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Python Enhancement Proposals

PEP 639 – Improving License Clarity with Better Package Metadata

Philippe Ombredanne <pombredanne at>, C.A.M. Gerlach <CAM.Gerlach at Gerlach.CAM>, Karolina Surma <karolina.surma at>
Brett Cannon <brett at>
Discourse thread
Standards Track
15-Aug-2019, 17-Dec-2021, 10-May-2024

Table of Contents


This PEP defines a specification how licenses are documented in the Python projects.

To achieve that, it:

This will make license declaration simpler and less ambiguous for package authors to create, end users to understand, and tools to programmatically process.

The changes will update the Core Metadata specification to version 2.4.


This PEP’s scope is limited to covering new mechanisms for documenting the license of a distribution package, specifically defining:

The changes that this PEP requires have been designed to minimize impact and maximize backward compatibility.


This PEP doesn’t recommend any particular license to be chosen by any particular package author.

If projects decide not to use the new fields, no additional restrictions are imposed by this PEP when uploading to PyPI.

This PEP also is not about license documentation for individual files, though this is a surveyed topic in an appendix, nor does it intend to cover cases where the source distribution and binary distribution packages don’t have the same licenses.


Software must be licensed in order for anyone other than its creator to download, use, share and modify it. Today, there are multiple fields where licenses are documented in Core Metadata, and there are limitations to what can be expressed in each of them. This often leads to confusion both for package authors and end users, including distribution re-packagers.

This has triggered a number of license-related discussions and issues, including on outdated and ambiguous PyPI classifiers, license interoperability with other ecosystems, too many confusing license metadata options, limited support for license files in the Wheel project, and the lack of precise license metadata.

As a result, on average, Python packages tend to have more ambiguous and missing license information than other common ecosystems. This is supported by the statistics page of the ClearlyDefined project, an Open Source Initiative effort to help improve licensing clarity of other FOSS projects, covering all packages from PyPI, Maven, npm and Rubygems.

The current license classifiers could be extended to include the full range of the SPDX identifiers while deprecating the ambiguous classifiers (such as License :: OSI Approved :: BSD License).

However, there are multiple arguments against such an approach:

  • It requires a great effort to duplicate the SPDX license list and keep it in sync.
  • It is a hard break in backward compatibility, forcing package authors to update to new classifiers immediately when PyPI deprecates the old ones.
  • It only covers packages under a single license; it doesn’t address projects that vendor dependencies (e.g. Setuptools), offer a choice of licenses (e.g. Packaging) or were relicensed, adapt code from other projects or contain fonts, images, examples, binaries or other assets under other licenses.
  • It requires both authors and tools understand and implement the PyPI-specific classifier system.
  • It does not provide as clear an indicator that a package has adopted the new system, and should be treated accordingly.


A survey was conducted to map the existing license metadata definitions in the Python ecosystem and a variety of other packaging systems, Linux distributions, language ecosystems and applications.

The takeaways from the survey have guided the recommendations of this PEP:

  • SPDX and SPDX-like syntaxes are the most popular license expressions in many modern package systems.
  • Most Free and Open Source Software licenses require package authors to include their full text in a Distribution Package.

Therefore, this PEP introduces two new Core Metadata fields:

  • License-Expression that provides an unambiguous way to express the license of a package using SPDX license expressions.
  • License-File that offers a standardized way to include the full text of the license(s) with the package when distributed, and allows other tools consuming the Core Metadata to locate a distribution archive’s license files.

Furthermore, this specification builds upon existing practice in the Setuptools and Wheel projects. An up-to-date version of the current draft of this PEP is implemented in the Hatch packaging tool, and an earlier draft of the license files portion is implemented in Setuptools.


