PEP: 639
Title: Metadata for Python Software Packages 2.2
Version: $Revision$
Last-Modified: $Date$
Author: Philippe Ombredanne <pombredanne at nexb.com>
Sponsor: Paul Moore <p.f.moore at gmail.com>
BDFL-Delegate: Paul Moore <p.f.moore at gmail.com>
Discussions-To: https://discuss.python.org/t/2154
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 15-Aug-2019
Python-Version: 3.x
Post-History:
Replaces: 566
Resolution:


Abstract
========

This PEP describes the changes between versions 2.1 and 2.2 of the `Core
Metadata Specification` [#cms]_ for Python packages. Version 2.1 is specified in
PEP 566.

The primary change introduced in this PEP updates how licenses are documented in
core metadata via the ``License`` field with license expression strings using
SPDX license identifiers [#spdxlist]_ such that license documentation is simpler and less
ambiguous:

- for package authors to create,
- for package users to read and understand, and,
- for tools to process package license information mechanically.

The other changes include:

- specifying a ``License-File`` field which is already used by ``wheel`` and
  ``setuptools`` to include license files in built distributions.
- defining how tools can validate license expressions and report warnings to
  users for invalid expressions (but still accept any string as ``License``).


Goals
=====

This PEP's scope is limited strictly to how we document the license of a
distribution:

- with an improved and structured way to document a license expression, and,
- by including license texts in a built package.

The core metadata specification updates that are part of this PEP, have been
designed to have minimal impact and to be backward compatible with v2.1.
These changes utilize emerging new ways to document licenses that are already
in use in some tools (e.g. by adding the ``Licence-File`` field already used in
``wheel`` and ``setuptools``) or by some package authors (e.g. storing an SPDX
license expression in the existing ``License`` field).

In addition to an update to the metadata specification, this PEP contains:

- recommendations for publishing tools on how to validate the ``License``
  and ``Classifier`` fields and report informational warnings when a package uses
  an older, non-structured style of license documentation conventions.

- informational appendixes that contain surveys of how we document licenses
  today in Python packages and elsewhere, and a reference Python library to
  parse, validate and build correct license expressions.

It is the intent of the PEP authors to work closely with tool authors to
implement to recommendations for validation and warnings specified in this
PEP.


Non-Goals
=========

This PEP is neutral regarding the choice of various licenses.

In particular, the SPDX license expression syntax proposed in this PEP provides
simpler and more expressive conventions to document more accurately any kind of
license that applies to a Python package, whether under an open source, free or
libre software license or a proprietary license.

This PEP makes no recommendation for certain licenses and does not require the use of
specific license documentation conventions. This PEP also does not impose any
restrictions when uploading to PyPI.

Instead, this PEP is intended to document common practices already in use,
and recommends that publishing tools should encourage users via informational
warnings when they do not follow this PEP's recommendations.

This PEP is not about documenting license in code files, even though this is a
surveyed topic in the appendix.


Possible future PEPs
--------------------

It is the intention of the authors of this PEP to consider the submission of
related but separate PEPs in the future such as:

- make ``License`` and new ``License-File`` fields mandatory including
  stricter enforcement in tools and PyPI publishing.

- require uploads to PyPI to use only FOSS (Free and Open Source software)
  licenses.


Motivation
==========

Software is licensed and providing accurate licensing information to Python
packages users is an important matter.  Today, there are multiple places where
license is documented in package metadata and there are limitations to what can
be documented. This is often leading to confusion or a lack of clarity both for
package authors and package users.

Several package authors have expressed difficulty and/or frustrations with the
possibilities to express licensing in package metadata. This also applies to
Linux and BSD* distribution packagers. This has triggered several license-related
discussions and issues and in particular:

- https://github.com/pypa/trove-classifiers/issues/17
- https://github.com/pypa/interoperability-peps/issues/46
- https://github.com/pypa/packaging-problems/issues/41
- https://github.com/pypa/wheel/issues/138
- https://github.com/pombredanne/spdx-pypi-pep/issues/1

On average, Python packages tend to have more ambiguous or missing license
information than other common application package formats (such as npm, Maven or
Gem) as can be seen in the statistics [#cdstats]_ page of the ClearlyDefined
[#cd]_ project that cover all packages from PyPI, Maven, npm and Rubygems.
ClearlyDefined is an open source project to help improve clarity of other open
source projects that is incubating at the OSI (Open Source Initiative) [#osi]_.


