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Python Enhancement Proposals

PEP 561 – Distributing and Packaging Type Information

Ethan Smith <ethan at>
Standards Track
Packaging, Typing
10-Sep-2017, 12-Sep-2017, 06-Oct-2017, 26-Oct-2017, 12-Apr-2018

Table of Contents


This PEP is a historical document: see Type information in libraries for up-to-date specs and documentation. Canonical typing specs are maintained at the typing specs site; runtime typing behaviour is described in the CPython documentation.


See the typing specification update process for how to propose changes to the typing spec.


PEP 484 introduced type hinting to Python, with goals of making typing gradual and easy to adopt. Currently, typing information must be distributed manually. This PEP provides a standardized means to leverage existing tooling to package and distribute type information with minimal work and an ordering for type checkers to resolve modules and collect this information for type checking.


Currently, package authors wish to distribute code that has inline type information. Additionally, maintainers would like to distribute stub files to keep Python 2 compatibility while using newer annotation syntax. However, there is no standard method to distribute packages with type information. Also, if one wished to ship stub files privately the only method available would be via setting MYPYPATH or the equivalent to manually point to stubs. If the package can be released publicly, it can be added to typeshed [1]. However, this does not scale and becomes a burden on the maintainers of typeshed. In addition, it ties bug fixes in stubs to releases of the tool using typeshed.

PEP 484 has a brief section on distributing typing information. In this section the PEP recommends using shared/typehints/pythonX.Y/ for shipping stub files. However, manually adding a path to stub files for each third party library does not scale. The simplest approach people have taken is to add site-packages to their MYPYPATH, but this causes type checkers to fail on packages that are highly dynamic (e.g. sqlalchemy and Django).

Definition of Terms

The definition of “MAY”, “MUST”, and “SHOULD”, and “SHOULD NOT” are to be interpreted as described in RFC 2119.

“inline” - the types are part of the runtime code using PEP 526 and PEP 3107 syntax (the filename ends in .py).

“stubs” - files containing only type information, empty of runtime code (the filename ends in .pyi).

“Distributions” are the packaged files which are used to publish and distribute a release. (PEP 426)

“Module” a file containing Python runtime code or stubbed type information.

“Package” a directory or directories that namespace Python modules. (Note the distinction between packages and distributions. While most distributions are named after the one package they install, some distributions install multiple packages.)


There are several motivations and methods of supporting typing in a package. This PEP recognizes three types of packages that users of typing wish to create:

  1. The package maintainer would like to add type information inline.
  2. The package maintainer would like to add type information via stubs.
  3. A third party or package maintainer would like to share stub files for a package, but the maintainer does not want to include them in the source of the package.

This PEP aims to support all three scenarios and make them simple to add to packaging and deployment.

The two major parts of this specification are the packaging specifications and the resolution order for resolving module type information. The type checking spec is meant to replace the shared/typehints/pythonX.Y/ spec of PEP 484.

New third party stub libraries SHOULD distribute stubs via the third party packaging methods proposed in this PEP in place of being added to typeshed. Typeshed will remain in use, but if maintainers are found, third party stubs in typeshed MAY be split into their own package.

Packaging Type Information

In order to make packaging and distributing type information as simple and easy as possible, packaging and distribution is done through existing frameworks.

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing. This marker applies recursively: if a top-level package includes it, all its sub-packages MUST support type checking as well. To have this file installed with the package, maintainers can use existing packaging options such as package_data in distutils, shown below.

Distutils option example:

    package_data = {
        'foopkg': ['py.typed'],

For namespace packages (see PEP 420), the py.typed file should be in the submodules of the namespace, to avoid conflicts and for clarity.

This PEP does not support distributing typing information as part of module-only distributions or single-file modules within namespace packages.

The single-file module should be refactored into a package and indicate that the package supports typing as described above.

Stub-only Packages

For package maintainers wishing to ship stub files containing all of their type information, it is preferred that the *.pyi stubs are alongside the corresponding *.py files. However, the stubs can also be put in a separate package and distributed separately. Third parties can also find this method useful if they wish to distribute stub files. The name of the stub package MUST follow the scheme foopkg-stubs for type stubs for the package named foopkg. Note that for stub-only packages adding a py.typed marker is not needed since the name *-stubs is enough to indicate it is a source of typing information.