The keywords “MUST”, “MUST NOT”, “REQUIRED”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

License terms

The license-related terminology draws heavily from the SPDX Project, particularly license identifier and license expression.

license classifier
A PyPI Trove classifier (as described in the Core Metadata specification) which begins with License ::.
license expression
SPDX expression
A string with valid SPDX license expression syntax including one or more SPDX license identifier(s), which describes a Project’s license(s) and how they inter-relate. Examples: GPL-3.0-or-later, MIT AND (Apache-2.0 OR BSD-2-clause)
license identifier
SPDX identifier
A valid SPDX short-form license identifier, as described in the Add License-Expression field section of this PEP. This includes all valid SPDX identifiers and the custom LicenseRef-[idstring] strings conforming to the SPDX specification, clause 10.1. Examples: MIT, GPL-3.0-only, LicenseRef-My-Custom-License
root license directory
license directory
The directory under which license files are stored in a project source tree, distribution archive or installed project. Also, the root directory that their paths recorded in the License-File Core Metadata field are relative to. Defined to be the project root directory for a project source tree or source distribution; and a subdirectory named licenses of the directory containing the built metadata— i.e., the .dist-info/licenses directory— for a Built Distribution or installed project.


The changes necessary to implement this PEP include:

Note that the guidance on errors and warnings is for tools’ default behavior; they MAY operate more strictly if users explicitly configure them to do so, such as by a CLI flag or a configuration option.

SPDX license expression syntax

This PEP adopts the SPDX license expression syntax as documented in the SPDX specification, either Version 2.2 or a later compatible version.

A license expression can use the following license identifiers:

  • Any SPDX-listed license short-form identifiers that are published in the SPDX License List, version 3.17 or any later compatible version. Note that the SPDX working group never removes any license identifiers; instead, they may choose to mark an identifier as “deprecated”.
  • The custom LicenseRef-[idstring] string(s), where [idstring] is a unique string containing letters, numbers, . and/or -, to identify licenses that are not included in the SPDX license list. The custom identifiers must follow the SPDX specification, clause 10.1 of the given specification version.

Examples of valid SPDX expressions:

MIT AND (Apache-2.0 OR BSD-2-Clause)
MIT OR GPL-2.0-or-later OR (FSFUL AND BSD-2-Clause)
GPL-3.0-only WITH Classpath-Exception-2.0 OR BSD-3-Clause
LicenseRef-Special-License OR CC0-1.0 OR Unlicense

Examples of invalid SPDX expressions:

Apache-2.0 OR 2-BSD-Clause
LicenseRef-License with spaces

Core Metadata

The error and warning guidance in this section applies to build and publishing tools; end-user-facing install tools MAY be less strict than mentioned here when encountering malformed metadata that does not conform to this specification.

As it adds new fields, this PEP updates the Core Metadata version to 2.4.

Add License-Expression field

The License-Expression optional Core Metadata field is specified to contain a text string that is a valid SPDX license expression, as defined above.

Build and publishing tools SHOULD check that the License-Expression field contains a valid SPDX expression, including the validity of the particular license identifiers (as defined above). Tools MAY halt execution and raise an error when an invalid expression is found. If tools choose to validate the SPDX expression, they also SHOULD store a case-normalized version of the License-Expression field using the reference case for each SPDX license identifier and uppercase for the AND, OR and WITH keywords. Tools SHOULD report a warning and publishing tools MAY raise an error if one or more license identifiers have been marked as deprecated in the SPDX License List.

For all newly-uploaded distribution archives that include a License-Expression field, the Python Package Index (PyPI) MUST validate that they contain a valid, case-normalized license expression with valid identifiers (as defined above) and MUST reject uploads that do not. Custom license identifiers which conform to the SPDX specification are considered valid. PyPI MAY reject an upload for using a deprecated license identifier, so long as it was deprecated as of the above-mentioned SPDX License List version.

Add License-File field

License-File is an optional Core Metadata field. Each instance contains the string representation of the path of a license-related file. The path is located within the project source tree, relative to the project root directory. It is a multi-use field that may appear zero or more times and each instance lists the path to one such file. Files specified under this field could include license text, author/attribution information, or other legal notices that need to be distributed with the package.

As specified by this PEP, its value is also that file’s path relative to the root license directory in both installed projects and the standardized Distribution Package types.