Rationale
=========

A mini survey of existing license metadata definitions in use in the Python
ecosystem today and documented in several other system/distro and application
package formats is provided in Appendix 2 of this PEP.

There are a few takeaways from the survey:

- Most package formats use a single ``License`` field.

- Many modern package formats use some form of license expression syntax to
  optionally combine more than one license identifiers together. SPDX and
  SPDX-like syntaxes are the most popular in use.

- SPDX license identifiers are becoming a de-facto way to reference common licenses
  everywhere, whether or not a license expression syntax is used.

- Several package formats support documenting both a license expression and
  the paths of the corresponding files that contain the license text. Most free
  and open source software licenses require to include their full text in a
  distribution.

These considerations have guided the design and recommendations of this PEP.

The reuse of the ``License`` field with license expressions will provide an
intuitive and more structured way to express the license of a distribution using
a well-defined syntax and well-known license identifiers.

Over time, recommending the usage of these expressions will help Python package
publishers improve the clarity of their license documentation to the benefit of
packages authors, consumers and redistributors.


Core Metadata Specification updates
===================================

The canonical source for the names and semantics of each of the supported
metadata fields is the Core Metadata Specification [#cms]_ document.

The details of the updates considered to the Core Metadata Specification [#cms]_
document as part of this PEP are detailed here and will be added to the
canonical source once this PEP is approved.


Added in Version 2.2
--------------------

License-File (multiple use)
:::::::::::::::::::::::::::

The License-File is a string that is a ``.dist-info`` relative path to a license
file. The license file content MUST be UTF-8 encoded text.

Build tools SHOULD honor this field and include the corresponding license
file(s) in the built package.


Changed in Version 2.2
----------------------

License (optional)
::::::::::::::::::

Text indicating the license covering the distribution. This text can be either a
valid license expression as defined here or any free text.

Publishing tools SHOULD issue an informational warning if this field is empty,
missing, or is not a valid license expression as defined here. Build tools MAY
issue a similar warning.


License Expression syntax
'''''''''''''''''''''''''

A license expression is a string using the SPDX license expression syntax as
documented in the SPDX specification [#spdx]_ using either Version 2.2
[#spdx22]_ or a later compatible version. SPDX is a working group at the Linux
Foundation that defines a standard way to exchange package information.

When used in the ``License`` field and as a specialization of the SPDX license
expression definition, a license expression can use the following license
identifiers:

- any SPDX-listed license short-form identifiers that are published in the
  SPDX License List [#spdxlist]_ using either Version 3.10 of this list or any
  later compatible version. Note that the SPDX working group never removes any
  license identifiers: instead they may only mark an identifier as "obsolete".

- the ``LicenseRef-Public-Domain`` and ``LicenseRef-Proprietary`` strings to support
  generic identifiers that are not available in the SPDX license list.

When processing the ``License`` field to determine if it contains a valid license
expression, tools:

- MUST ignore the case of the ``License`` field

- SHOULD report an informational warning if one or more of the following applies:

  - the field does not contain a license expression,
  - the license expression syntax is invalid,
  - the license expression syntax is valid but some license identifiers are
    unknown as defined here or the license identifiers have been marked as
    deprecated in the SPDX License List [#spdxlist]_

- SHOULD store a case-normalized version of the ``License`` field using the
  reference case for each SPDX license identifier and uppercase for the AND, OR
  and WITH keywords. And SHOULD report an informational warning if the reference
  case is not used.

License expression examples::

    License: MIT

    License: BSD-3-Clause

    License: MIT OR GPL-2.0-or-later OR (FSFUL AND BSD-2-Clause)

    License: GPL-3.0-only WITH Classpath-Exception-2.0 OR BSD-3-Clause

    License: This software may only be obtained by sending the
            author a postcard, and then the user promises not
            to redistribute it.

    License: LicenseRef-Proprietary AND LicenseRef-Public-Domain


Classifier (multiple use)
:::::::::::::::::::::::::

Each entry is a string giving a single classification value for the
distribution. Classifiers are described in PEP 301.

Examples::

    Classifier: Development Status :: 4 - Beta
    Classifier: Environment :: Console (Text Based)

Tools SHOULD issue an informational warning if this field contains a licensing
related classifier string starting with the ``License ::`` prefix and SHOULD
suggest the use of a license expression in the ``License`` field instead.

