghlan at gmail.com>, Daniel Holth <dholth at gmail.com>, Donald Stufft <donald at stufft.io>
BDFL-Delegate:Nick Coghlan <ncoghlan@gmail.com>
Discussions-To:Distutils SIG <distutils-sig at 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

Contents

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 [3].

"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 [3] 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, 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.

Note that these metadata files SHOULD NOT be processed if the version of the containing location is too low to indicate that they are valid. Specifically, unversioned sdist archives, unversioned installation database directories and version 1.0 of the wheel specification do not cover pydist.json files.

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 description of the distribution metadata is available.

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) 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 or local version identifier, as defined in PEP 440. Version identifiers 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.

Index servers MAY place restrictions on the use of local version identifiers as described in PEP 440.

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 labels

Source labels are text strings with minimal defined semantics. They are intended to allow the original source code to be unambiguously identified, even if an integrator has applied additional local modifications to a particular distribution.

To ensure source labels can be readily incorporated as part of file names and URLs, and to avoid formatting inconsistencies in hexadecimal hash representations they MUST be limited to the following set of permitted characters:

  • Lowercase ASCII letters ([a-z])
  • ASCII digits ([0-9])
  • underscores (_)
  • hyphens (-)
  • periods (.)
  • plus signs (+)

Source labels MUST start and end with an ASCII letter or digit.

A source label for a project MUST NOT match any defined version for that project. This restriction ensures that there is no ambiguity between version identifiers and source labels.

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 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.

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

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 [3]; 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"

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.

Two key names are reserved and MUST NOT be used by extensions, except as described below:

  • extension_version
  • installer_must_handle

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.

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.

Extension versioning

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.

Required extension handling

A project may consider correct handling of some extensions to be essential to correct installation of the software. This is indicated by setting the installer_must_handle field to true. Setting it to false or omitting it altogether indicates that processing the extension when installing the distribution is not considered mandatory by the developers.

Installation tools MUST fail if installer_must_handle is set to true for an extension and the tool does not have any ability to process that particular extension (whether directly or through a tool-specific plugin system).

If an installation tool encounters a required extension it doesn't understand when attempting to install from a wheel archive, it MAY fall back on attempting to install from source rather than failing entirely.

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, which adds the bdist_wheel command to setuptools
  • the Warehouse project, which will eventually be migrated to the Python Packaging Authority as the next generation Python Package Index implementation
  • the distlib project 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 constraint mechanism to define supported environments and ensure compatibility between independently built binary components at installation time
  • Updated obsolescence mechanism
  • 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 [4] 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.

The specific choice of pydist.json as the preferred file name relates to the fact that the metadata described in these files applies to the distribution as a whole, rather than to any particular build. Additional metadata formats may be defined in the future to hold information that can only be determined after building a binary distribution for a particular target environment.

Changing the version scheme

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

Source labels

The new source label support is intended to make it clearer that the constraints on public version identifiers are there primarily to aid in the creation of reliable automated dependency analysis tools. Projects are free to use whatever versioning scheme they like internally, so long as they are able to translate it to something the dependency analysis tools will understand.

Source labels also make it straightforward to record specific details of a version, like a hash or tag name that allows the release to be reconstructed from the project version control system.

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 and hints for automatic conversion to Linux system packages.

The ability to declare an extension as required is included primarily to allow the definition of the metadata hooks extension to be deferred until some time after the initial adoption of the metadata 2.0 specification. If a distribution needs a postinstall hook to run in order to complete the installation successfully, then earlier versions of tools should fall back to installing from source rather than installing from a wheel file and then failing to run the expected postinstall hook.

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

This feature is provided by the python.constraints extension in PEP 459.

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.

The constraints mechanism also allows additional information to be conveyed through metadata extensions and then checked for consistency at install time.

For the moment, the old Requires-External field has been removed entirely. 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, we may 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.

Support for metadata hooks

While a draft proposal for a metadata hook system has been created, that proposal is not part of the initial set of standard metadata extensions in PEP 459.

A metadata hook system would 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 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.

For the time being, any such system is being left to the realm of tool specific metadata extensions. This does mean that affected projects may choose not to publish wheel files, instead continuing to rely on source distributions until the relevant extension is well defined and widely supported.

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).

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/
[2]PEP 301: http://www.python.org/dev/peps/pep-0301/
[3](1, 2, 3) http://pypi.python.org/pypi/
[4]https://www.youtube.com/watch?v=CSe38dzJYkY