If a License-File is listed in a Source Distribution or Built Distribution’s Core Metadata:

  • That file MUST be included in the distribution archive at the specified path relative to the root license directory.
  • That file MUST be installed with the project at that same relative path.
  • The specified relative path MUST be consistent between project source trees, source distributions (sdists), built distributions (Wheels) and installed projects.
  • Inside the root license directory, packaging tools MUST reproduce the directory structure under which the source license files are located relative to the project root.
  • Path delimiters MUST be the forward slash character (/), and parent directory indicators (..) MUST NOT be used.
  • License file content MUST be UTF-8 encoded text.

Build tools MAY and publishing tools SHOULD produce an informative warning if a built distribution’s metadata contains no License-File entries, and publishing tools MAY but build tools MUST NOT raise an error.

For all newly-uploaded distribution archives that include one or more License-File fields in their Core Metadata and declare a Metadata-Version of 2.4 or higher, PyPI SHOULD validate that all specified files are present in that distribution archives, and MUST reject uploads that do not validate.

Deprecate License field

The legacy unstructured-text License Core Metadata field is deprecated and replaced by the new License-Expression field. The fields are mutually exclusive. Tools which generate Core Metadata MUST NOT create both these fields. Tools which read Core Metadata, when dealing with both these fields present at the same time, MUST read the value of License-Expression and MUST disregard the value of the License field.

If only the License field is present, tools MAY issue a warning informing users it is deprecated and recommending License-Expression instead.

For all newly-uploaded distribution archives that include a License-Expression field, the Python Package Index (PyPI) MUST reject any that specify both License and License-Expression fields.

The License field may be removed from a new version of the specification in a future PEP.

Deprecate license classifiers

Using license classifiers in the Classifier Core Metadata field (described in the Core Metadata specification) is deprecated and replaced by the more precise License-Expression field.

If the License-Expression field is present, build tools SHOULD and publishing tools MUST raise an error if one or more license classifiers is included in a Classifier field, and MUST NOT add such classifiers themselves.

Otherwise, if this field contains a license classifier, build tools MAY and publishing tools SHOULD issue a warning informing users such classifiers are deprecated, and recommending License-Expression instead. For compatibility with existing publishing and installation processes, the presence of license classifiers SHOULD NOT raise an error unless License-Expression is also provided.

For all newly-uploaded distributions that include a License-Expression field, the Python Package Index (PyPI) MUST reject any that also specify any license classifiers.

New license classifiers MUST NOT be added to PyPI; users needing them SHOULD use the License-Expression field instead. License classifiers may be removed from a new version of the specification in a future PEP.

Project source metadata

This PEP specifies changes to the project’s source metadata under a [project] table in the pyproject.toml file.

Add string value to license key

license key in the [project] table is defined to contain a top-level string value. It is a valid SPDX license expression as defined in this PEP. Its value maps to the License-Expression field in the core metadata.

Build tools SHOULD validate and perform case normalization of the expression as described in the Add License-Expression field section, outputting an error or warning as specified.


license = "MIT"

license = "MIT AND (Apache-2.0 OR BSD-2-clause)"

license = "MIT OR GPL-2.0-or-later OR (FSFUL AND BSD-2-Clause)"

license = "LicenseRef-Proprietary"

Add license-files key

A new license-files key is added to the [project] table for specifying paths in the project source tree relative to pyproject.toml to file(s) containing licenses and other legal notices to be distributed with the package. It corresponds to the License-File fields in the Core Metadata.

Its value is a table, which if present MUST contain one of two optional, mutually exclusive subkeys, paths and globs; if both are specified, tools MUST raise an error. Both are arrays of strings; the paths subkey contains verbatim file paths, and the globs subkey valid glob patterns, which MUST be parsable by the glob module in the Python standard library.

Path delimiters MUST be the forward slash character (/), and parent directory indicators (..) MUST NOT be used. Tools MUST assume that license file content is valid UTF-8 encoded text, and SHOULD validate this and raise an error if it is not.

If the paths subkey is a non-empty array, build tools:

  • MUST treat each value as a verbatim, literal file path, and MUST NOT treat them as glob patterns.
  • MUST include each listed file in all distribution archives.
  • MUST NOT match any additional license files beyond those explicitly statically specified by the user under the paths subkey.
  • MUST list each file path under a License-File field in the Core Metadata.
  • MUST raise an error if one or more paths do not correspond to a valid file in the project source that can be copied into the distribution archive.