If the ``License`` field is present and contains a valid license expression,
publishing tools MUST NOT also provide any licensing related classifier entries
[#classif]_.

However, for compatibility with existing publishing and installation processes,
licensing-related classifier entries SHOULD continue to be accepted if the
``License`` field is absent or does not contain a valid license expression.

Publishing tools MAY infer a license expression from the provided classifier
entries if they are able to do so unambiguously.

However, no new licensing related classifiers will be added, with anyone
requesting them being directed to use a license expression in the ``License``
field instead. Note that the licensing related classifiers may be deprecated in
a future PEP.


Mapping Legacy Classifiers to New License Expressions
'''''''''''''''''''''''''''''''''''''''''''''''''''''

Publishing tools MAY infer or suggest an equivalent license expression from
the provided ``License`` or ``Classifier`` information if they are able to do so
unambiguously. For instance, if a package only has this license classifier::

    Classifier: License :: OSI Approved :: MIT License

Then the corresponding value for a ``License`` field using a valid license
expression to suggest would be::

    License: MIT


Here are mappings guidelines for the legacy classifiers:

- Classifier ``License :: Other/Proprietary License`` becomes License:
  ``LicenseRef-Proprietary`` expression.

- Classifier ``License :: Public Domain`` becomes License: ``LicenseRef-Public-Domain``
  expression, though tools should encourage the use of more explicit and legally
  portable licenses identifiers such as  ``CC0-1.0`` [#cc0]_, the ``Unlicense``
  [#unlic]_: the meaning associated with the term "public domain" is thoroughly
  dependent on the specific legal jurisdiction involved and some jurisdictions
  have no concept of Public Domain as it exists in the USA.

- The generic and ambiguous classifiers ``License :: OSI Approved``
  and ``License :: DFSG approved`` do not have an equivalent license expression.

- The generic and sometimes ambiguous classifiers
  ``License :: Free For Educational Use``, ``License :: Free For Home Use``,
  ``License :: Free for non-commercial use``, ``License :: Freely Distributable``,
  ``License :: Free To Use But Restricted``, and ``License :: Freeware`` are mapped
  to the generic License: ``LicenseRef-Proprietary`` expression.

- Classifiers ``License :: GUST*`` have no mapping to SPDX license identifierss
  for now and no package uses them in PyPI as of the writing of this PEP.

The remainder of the classifiers using a ``License ::`` prefix map to a simple
single license expression using the corresponding SPDX license identifiers.

When multiple license-related classifiers are used, their relation is
ambiguous and it is typically not possible to determine if all the licenses
apply or if there is a choice that is possible among the licenses. In this case,
tools cannot infer reliably a license expression to suggest using only the
legacy classifier usage.


Summary of Differences From PEP 566
===================================

* Metadata-Version is now 2.2.
* Added one new field: ``License-File``
* Updated the documentation of two fields: ``License`` and ``Classifier``


Backwards Compatibility
=======================

The reuse of the ``License`` field means that we keep backward compatibility. The
specification of the ``License-File`` field is only writing down the practices
of the ``wheel`` and ``setuptools`` tools and is backward compatible with their
support for that field.

The "soft" validation of the ``License`` field when it does not contain a valid
license expression and when the ``Classifier`` field is used with legacy
license-related classifiers means that we can gently prepare users for a
possible strict and incompatible validation of these fields in the future.


Security Implications
=====================

This PEP has no foreseen security implications: the License field is
a plain string and the License-File(s) are file paths. None of them introduces
any new security concern.


How to Teach Users to Use License Expressions
=============================================

The simple cases are simple: a single license id is a valid license expression
and a large majority of packages use a single license.

The plan to teach users of packaging tools how to use the license with a valid
license expressions is to have tool issue warning messages when they detect an
incorrect license expressions or when a license-related classifier is used in
the ``Classifier`` field.

With a warning message that does not terminate processing, publishing tools will
gently teach users on how to provide correct license expressions over time.

Tools may also help with the conversion and suggest a license expression in some
cases:

1. The section `Mapping Legacy Classifiers to New License expressions` provides
   tools authors with guidelines on how to suggest a license expression from
   legacy classifiers.

2. Tools may also be able to infer and suggest how to update an existing
   incorrect ``License`` value and convert that to a correct license expression.
   For instance a tool may suggest to correct a ``License`` field from ``Apache2``
   (which is not a valid license expression as defined in this PEP) to
   ``Apache-2.0`` (which is a valid license expression using an SPDX license id as
   defined in this PEP).


