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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>
Brett Cannon <brett at>
Discourse thread
Standards Track
15-Aug-2019, 17-Dec-2021

Table of Contents


This PEP defines a specification for how licenses are documented in the core metadata, with license expression strings using SPDX identifiers in a new License-Expression field. This will make license declarations simpler and less ambiguous for package authors to create, end users to read and understand, and tools to programmatically process.

The PEP also:

The changes in this PEP will update the core metadata to version 2.4, modify the project (source) metadata specification, and make minor additions to the source distribution (sdist), built distribution (wheel) and installed project standards.


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

  • A means of specifying a SPDX license expression.
  • A method of including license texts in distributions and installed projects.

The changes to the core metadata specification that this PEP requires have been designed to minimize impact and maximize backward compatibility. This specification builds off of existing ways to document licenses that are already in use in popular tools (e.g. adding support to core metadata for the License-File field already used in the Wheel and Setuptools projects) and by some package authors (e.g. storing an SPDX license expression in the existing License field).

In addition to these proposed changes, this PEP contains guidance for tools handling and converting these metadata, a tutorial for package authors covering various common use cases, detailed examples of them in use, and a comprehensive survey of license documentation in Python and other languages.

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


This PEP is neutral regarding the choice of license by any particular package author. This PEP makes no recommendation for specific licenses, and does not require the use of a particular license documentation convention.

Rather, the SPDX license expression syntax proposed in this PEP provides a simpler and more expressive mechanism to accurately document any kind of license that applies to a Python package, whether it is open source, free/libre, proprietary, or a combination of such.

This PEP also does not impose any additional restrictions when uploading to PyPI, unless projects choose to make use of the new fields.

Instead, it is intended to document best practices already in use, extend them to use a new formally-specified and supported mechanism, and provide guidance for packaging tools on how to hand the transition and inform users accordingly.

This PEP also is not about license documentation in files inside projects, though this is a surveyed topic in an appendix, and nor does it intend to cover cases where the source 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, so providing accurate license information to Python package users is an important matter. 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 and a lack of clarity, both for package authors and end users.

Many package authors have expressed difficulty and frustrations due to the limited capabilities to express licensing in project metadata, and this creates further trouble for Linux and BSD 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 clear, precise and standardized license metadata.

The current license classifiers address some common cases, and could be extended to include the full range of current SPDX identifiers while deprecating the many ambiguous classifiers (including some popular and problematic ones, such as License :: OSI Approved :: BSD License). However, this requires a substantial amount of effort to duplicate the SPDX license list and keep it in sync. Furthermore, it is effectively a hard break in backward compatibility, forcing a huge proportion of package authors to immediately update to new classifiers (in most cases, with many possible choices that require closely examining the project’s license) immediately when PyPI deprecates the old ones.

Furthermore, this only covers simple packages entirely under a single license; it doesn’t address the substantial fraction of common 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 also requires both authors and tools understand and implement the PyPI-specific bespoke classifier system, rather than using short, easy to add and standardized SPDX identifiers in a simple text field, as increasingly widely adopted by most other packaging systems to reduce the overall burden on the ecosystem. Finally, this does not provide as clear an indicator that a package has adopted the new system, and should be treated accordingly.

On average, Python packages tend to have more ambiguous and missing license information than other common ecosystems (such as npm, Maven or Gem). This is supported by the statistics page of the ClearlyDefined project, an Open Source Initiative incubated effort to help improve licensing clarity of other FOSS projects, covering all packages from PyPI, Maven, npm and Rubygems.


A survey of existing license metadata definitions in use in the Python ecosystem today is provided in an appendix of this PEP, and license documentation in a variety of other packaging systems, Linux distros, languages ecosystems and applications is surveyed in another appendix.

There are a few takeaways from the survey, which have guided the design and recommendations of this PEP:

  • Most package formats use a single License field.
  • Many modern package systems use some form of license expression syntax to optionally combine more than one license identifier together. SPDX and SPDX-like syntaxes are the most popular in use.
  • SPDX license identifiers are becoming the de facto way to reference common licenses everywhere, whether or not a full 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 package authors to include their full text in a distribution.

The use of a new License-Expression field will provide an intuitive, structured and unambiguous way to express the license of a package using a well-defined syntax and well-known license identifiers. Similarly, a formally-specified License-File field offers a standardized way to ensure that the full text of the license(s) are included with the package when distributed, as legally required, and allows other tools consuming the core metadata to unambiguously locate a distribution’s license files.