If the globs subkey is a non-empty array, build tools:

  • MUST treat each value as a glob pattern, and MUST raise an error if the pattern contains invalid glob syntax.
  • MUST include all files matched by at least one listed pattern in all distribution archives.
  • MAY exclude files matched by glob patterns that can be unambiguously determined to be backup, temporary, hidden, OS-generated or VCS-ignored.
  • MUST list each matched file path under a License-File field in the Core Metadata.
  • SHOULD issue a warning and MAY raise an error if no files are matched.
  • MAY issue a warning if any individual user-specified pattern does not match at least one file.

If the license-files key is present, and the paths or globs subkey is set to a value of an empty array, then tools MUST NOT include any license files and MUST NOT raise an error.

If the license-files key is not present and not explicitly marked as dynamic, tools MUST assume a default value of the following:

license-files.globs = ["LICEN[CS]E*", "COPYING*", "NOTICE*", "AUTHORS*"]

In this case, tools MAY issue a warning if no license files are matched, but MUST NOT raise an error.

If the license-files key is marked as dynamic (and not present), to preserve consistent behavior with current tools and help ensure the packages they create are legally distributable, build tools SHOULD default to including at least the license files matching the above patterns, unless the user has explicitly specified their own.

Examples of valid license files declaration:

license-files = { globs = ["LICEN[CS]E*", "AUTHORS*"] }

license-files.paths = ["licenses/LICENSE.MIT", "licenses/LICENSE.CC0"]

license-files = { paths = [] }

license-files.globs = []

Examples of invalid license files declaration:

license-files.globs = ["LICEN[CS]E*", "AUTHORS*"]
license-files.paths = ["LICENSE.MIT"]

Reason: license-files.paths and license-files.globs are mutually exclusive.

license-files = { paths = ["..\LICENSE.MIT"] }

Reason: .. must not be used. \ is an invalid path delimiter, / must be used.

license-files = { globs = ["LICEN{CSE*"] }

Reason: “LICEN{CSE*” is not a valid glob.

Deprecate license key table subkeys

Table values for the license key in the [project] table, including the text and file table subkeys, are now deprecated. If the new license-files key is present, build tools MUST raise an error if the license key is defined and has a value other than a single top-level string.

If the new license-files key is not present and the text subkey is present in a license table, tools SHOULD issue a warning informing users it is deprecated and recommending a license expression as a top-level string key instead.

Likewise, if the new license-files key is not present and the file subkey is present in the license table, tools SHOULD issue a warning informing users it is deprecated and recommending the license-files key instead.

If the specified license file is present in the source tree, build tools SHOULD use it to fill the License-File field in the core metadata, and MUST include the specified file as if it were specified in a license-file.paths field. If the file does not exist at the specified path, tools MUST raise an informative error as previously specified. However, tools MUST also still assume the specified default value for the license-files key and also include, in addition to a license file specified under the license.file subkey, any license files that match the specified list of patterns.

Table values for the license key MAY be removed from a new version of the specification in a future PEP.

License files in project formats

A few additions will be made to the existing specifications.

Project source trees
Per Project source metadata section, the Declaring Project Metadata specification will be updated to reflect that license file paths MUST be relative to the project root directory; i.e. the directory containing the pyproject.toml (or equivalently, other legacy project configuration, e.g., setup.cfg, etc).
Source distributions (sdists)
The sdist specification will be updated to reflect that if the Metadata-Version is 2.4 or greater, the sdist MUST contain any license files specified by the License-File field in the PKG-INFO at their respective paths relative to the of the sdist (containing the pyproject.toml and the PKG-INFO Core Metadata).
Built distributions (wheels)
The Wheel specification will be updated to reflect that if the Metadata-Version is 2.4 or greater and one or more License-File fields is specified, the .dist-info directory MUST contain a licenses subdirectory, which MUST contain the files listed in the License-File fields in the METADATA file at their respective paths relative to the licenses directory.
Installed projects
The Recording Installed Projects specification will be updated to reflect that if the Metadata-Version is 2.4 or greater and one or more License-File fields is specified, the .dist-info directory MUST contain a licenses subdirectory which MUST contain the files listed in the License-File fields in the METADATA file at their respective paths relative to the licenses directory, and that any files in this directory MUST be copied from wheels by install tools.