Reference Implementation
========================

Tools will need to support parsing and validating license expressions in the
``License`` field.

The ``license-expression`` library [#licexp]_ is a reference Python
implementation for a library that handles license expressions including
parsing, validating and formatting license expressions using flexible lists of
license symbols (including SPDX license identifiers and any extra identifiers
referenced here). It is licensed under the Apache-2.0 license and is used in a
few projects such as the SPDX Python tools [#spdxpy]_, the ScanCode toolkit
[#scancodetk]_ and the Free Software Foundation Europe (FSFE) Reuse project
[#reuse]_.


Rejected ideas
==============

1. use a new ``License-Expression`` field and deprecate the ``License`` field.

Adding a new field would introduce backward incompatible changes when the
``License`` field would be retired later and require to have a more complex
validation. The use of such a field would further introduce a new concept that
is not seen anywhere else in any other package metadata (e.g. a new a field only
for license expression) and possibly be a source of confusion. Also, users are
less likely to start using a new field than make small adjustments to their use
of existing fields.


2. mapping licenses used in the license expression to specific files in the
   license files (or vice versa).

This would require using a mapping (two parallel lists would be too prone to
alignment errors) and a mapping would bring extra complication to how license
are documented by adding an additional nesting level.

A mapping would be needed as you cannot guarantee that all expressions (e.g. a
GPL with an exception may be in a single file) or all the license keys have a
single license file and that any expression does not have more than one. (e.g.
an Apache license ``LICENSE`` and its ``NOTICE`` file for instance are two
distinct file). Yet in most cases, there is a simpler `one license`, `one or
more license files`. In the rarer and more complex cases where there are many
licenses involved you can still use the proposed conventions at the cost of a
slight loss of clarity by not specifying which text file is for which license
identifier, but you are not forcing the more complex data model (e.g. a mapping)
on everyone that may not need it.

We could of course have data field with multiple possible value types (it’s a
string, it’s a list, it’s a mapping!) but this could be a source of confusion.
This is what has been done for instance in npm (historically) and in Rubygems
(still today) and as result you need to test the type of the metadata field
before using it in code and users are confused about when to use a list or a
string.


3. mapping licenses to specific source files and/or directories of source files
   (or vice versa).

File-level notices are not considered as part of the scope of this PEP and the
existing the ``SPDX-License-Identifier`` [#spdxids]_ convention can be used and
may not need further specification as a PEP.


Appendix 1. License Expression example
======================================

The current version of ``setuptools`` metadata [#setuptools5030]_ does not use
the ``License`` field. It uses instead this license-related information in
``setup.cfg``::

    license_file = LICENSE
    classifiers =
        License :: OSI Approved :: MIT License

The simplest migration to this PEP would consist of using this instead::

    license = MIT
    license_files =
        LICENSE

Another possibility would be to include the licenses of the third-party packages
bundled in that are vendored in the ``setuptools/_vendor/`` and
``pkg_resources/_vendor`` directories::

    appdirs==1.4.3
    packaging==20.4
    pyparsing==2.2.1
    ordered-set==3.1.1

These are using these license expressions::

    appdirs: MIT
    packaging: Apache-2.0 OR BSD-2-Clause
    pyparsing: MIT
    ordered-set: MIT

Therefore, a comprehensive license documentation covering both ``setuptools`` proper
and its vendored packages could contain these metadata, combining all the
license expressions in one expression::

    license = MIT AND (Apache-2.0 OR BSD-2-Clause)
    license_files =
        LICENSE.MIT
        LICENSE.packaging

Here we would assume that the ``LICENSE.MIT`` file contains the text of the MIT
license and the copyrights used by ``setuptools``, ``appdirs``, ``pyparsing`` and
``ordered-set``, and that the ``LICENSE.packaging`` file contains the texts of the
Apache and BSD license, its copyrights and its license choice notice [#packlic]_.


Appendix 2. Surveying how we document licenses today in Python
==============================================================

There are multiple ways used or recommended to document Python package
licenses today:


In Core metadata
----------------

There are two overlapping core metadata fields to document a license: the
license-related ``Classifier`` strings [#classif]_ prefixed with ``License ::`` and
the ``License`` field as free text [#licfield]_.