While dramatically simplifying and improving the present Python license metadata story, this specification standardizes and builds upon existing practice in the Setuptools and Wheel projects. Furthermore, an up-to-date version of the current draft of this PEP is already successfully implemented in the popular PyPA Hatch packaging tool, and an earlier draft of the license files portion is implemented in Setuptools.

Over time, encouraging the use of these fields and deprecating the ambiguous, duplicative and confusing legacy alternatives will help Python software publishers improve the clarity, accuracy and portability of their licensing practices, to the benefit of package authors, consumers and redistributors alike.


This PEP seeks to clearly define the terms it uses, given that some have multiple established meanings (e.g. import vs. distribution package, wheel format vs. Wheel project); are related and often used interchangeably, but have critical distinctions in meaning (e.g. [project] key vs. core metadata field); are existing concepts that don’t have formal terms/definitions (e.g. project/source metadata vs. distribution/built metadata, build vs. publishing tools), or are new concepts introduced here (e.g. license expression/identifier).

This PEP also uses terms defined in the PyPA PyPUG Glossary (specifically built/binary distribution, distribution package, project and source distribution), and by the SPDX Project (license identifier, license expression).

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.

Terms are listed here in their full versions; related words (Rel:) are in parenthesis, including short forms (Short:), sub-terms (Sub:) and common synonyms for the purposes of this PEP (Syn:).

Core Metadata (Syn: Package Metadata, Sub: Distribution Metadata)
The PyPA specification and the set of metadata fields it defines that describe key static attributes of distribution packages and installed projects.

The distribution metadata refers to, more specifically, the concrete form core metadata takes when included inside a distribution archive (PKG-INFO in a sdist and METADATA in a wheel) or installed project (METADATA).

Core Metadata Field (Short: Metadata Field/Field)
A single key-value pair, or sequence of such with the same key, as defined by the core metadata specification. Notably, distinct from a pyproject.toml [project] table key.
Distribution Package (Sub: Package, Distribution Archive)
(See PyPUG) In this PEP, package is used to refer to the abstract concept of a distributable form of a Python project, while distribution more specifically references the physical distribution archive.
License Classifier
A PyPI Trove classifier (as described in the core metadata specification) which begins with License ::, currently used to indicate a project’s license status by including it as a Classifier in the core metadata.
License Expression (Syn: SPDX Expression)
A string with valid SPDX license expression syntax including any SPDX license identifiers as defined here, which describes a project’s license(s) and how they relate to one another. Examples: GPL-3.0-or-later, MIT AND (Apache-2.0 OR BSD-2-clause)
License Identifier (Syn: License ID/SPDX Identifier)
A valid SPDX short-form license identifier, as described in the Add License-Expression field section of this PEP; briefly, this includes all valid SPDX identifiers and the LicenseRef-Public-Domain and LicenseRef-Proprietary strings. Examples: MIT, GPL-3.0-only
Project (Sub: Project Source Tree, Installed Project)
(See PyPUG) Here, a project source tree refers to the on-disk format of a project used for development, while an installed project is the form a project takes once installed from a distribution, as specified by PyPA.
Project Source Metadata (Sub: Project Table Metadata, Key, Subkey)
Core metadata defined by the package author in the project source tree, as top-level keys in the [project] table of a pyproject.toml file, in the [metadata] table of setup.cfg, or the equivalent for other build tools.

The Project Table Metadata, or pyproject.toml [project] metadata, refers specifically to the former, as defined by the PyPA Declaring Project Metadata specification and originally specified in PEP 621. A Project Table Key, or an unqualified key refers specifically to a top-level [project] key (notably, distinct from a core metadata field), while a subkey refers to a second-level key in a table-valued [project] key.

Root License Directory (Short: License Directory)
The directory under which license files are stored in a project/distribution and the root directory that their paths, as recorded under the License-File core metadata fields, are relative to. Defined here to be the project root directory for source trees and source distributions, and a subdirectory named licenses of the directory containing the core metadata (i.e., the .dist-info/licenses directory) for built distributions and installed projects.
Tool (Sub: Packaging Tool, Build Tool, Install Tool, Publishing Tool)
A program, script or service executed by the user or automatically that seeks to conform to the specification defined in this PEP.

A packaging tool refers to a tool used to build, publish, install, or otherwise directly interact with Python packages.

A build tool is a packaging tool used to generate a source or built distribution from a project source tree or sdist, when directly invoked as such (as opposed to by end-user-facing install tools). Examples: Wheel project, PEP 517 backends via build or other package-developer-facing frontends, calling directly.

An install tool is a packaging tool used to install a source or built distribution in a target environment. Examples include the PyPA pip and installer projects.

A publishing tool is a packaging tool used to upload distribution archives to a package index, such as Twine for PyPI.