Converting legacy metadata

Tools MUST NOT use the contents of the license.text [project] key (or equivalent tool-specific format), license classifiers or the value of the Core Metadata License field to fill the top-level string value of the license key or the Core Metadata License-Expression field without informing the user and requiring unambiguous, affirmative user action to select and confirm the desired license expression value before proceeding.

Tool authors, who need to automatically convert license classifiers to SPDX identifiers, can use the recommendation prepared by the PEP authors.

Backwards Compatibility

Adding a new License-Expression Core Metadata field and a top-level string value for the license key in the pyproject.toml [project] table unambiguously means support for the specification in this PEP. This avoids the risk of new tooling misinterpreting a license expression as a free-form license description or vice versa.

The legacy deprecated Core Metadata License field, license key table subkeys (text and file) in the pyproject.toml [project] table and license classifiers retain backwards compatibility. A removal is left to a future PEP and a new version of the Core Metadata specification.

Specification of the new License-File Core Metadata field and adding the files in the distribution codifies the existing practices of many packaging tools. It is designed to be largely backwards-compatible with their existing use of that field. The new license-files key in the [project] table of pyproject.toml will only have an effect once users and tools adopt it.

This PEP specifies that license files should be placed in a dedicated licenses subdir of .dist-info directory. This is new and ensures that wheels following this PEP will have differently-located licenses relative to those produced via the previous installer-specific behavior. This is further supported by a new metadata version.

This also resolves current issues where license files are accidentally replaced if they have the same names in different places, making wheels undistributable without noticing. It also prevents conflicts with other metadata files in the same directory.

The additions will be made to the source distribution (sdist), built distribution (wheel) and installed project specifications. They document behaviors allowed under their current specifications, and gate them behind the new metadata version.

This PEP proposes PyPI implement validation of the new License-Expression and License-File fields, which has no effect on new and existing packages uploaded unless they explicitly opt in to using these new fields and fail to follow the specification correctly. Therefore, this does not have a backward compatibility impact, and guarantees forward compatibility by ensuring all distributions uploaded to PyPI with the new fields conform to the specification.

Security Implications

This PEP has no foreseen security implications: the License-Expression field is a plain string and the License-File fields are file paths. Neither introduces any known new security concerns.

How to Teach This

A majority of packages use a single license which makes the case simple: a single license identifier is a valid license expression.

Users of packaging tools will learn the valid license expression of their package through the messages issued by the tools when they detect invalid ones, or when the deprecated License field or license classifiers are used.

If an invalid License-Expression is used, the users will not be able to publish their package to PyPI and an error message will help them understand they need to use SPDX identifiers. It will be possible to generate a distribution with incorrect license metadata, but not to publish one on PyPI or any other index server that enforces License-Expression validity. For authors using the now-deprecated License field or license classifiers, packaging tools may warn them and inform them of the replacement, License-Expression.

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

Reference Implementation

Tools will need to support parsing and validating license expressions in the License-Expression field if they decide to implement this part of the specification. It’s up to the tools whether they prefer to implement the validation on their side (e.g. like hatch) or use one of the available Python libraries (e.g. license-expression). This PEP does not mandate using any specific library and leaves it to the tools authors to choose the best implementation for their projects.

Rejected Ideas

Many alternative ideas were proposed and after a careful consideration, rejected. The exhaustive list including the rationale for rejecting can be found in a separate page.


A list of auxilliary documents is provided:


  • Alyssa Coghlan
  • Kevin P. Fleming
  • Pradyun Gedam
  • Oleg Grenrus
  • Dustin Ingram
  • Chris Jerdonek
  • Cyril Roelandt
  • Luis Villa
  • Seth M. Larson
  • Ofek Lev


Last modified: 2024-06-28 22:20:47 GMT