The core metadata documentation ``License`` field documentation is currently::

    License (optional)
    ::::::::::::::::::

    Text indicating the license covering the distribution where the license
    is not a selection from the "License" Trove classifiers. See
    "Classifier" below.  This field may also be used to specify a
    particular version of a license which is named via the ``Classifier``
    field, or to indicate a variation or exception to such a license.

    Examples::

        License: This software may only be obtained by sending the
                author a postcard, and then the user promises not
                to redistribute it.

        License: GPL version 3, excluding DRM provisions

Even though there are two fields, it is at times difficult to convey anything
but simpler licensing. For instance some classifiers lack accuracy (GPL
without a version) and when you have multiple License-related classifiers it is
not clear if this is a choice or all these apply and which ones. Furthermore,
the list of available license-related classifiers is often out-of-date.


In the PyPA ``sampleproject``
-----------------------------

The latest PyPA ``sampleproject`` recommends only to use classifiers in
``setup.py`` and does not list the ``license`` field in its example
``setup.py`` [#samplesetup]_.


The License Files in wheels and setuptools
------------------------------------------

Beyond a license code or qualifier, license text files are documented and
included in a built package either implicitly or explicitly and this is another
possible source of confusion:

- In wheels [#wheels]_ license files are automatically added to the ``.dist-info``
  directory if they match one of a few common license file name patterns (such
  as LICENSE*, COPYING*). Alternatively a package author can specify a list of
  license files paths to include in the built wheel using in the
  ``license_files`` field in the ``[metadata]`` section of the project's
  ``setup.cfg``. Previously this was a (singular) ``license_file`` file attribute
  that is now deprecated but is still in common use. See [#pipsetup]_ for
  instance.

- In ``setuptools`` [#setuptoolssdist]_, a ``license_file`` attribute is used to add
  a single license file to a source distribution. This singular version is
  still honored by ``wheels`` for backward compatibility.

- Using a LICENSE.txt file is encouraged in the packaging guide [#packaging]_
  paired with a ``MANIFEST.in`` entry to ensure that the license file is included
  in a built source distribution (sdist).

Note: the License-File field proposed in this PEP already exists in ``wheel`` and
``setuptools`` with the same behaviour as explained above. This PEP is only
recognizing and documenting the existing practice as used in ``wheel`` (with the
``license_file`` and ``license_files`` ``setup.cfg`` ``[metadata]`` entries) and in
``setuptools`` ``license_file`` ``setup()`` argument.


In Python code files
--------------------

(Note: Documenting licenses in source code is not in the scope of this PEP)

Beside using comments and/or ``SPDX-License-Identifier`` conventions, the license
is sometimes documented in Python code file using `dunder` variables typically
named after one of the lower cased Core metadata field such as ``__license__``
[#pycode]_.

This convention (dunder global variables) is recognized by the built-in ``help()``
function and the standard ``pydoc`` module. The dunder variable(s) will show up in
the ``help()`` DATA section for a module.


In some other Python packaging tools
------------------------------------

- `Conda package manifest` [#conda]_ has support for ``license`` and ``license_file``
  fields as well as a ``license_family`` license grouping field.

- ``flit`` [#flit]_ recommends to use classifiers instead of License (as per the
  current metadata spec).

- ``pbr`` [#pbr]_ uses similar data as setuptools but always stored setup.cfg.

- ``poetry`` [#poetry]_ specifies the use of the ``license`` field in
  ``pyproject.toml`` with SPDX license identifiers.


Appendix 3. Surveying how other package formats document licenses
=================================================================

Here is a survey of how things are done elsewhere.

License in Linux distribution packages
---------------------------------------

Note: in most cases the license texts of the most common licenses are included
globally once in a shared documentation directory (e.g. /usr/share/doc).

- Debian document package licenses with machine readable copyright files
  [#dep5]_. This specification defines its own license expression syntax that is
  very similar to the SDPX syntax and use its own list of license identifiers
  for common licenses also closely related to SPDX identifiers.

- Fedora RPM packages [#fedora]_ specifies how to include ``License Texts``
  [#fedoratext]_ and how use a ``License`` field [#fedoralic]_ that must be filled
  with an appropriate license Short License identifier(s) from an extensive list
  of "Good Licenses" identifiers [#fedoralist]_. Fedora also defines its own
  license expression syntax very similar to the SDPX syntax.

- OpenSuse RPMs packages [#opensuse]_ use SPDX license expressions with a
  either SPDX license identifiers and a list of extra license identifiers
  [#opensuselist]_.