Wheel (Short: wheel, Rel: wheel format, Wheel project)
Here, wheel, the standard built distribution format introduced in PEP 427 and specified by the PyPA, will be referred to in lowercase, while the Wheel project, its reference implementation, will be referred to as such with Wheel in Title Case.


The changes necessary to implement the improved license handling outlined in this PEP include those in both distribution package metadata, as defined in the core metadata specification, and author-provided project source metadata, as defined in the project source metadata specification (and originally introduced in PEP 621).

Further, minor additions to the source distribution (sdist), built distribution (wheel) and installed project specifications will help document and clarify the already allowed, now formally standardized behavior in these respects. Finally, guidance is established for tools handling and converting legacy license metadata to license expressions, to ensure the results are consistent, correct and unambiguous.

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.

Core metadata

The PyPA Core Metadata specification defines the names and semantics of each of the supported fields in the distribution metadata of Python distribution packages and installed projects.

This PEP adds the License-Expression field, adds the License-File field, deprecates the License field, and deprecates the license classifiers in the Classifier field.

The error and warning guidance in this section applies to build and publishing tools; end-user-facing install tools MAY be more lenient 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 to version 2.4.

Add License-Expression field

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

Publishing tools SHOULD issue an informational warning if this field is missing, and MAY raise an error. Build tools MAY issue a similar warning, but MUST NOT raise an error.

A license expression is a string using the SPDX license expression syntax as documented in the SPDX specification, either Version 2.2 or a later compatible version.

When used in the License-Expression 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, 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 LicenseRef-Public-Domain and LicenseRef-Proprietary strings, to identify licenses that are not included in the SPDX license list.

When processing the License-Expression field to determine if it contains a valid license expression, build and publishing tools:

  • SHOULD halt execution and raise an error if:
    • The field does not contain a valid license expression
    • One or more license identifiers are not valid (as defined above)
  • SHOULD report an informational warning, and publishing tools MAY raise an error, if one or more license identifiers have been marked as deprecated in the SPDX License List.
  • MUST 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.
  • SHOULD report an informational warning, and MAY raise an error if the normalization process results in changes to the License-Expression field contents.

For all newly-upload distributions that include a License-Expression field, the Python Package Index (PyPI) MUST validate that it contains a valid, case-normalized license expression with valid identifiers (as defined here) and MUST reject uploads that do not. 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

Each instance of the License-File optional field is specified to contain the string representation of the path in the project source tree, relative to the project root directory, of a license-related file. It is a multi-use field that may appear zero or more times, each instance listing 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. In other legacy, non-standard or new distribution package formats and mechanisms of accessing and storing core metadata, the value MAY correspond to the license file path relative to a format-defined root license directory. Alternatively, it MAY be treated as a unique abstract key to access the license file contents by another means, as specified by the format.

If a License-File is listed in a source or built distribution’s core metadata, that file MUST be included in the distribution at the specified path relative to the root license directory, and MUST be installed with the distribution 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. Therefore, 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 packages that include one or more License-File fields and declare a Metadata-Version of 2.4 or higher, PyPI SHOULD validate that the specified files are present in all uploaded distributions, and MUST reject uploads that do not validate.

Deprecate License field

The legacy unstructured-text License field is deprecated and replaced by the new License-Expression field. Build and publishing tools MUST raise an error if both these fields are present and their values are not identical, including capitalization and excluding leading and trailing whitespace.

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

For all newly-uploaded distributions that include a License-Expression field, the Python Package Index (PyPI) MUST reject any that specify a License field and the text of which is not identical to that of License-Expression, as defined in this section.

Along with license classifiers, 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 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. Along with the License field, license classifiers may be removed from a new version of the specification in a future PEP.

Project source metadata

As originally introduced in PEP 621, the PyPA Declaring Project Metadata specification defines how to declare a project’s source metadata under a [project] table in the pyproject.toml file for build tools to consume and output distribution core metadata.

This PEP adds a top-level string value for the license key, adds the new license-files key and deprecates the table value for the license key along with its corresponding table subkeys, text and file.

Add string value to license key

A top-level string value is defined for the license key in the [project] table, which is specified to be a valid SPDX license expression, as defined previously. Its value maps to the License-Expression field in the core metadata.

Build tools SHOULD validate the expression as described in the Add License-Expression field section, outputting an error or warning as specified. When generating the core metadata, tools MUST perform case normalization.

If a top-level string value for the license key is present and valid, for purposes of backward compatibility tools MAY back-fill the License core metadata field with the normalized value of the license key.

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.

