Basic example

The Setuptools project itself, as of version 59.1.1, does not use the License field in its own project source metadata. Further, it no longer explicitly specifies license_file/license_files as it did previously, since Setuptools relies on its own automatic inclusion of license-related files matching common patterns, such as the LICENSE file it uses.

It includes the following license-related metadata in its setup.cfg:

[metadata]
classifiers =
    License :: OSI Approved :: MIT License

The simplest migration to PEP 639 would consist of using this instead:

[metadata]
license_expression = MIT

Or, in the [project] table of pyproject.toml:

[project]
license = "MIT"

The output core metadata for the distribution packages would then be:

License-Expression: MIT
License-File: LICENSE

The LICENSE file would be stored at /setuptools-${VERSION}/LICENSE in the sdist and /setuptools-${VERSION}.dist-info/licenses/LICENSE in the wheel, and unpacked from there into the site directory (e.g. site-packages) on installation; / is the root of the respective archive and ${VERSION} the version of the Setuptools release in the core metadata.

Advanced example

Suppose Setuptools were to include the licenses of the third-party projects that are vendored in the setuptools/_vendor/ and pkg_resources/_vendor directories; specifically:

packaging==21.2
pyparsing==2.2.1
ordered-set==3.1.1
more_itertools==8.8.0

The license expressions for these projects are:

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

A comprehensive license expression covering both Setuptools proper and its vendored dependencies would contain these metadata, combining all the license expressions into one. Such an expression might be:

MIT AND (Apache-2.0 OR BSD-2-Clause)

In addition, per the requirements of the licenses, the relevant license files must be included in the package. Suppose the LICENSE file contains the text of the MIT license and the copyrights used by Setuptools, pyparsing, more_itertools and ordered-set; and the LICENSE* files in the setuptools/_vendor/packaging/ directory contain the Apache 2.0 and 2-clause BSD license text, and the Packaging copyright statement and license choice notice.

Specifically, we assume the license files are located at the following paths in the project source tree (relative to the project root and pyproject.toml):

LICENSE
setuptools/_vendor/packaging/LICENSE
setuptools/_vendor/packaging/LICENSE.APACHE
setuptools/_vendor/packaging/LICENSE.BSD

Putting it all together, our setup.cfg would be:

[metadata]
license_expression = MIT AND (Apache-2.0 OR BSD-2-Clause)
license_files =
    LICENSE
    setuptools/_vendor/packaging/LICENSE
    setuptools/_vendor/packaging/LICENSE.APACHE
    setuptools/_vendor/packaging/LICENSE.BSD

In the [project] table of pyproject.toml, with license files specified explicitly via the paths subkey, this would look like:

[project]
license = "MIT AND (Apache-2.0 OR BSD-2-Clause)"
license-files.paths = [
    "LICENSE",
    "setuptools/_vendor/LICENSE",
    "setuptools/_vendor/LICENSE.APACHE",
    "setuptools/_vendor/LICENSE.BSD",
]

Or alternatively, matched via glob patterns, this could be:

[project]
license = "MIT AND (Apache-2.0 OR BSD-2-Clause)"
license-files.globs = [
    "LICENSE*",
    "setuptools/_vendor/LICENSE*",
]

With either approach, the output core metadata in the distribution would be:

License-Expression: MIT AND (Apache-2.0 OR BSD-2-Clause)
License-File: LICENSE
License-File: setuptools/_vendor/packaging/LICENSE
License-File: setuptools/_vendor/packaging/LICENSE.APACHE
License-File: setuptools/_vendor/packaging/LICENSE.BSD

In the resulting sdist, with / as the root of the archive and ${VERSION} the version of the Setuptools release specified in the core metadata, the license files would be located at the paths:

/setuptools-${VERSION}/LICENSE
/setuptools-${VERSION}/setuptools/_vendor/packaging/LICENSE
/setuptools-${VERSION}/setuptools/_vendor/packaging/LICENSE.APACHE
/setuptools-${VERSION}/setuptools/_vendor/packaging/LICENSE.BSD

In the built wheel, with / being the root of the archive and {version} as the previous, the license files would be stored at:

/setuptools-${VERSION}.dist-info/licenses/LICENSE
/setuptools-${VERSION}.dist-info/licenses/setuptools/_vendor/packaging/LICENSE
/setuptools-${VERSION}.dist-info/licenses/setuptools/_vendor/packaging/LICENSE.APACHE
/setuptools-${VERSION}.dist-info/licenses/setuptools/_vendor/packaging/LICENSE.BSD

Finally, in the installed project, with site-packages being the site dir and {version} as the previous, the license files would be installed to:

site-packages/setuptools-${VERSION}.dist-info/licenses/LICENSE
site-packages/setuptools-${VERSION}.dist-info/licenses/setuptools/_vendor/packaging/LICENSE
site-packages/setuptools-${VERSION}.dist-info/licenses/setuptools/_vendor/packaging/LICENSE.APACHE
site-packages/setuptools-${VERSION}.dist-info/licenses/setuptools/_vendor/packaging/LICENSE.BSD

Expression examples

Some additional examples of valid License-Expression values:

License-Expression: MIT
License-Expression: BSD-3-Clause
License-Expression: MIT AND (Apache-2.0 OR BSD-2-clause)
License-Expression: MIT OR GPL-2.0-or-later OR (FSFUL AND BSD-2-Clause)
License-Expression: GPL-3.0-only WITH Classpath-Exception-2.0 OR BSD-3-Clause
License-Expression: LicenseRef-Public-Domain OR CC0-1.0 OR Unlicense
License-Expression: LicenseRef-Proprietary