- Gentoo ebuild use a ``LICENSE`` variable [#gentoo]_. This field is specified
  in GLEP-0023 [#glep23]_ and in the Gentoo development manual [#gentoodev]_.
  Gentoo also defines a license expressions syntax and a list of allowed
  licenses. The expression syntax is rather different from SPDX.

- FreeBSD package Makefile [#freebsd]_ provide a ``LICENSE`` and a
  ``LICENSE_FILE`` field with a list of custom license symbols. For
  non-standard licenses, FreeBSD recommend to use ``LICENSE=UNKNOWN`` and add
  ``LICENSE_NAME`` and ``LICENSE_TEXT`` fields, as well as sophisticated
  ``LICENSE_PERMS`` to qualify the license permissions and ``LICENSE_GROUPS``
  to document a license grouping. The ``LICENSE_COMB`` allows to document more
  than one license and how they apply together, forming a custom license
  expression syntax. FreeBSD also recommends the use of
  ``SPDX-License-Identifier`` in source code files.

- Archlinux PKGBUILD [#archinux]_ define its own license identifiers
  [#archlinuxlist]_. The value ``'unknown'`` can be used if the license is not
  defined.

- OpenWRT ipk packages [#openwrt]_ use the ``PKG_LICENSE`` and
  ``PKG_LICENSE_FILES`` variables and recommend the use of SPDX License
  identifiers.

- NixOS uses SPDX identifiers [#nixos]_ and some extras license identifiers in
  its license field.

- GNU Guix (based on NixOS) has a single License field, uses its own license
  symbols list [#guix]_ and specifies to use one license or a list of licenses
  [#guixlic]_.

- Alpine Linux apk packages [#alpine]_ recommend using SPDX identifiers in its
  license field.


License in Language and Application packages
--------------------------------------------

- In Java, Maven POM [#maven]_ defines a licenses XML tag with a list of license
  items each with a name, URL, comments and "distribution" type. This is not
  mandatory and the content of each field is not specified.

- JavaScript npm package.json [#npm]_ use a single license field with SPDX
  license expression or the ``UNLICENSED`` id if no license is specified.
  A license file can be referenced as an alternative using "SEE LICENSE IN
  <filename>" in the single ``license`` field.

- Rubygems gemspec [#gem]_ specifies either a singular license string for a list
  of licenses strings. The relationship between multiple licenses in a list is
  not specified. They recommend using SPDX license identifiers.

- CPAN Perl modules [#perl]_ use a single license field which is either a single
  string or a list of strings. The relationship between the licenses in a list
  is not specified. There is a list of support own license identifiers plus
  these generic identifiers: ``open_source``, ``restricted``, ``unrestricted``,
  ``unknown``.

- Rust Cargo [#cargo]_ specifies the use of an SPDX license expression (v2.1) in
  the ``license`` field. It also supports an alternative expression syntax using
  slash-separated SPDX license identifiers. There is also a ``license_file``
  field. The crates.io package registry [#cratesio]_ requires that either
  ``license`` or ``license_file`` fields are set when you upload a package.

- PHP Composer composer.json [#composer]_ uses a ``license`` field with an SPDX
  License id or "proprietary". The ``license`` field is either a single string
  that can use something which resemble the SPDX license expression syntax with
  "and" and "or" keywords; or this is a list of strings if there is a choice of
  licenses (aka. a "disjunctive" choice of license).

- NuGet packages [#nuget]_ were using only a simple license URL and are now
  specifying to use an SPDX License expression and/or the path to a license
  file within the package. The NuGet.org repository states that they only
  accepts license expressions that are `approved by the Open Source Initiative
  or the Free Software Foundation.`

- Go language modules ``go.mod`` have no provision for any metadata beyond
  dependencies. Licensing information is left for code authors and other
  community package managers to document.

- Dart/Flutter spec [#flutter]_ recommends to use a single ``LICENSE`` file
  that should contain all the license texts each separated by a line with 80
  hyphens.

- JavaScript Bower [#bower]_ ``license`` field is either a single string or a list
  of strings using either SPDX license identifiers, a path or a URL to a
  license file.

- Cocoapods podspec [#cocoapod]_ ``license`` field is either a single string or a
  mapping with attributes of type, file and text keys. This is mandatory unless
  there is a LICENSE or LICENCE file provided.