Note: To avoid ambiguity, confusion and (per PEP 20, the Zen of Python) “more than one (obvious) way to do it”, allowing a flat array of strings as the value for the license-files key has been left out for now.

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.

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 minor additions will be made to the relevant existing specifications to document, standardize and clarify what is already currently supported, allowed and implemented behavior, as well as explicitly mention the root license directory the license files are located in and relative to for each format, per the Add License-File field section.

Project source trees
As described in the 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 for Metadata-Version is 2.4 or greater, the sdist MUST contain any license files specified by License-File in the PKG-INFO at their respective paths relative to the top-level directory 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, dedicated License-Expression core metadata field and a top-level string value for the license key reserved for this purpose in the pyproject.toml [project] table unambiguously signals 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, and raises an error if and only if the user affirmatively upgrades to the latest metadata version and adds the new field/key.

The legacy License core metadata field and the license key table subkeys (text and file) in the pyproject.toml [project] table will be deprecated along with the license classifiers, retaining backwards compatibility while gently preparing users for their future removal. Such a removal would follow a suitable transition period, and be left to a future PEP and a new version of the core metadata specification.

Formally specifying the new License-File core metadata field and the inclusion of the listed files in the distribution merely codifies and refines the existing practices in popular packaging tools, including the Wheel and Setuptools projects, and is designed to be largely backwards-compatible with their existing use of that field. Likewise, the new license-files key in the [project] table of pyproject.toml standardizes statically specifying the files to include, as well as the default behavior, and allows other tools to make use of them, while only having an effect once users and tools expressly adopt it.

Due to requiring license files not be flattened into .dist-info and specifying that they should be placed in a dedicated licenses subdir, wheels produced following this change will have differently-located licenses relative to those produced via the previous unspecified, installer-specific behavior, but as until this PEP there was no way of discovering these files or accessing them programmatically, and this will be further discriminated by a new metadata version, there aren’t any foreseen mechanism for this to pose a practical issue.

Furthermore, this resolves existing compatibility issues with the current ad hoc behavior, namely license files being silently clobbered if they have the same names as others at different paths, unknowingly rendering the wheel undistributable, and conflicting with the names of other metadata files in the same directory. Formally specifying otherwise would in fact block full forward compatibility with additional standard or installer-specified files and directories added to .dist-info, as they too could conflict with the names of existing licenses.

While minor additions will be made to the source distribution (sdist), built distribution (wheel) and installed project specifications, all of these are merely documenting, clarifying and formally specifying behaviors explicitly allowed under their current respective specifications, and already implemented in practice, and gating them behind the explicit presence of both the new metadata versions and the new fields. In particular, sdists may contain arbitrary files following the project source tree layout, and formally mentioning that these must include the license files listed in the metadata merely documents and codifies existing Setuptools practice. Likewise, arbitrary installer-specific files are allowed in the .dist-info directory of wheels and copied to installed projects, and again this PEP just formally clarifies and standardizes what is already being done.

Finally, while this PEP does propose PyPI implement validation of the new License-Expression and License-File fields, this has no effect on existing packages, nor any effect on any new distributions uploaded unless they explicitly choose to opt in to using these new fields while not following the requirements in the specification. Therefore, this does not have a backward compatibility impact, and in fact ensures forward compatibility with any future changes by ensuring all distributions uploaded to PyPI with the new fields are valid and 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

The simple cases are simple: a single license identifier 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 express their package’s license with a valid license expression is to have tools issue informative messages when they detect invalid license expressions, or when the deprecated License field or license classifiers are used.

An immediate, descriptive error message if an invalid License-Expression is used will help users understand they need to use SPDX identifiers in this field, and catch them if they make a mistake. For authors still using the now-deprecated, less precise and more redundant License field or license classifiers, packaging tools will warn them and inform them of the modern replacement, License-Expression. Finally, for users who may have forgotten or not be aware they need to do so, publishing tools will gently guide them toward including license and license-files in their project source metadata.

Tools may also help with the conversion and suggest a license expression in many, if not most common cases:

  • The appendix Appendix: Mapping License Classifiers to SPDX Identifiers provides tool authors with recommendation on how to suggest a license expression produced from legacy classifiers.
  • Tools may also be able to infer and suggest how to update an existing License value in project source metadata and convert that to a license expression, as also specified in this PEP. For instance, a tool may suggest converting a value of MIT in the license.text key in [project] (or the equivalent in tool-specific formats) to a top-level string value of the license key (or equivalent). Likewise, a tool could suggest converting from a License of Apache2 (which is not a valid license expression as defined in this PEP) to a License-Expression of Apache-2.0 (the equivalent valid license expression using an SPDX license identifier).