- Haskell Cabal [#cabal]_ accepts an SPDX license expression since version 2.2.
  The version of the SPDX license list used is a function of the ``cabal`` version.
  The specification also provides a mapping between pre-SPDX Legacy license
  Identifiers and SPDX identifiers. Cabal also specifies a ``license-file(s)``
  field that lists license files that will be installed with the package.

- Erlang/Elixir mix/hex package [#mix]_ specifies a ``licenses`` field as a
  required list of license strings and recommends to use SPDX License
  identifiers.

- D lang dub package [#dub]_ defines its own list of license identifiers and
  its own license expression syntax and both are similar to SPDX the conventions.

- R Package DESCRIPTION [#cran]_ defines its own sophisticated license
  expression syntax and list of licenses identifiers. R has a unique way to
  support specifiers for license versions such as ``LGPL (>= 2.0, < 3)`` in its
  license expression syntax.


Conventions used by other ecosystems
------------------------------------

- ``SPDX-License-Identifier`` [#spdxids]_ is a simple convention to document the
  license inside a code file.

- The Free Software Foundation (FSF) promotes using SPDX license identifiers
  for clarity in the GPL and other versioned free software licenses [#gnu]_
  [#fsf]_.

- The Free Software Foundation Europe (FSFE) REUSE project [#reuse]_ promotes
  using ``SPDX-License-Identifier``.

- The Linux kernel uses ``SPDX-License-Identifier`` and parts of the FSFE REUSE
  conventions to document its licenses [#linux]_.

- U-Boot spearheaded using ``SPDX-License-Identifier`` in code and now follows the
  Linux ways [#uboot]_.

- The Apache Software Foundation projects use RDF DOAP [#apache]_ with a single
  license field pointing to SPDX license identifiers.

- The Eclipse Foundation promotes using ``SPDX-license-Identifiers`` [#eclipse]_

- The ClearlyDefined project [#cd]_ promotes using SPDX license identifiers and
  expressions to improve license clarity.

- The Android Open Source Project [#android]_ use ``MODULE_LICENSE_XXX`` empty
  tag files where ``XXX`` is a license code such as BSD, APACHE, GPL, etc. And
  side-by-side with this ``MODULE_LICENSE`` file there is a ``NOTICE`` file
  that contains license and notices texts.


References
==========

This document specifies version 2.2 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, while not formally accepted, was specified in PEP 426.
- Version 2.1 is specified in PEP 566.

.. [#cms] https://packaging.python.org/specifications/core-metadata
.. [#cdstats] https://clearlydefined.io/stats
.. [#cd] https://clearlydefined.io
.. [#osi] http://opensource.org
.. [#classif] https://pypi.org/classifiers
.. [#spdxlist] https://spdx.org/licenses
.. [#spdx] https://spdx.org
.. [#spdx22] https://spdx.github.io/spdx-spec/appendix-IV-SPDX-license-expressions/
.. [#wheels] https://github.com/pypa/wheel/blob/b8b21a5720df98703716d3cd981d8886393228fa/docs/user_guide.rst#including-license-files-in-the-generated-wheel-file
.. [#reuse] https://reuse.software/
.. [#licexp] https://github.com/nexB/license-expression/
.. [#spdxpy] https://github.com/spdx/tools-python/
.. [#scancodetk] https://github.com/nexB/scancode-toolkit
.. [#licfield] https://packaging.python.org/guides/distributing-packages-using-setuptools/?highlight=MANIFEST.in#license
.. [#samplesetup] https://github.com/pypa/sampleproject/blob/52966defd6a61e97295b0bb82cd3474ac3e11c7a/setup.py#L98
.. [#pipsetup] https://github.com/pypa/pip/blob/476606425a08c66b9c9d326994ff5cf3f770926a/setup.cfg#L40
.. [#setuptoolssdist] https://github.com/pypa/setuptools/blob/97e8ad4f5ff7793729e9c8be38e0901e3ad8d09e/setuptools/command/sdist.py#L202
.. [#packaging] https://packaging.python.org/guides/distributing-packages-using-setuptools/?highlight=MANIFEST.in#license-txt
.. [#pycode] https://github.com/search?l=Python&q=%22__license__%22&type=Code
.. [#setuptools5030] https://github.com/pypa/setuptools/blob/v50.3.0/setup.cfg#L17
.. [#packlic] https://github.com/pypa/packaging/blob/19.1/LICENSE
.. [#conda] https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html#about-section
.. [#flit] https://github.com/takluyver/flit
.. [#poetry] https://poetry.eustace.io/docs/pyproject/#license
.. [#pbr] https://docs.openstack.org/pbr/latest/user/features.html
.. [#dep5] https://dep-team.pages.debian.net/deps/dep5/
.. [#fedora] https://docs.fedoraproject.org/en-US/packaging-guidelines/LicensingGuidelines/
.. [#fedoratext] https://docs.fedoraproject.org/en-US/packaging-guidelines/LicensingGuidelines/#_license_text
.. [#fedoralic] https://docs.fedoraproject.org/en-US/packaging-guidelines/LicensingGuidelines/#_valid_license_short_names
.. [#fedoralist] https://fedoraproject.org/wiki/Licensing:Main?rd=Licensing#Good_Licenses
.. [#opensuse] https://en.opensuse.org/openSUSE:Packaging_guidelines#Licensing
.. [#opensuselist] https://docs.google.com/spreadsheets/d/14AdaJ6cmU0kvQ4ulq9pWpjdZL5tkR03exRSYJmPGdfs/pub
.. [#gentoo] https://devmanual.gentoo.org/ebuild-writing/variables/index.html#license
.. [#glep23] https://www.gentoo.org/glep/glep-0023.html
.. [#gentoodev] https://devmanual.gentoo.org/general-concepts/licenses/index.html
.. [#freebsd] https://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/licenses.html
.. [#archinux] https://wiki.archlinux.org/index.php/PKGBUILD#license
.. [#archlinuxlist] https://wiki.archlinux.org/index.php/PKGBUILD#license
.. [#openwrt] https://openwrt.org/docs/guide-developer/packages#buildpackage_variables
.. [#nixos] https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix
.. [#guix] http://git.savannah.gnu.org/cgit/guix.git/tree/guix/licenses.scm
.. [#guixlic] https://guix.gnu.org/manual/en/html_node/package-Reference.html#index-license_002c-of-packages
.. [#alpine] https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package#license
.. [#maven] https://maven.apache.org/pom.html#Licenses
.. [#npm] https://docs.npmjs.com/files/package.json#license
.. [#gem] https://guides.rubygems.org/specification-reference/#license=
.. [#perl] https://metacpan.org/pod/CPAN::Meta::Spec#license
.. [#cargo] https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata
.. [#cratesio] https://doc.rust-lang.org/cargo/reference/registries.html#publish
.. [#composer] https://getcomposer.org/doc/04-schema.md#license
.. [#nuget] https://docs.microsoft.com/en-us/nuget/reference/nuspec#licenseurl
.. [#flutter] https://flutter.dev/docs/development/packages-and-plugins/developing-packages#adding-licenses-to-the-license-file
.. [#bower] https://github.com/bower/spec/blob/master/json.md#license
.. [#cocoapod] https://guides.cocoapods.org/syntax/podspec.html#license
.. [#cabal] https://cabal.readthedocs.io/en/latest/developing-packages.html#pkg-field-license
.. [#mix] https://hex.pm/docs/publish
.. [#dub] https://dub.pm/package-format-json.html#licenses
.. [#cran] https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Licensing
.. [#spdxids] https://spdx.org/using-spdx-license-identifier
.. [#gnu] https://www.gnu.org/licenses/identify-licenses-clearly.html
.. [#fsf] https://www.fsf.org/blogs/rms/rms-article-for-claritys-sake-please-dont-say-licensed-under-gnu-gpl-2
.. [#linux] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/license-rules.rst
.. [#uboot] https://www.denx.de/wiki/U-Boot/Licensing
.. [#apache] https://svn.apache.org/repos/asf/allura/doap_Allura.rdf
.. [#eclipse] https://www.eclipse.org/legal/epl-2.0/faq.php
.. [#android] https://github.com/aosp-mirror/platform_external_tcpdump/blob/master/MODULE_LICENSE_BSD
.. [#cc0] https://creativecommons.org/publicdomain/zero/1.0/
.. [#unlic] https://unlicense.org/


Copyright
=========

This document is placed in the public domain or under the CC0-1.0-Universal
license [#cc0]_, whichever is more permissive.


Acknowledgements
================

- Nick Coghlan
- Kevin P. Fleming
- Pradyun Gedam
- Oleg Grenrus
- Dustin Ingram
- Chris Jerdonek
- Cyril Roelandt
- Luis Villa



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