Reference Implementation

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

The license-expression library is a reference Python implementation that handles license expressions including parsing, formatting and validation, using flexible lists of license symbols (including SPDX license IDs and any extra identifiers included here). It is licensed under Apache-2.0 and is already used in several projects, including the SPDX Python Tools, the ScanCode toolkit and the Free Software Foundation Europe (FSFE) REUSE project.

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.

Open Issues

Should the License field be back-filled, or mutually exclusive?

At present, this PEP explicitly allows, but does not formally recommend or require, build tools to back-fill the License core metadata field with the verbatim text from the License-Expression field. This would presumably improve backwards compatibility and was suggested by some on the Discourse thread. On the other hand, allowing it does increase complexity and is less of a clean, consistent separation, preventing the License field from being completely mutually exclusive with the new License-Expression field and requiring that their values match.

As such, it would be very useful to have a more concrete and specific rationale and use cases for the back-filled data, and give fuller consideration to any potential benefits or drawbacks of this approach, in order to come to a final consensus on this matter that can be appropriately justified here.

Therefore, is the status quo expressed here acceptable, allowing tools leeway to decide this for themselves? Should this PEP formally recommend, or even require, that tools back-fill this metadata (which would presumably be reversed once a breaking revision of the metadata spec is issued)? Or should this not be explicitly allowed, discouraged or even prohibited?

Should custom license identifiers be allowed?

The current version of this PEP retains the behavior of only specifying the use of SPDX-defined license identifiers, as well as the explicitly defined custom identifiers LicenseRef-Public-Domain and LicenseRef-Proprietary to handle the two common cases where projects have a license, but it is not one that has a recognized SPDX license identifier.

For maximum flexibility, custom LicenseRef-<CUSTOM-TEXT> license identifiers could be allowed, which could potentially be useful for niche cases or corporate environments where LicenseRef-Proprietary is not appropriate or insufficiently specific, but relying on mainstream Python build tooling and the License-Expression metadata field is still desirable to use for this purpose.

This has the downsides, however, of not catching misspellings of the canonically defined license identifiers and thus producing license metadata that is not a valid match for what the author intended, as well as users potentially thinking they have to prepend LicenseRef in front of valid license identifiers, as there seems to be some previous confusion about. Furthermore, this encourages the proliferation of bespoke license identifiers, which obviates the purpose of enabling clear, unambiguous and well understood license metadata for which this PEP was created.

Indeed, for niche cases that need specific, proprietary custom licenses, they could always simply specify LicenseRef-Proprietary, and then include the actual license files needed to unambiguously identify the license regardless (if not using SPDX license identifiers) under the License-File fields. Requiring standards-conforming tools to allow custom license identifiers does not seem very useful, since standard tools will not recognize bespoke ones or know how to treat them. By contrast, bespoke tools, which would be required in any case to understand and act on custom identifiers, are explicitly allowed, with good reason (thus the SHOULD keyword) to not require that license identifiers conform to those listed here. Therefore, this specification still allows such use in private corporate environments or specific ecosystems, while avoiding the disadvantages of imposing them on all mainstream packaging tools.

As an alternative, a literal LicenseRef-Custom identifier could be defined, which would more explicitly indicate that the license cannot be expressed with defined identifiers and the license text should be referenced for details, without carrying the negative and potentially inappropriate implications of LicenseRef-Proprietary. This would avoid the main mentioned downsides (misspellings, confusion, license proliferation) of the approve approach of allowing an arbitrary LicenseRef, while addressing several of the potential theoretical scenarios cited for it.

On the other hand, as SPDX aims to (and generally does) encompass all FSF-recognized “Free” and OSI-approved “Open Source” licenses, and those sources are kept closely in sync and are now relatively stable, anything outside those bounds would generally be covered by LicenseRef-Proprietary, thus making LicenseRef-Custom less specific in that regard, and somewhat redundant to it. Furthermore, it may mislead authors of projects with complex/multiple licenses that they should use it over specifying a license expression.

At present, the PEP retains the existing approach over either of these, given the use cases and benefits were judged to be sufficiently marginal based on the current understanding of the packaging landscape. For both these proposals, however, if more concrete use cases emerge, this can certainly be reconsidered, either for this current PEP or a future one (before or in tandem with actually removing the legacy unstructured License metadata field). Not defining this now enables allowing it later (or still now, with custom packaging tools), without affecting backward compatibility, while the same is not so if they are allowed now and later determined to be unnecessary or too problematic in practice.


A list of auxilliary documents is provided:


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


Last modified: 2024-04-06 18:23:15 GMT