Third parties seeking to distribute stub files are encouraged to contact the maintainer of the package about distribution alongside the package. If the maintainer does not wish to maintain or package stub files or type information inline, then a third party stub-only package can be created.

In addition, stub-only distributions SHOULD indicate which version(s) of the runtime package are supported by indicating the runtime distribution’s version(s) through normal dependency data. For example, the stub package flyingcircus-stubs can indicate the versions of the runtime flyingcircus distribution it supports through install_requires in distutils-based tools, or the equivalent in other packaging tools. Note that in pip 9.0, if you update flyingcircus-stubs, it will update flyingcircus. In pip 9.0, you can use the --upgrade-strategy=only-if-needed flag. In pip 10.0 this is the default behavior.

For namespace packages (see PEP 420), stub-only packages should use the -stubs suffix on only the root namespace package. All stub-only namespace packages should omit __init__.pyi files. py.typed marker files are not necessary for stub-only packages, but similarly to packages with inline types, if used, they should be in submodules of the namespace to avoid conflicts and for clarity.

For example, if the pentagon and hexagon are separate distributions installing within the namespace package shapes.polygons The corresponding types-only distributions should produce packages laid out as follows:

└── polygons
    └── pentagon
        └── __init__.pyi

└── polygons
    └── hexagon
        └── __init__.pyi

Type Checker Module Resolution Order

The following is the order in which type checkers supporting this PEP SHOULD resolve modules containing type information:

  1. Stubs or Python source manually put in the beginning of the path. Type checkers SHOULD provide this to allow the user complete control of which stubs to use, and to patch broken stubs/inline types from packages. In mypy the $MYPYPATH environment variable can be used for this.
  2. User code - the files the type checker is running on.
  3. Stub packages - these packages SHOULD supersede any installed inline package. They can be found at foopkg-stubs for package foopkg.
  4. Packages with a py.typed marker file - if there is nothing overriding the installed package, and it opts into type checking, the types bundled with the package SHOULD be used (be they in .pyi type stub files or inline in .py files).
  5. Typeshed (if used) - Provides the stdlib types and several third party libraries.

If typecheckers identify a stub-only namespace package without the desired module in step 3, they should continue to step 4/5. Typecheckers should identify namespace packages by the absence of __init__.pyi. This allows different subpackages to independently opt for inline vs stub-only.

Type checkers that check a different Python version than the version they run on MUST find the type information in the site-packages/dist-packages of that Python version. This can be queried e.g. pythonX.Y -c 'import site; print(site.getsitepackages())'. It is also recommended that the type checker allow for the user to point to a particular Python binary, in case it is not in the path.

Partial Stub Packages

Many stub packages will only have part of the type interface for libraries completed, especially initially. For the benefit of type checking and code editors, packages can be “partial”. This means modules not found in the stub package SHOULD be searched for in parts four and five of the module resolution order above, namely inline packages and typeshed.

Type checkers should merge the stub package and runtime package or typeshed directories. This can be thought of as the functional equivalent of copying the stub package into the same directory as the corresponding runtime package or typeshed folder and type checking the combined directory structure. Thus type checkers MUST maintain the normal resolution order of checking *.pyi before *.py files.

If a stub package distribution is partial it MUST include partial\n in a py.typed file. For stub-packages distributing within a namespace package (see PEP 420), the py.typed file should be in the submodules of the namespace.

Type checkers should treat namespace packages within stub-packages as incomplete since multiple distributions may populate them. Regular packages within namespace packages in stub-package distributions are considered complete unless a py.typed with partial\n is included.


The proposed scheme of indicating support for typing is completely backwards compatible, and requires no modification to package tooling. A sample package with inline types is available [typed_package], as well as a [stub_package]. A sample package checker [pkg_checker] which reads the metadata of installed packages and reports on their status as either not typed, inline typed, or a stub package.

The mypy type checker has an implementation of PEP 561 searching which can be read about in the mypy docs [4].

[numpy-stubs] is an example of a real stub-only package for the numpy distribution.


This PEP would not have been possible without the ideas, feedback, and support of Ivan Levkivskyi, Jelle Zijlstra, Alyssa Coghlan, Daniel F Moisset, Andrey Vlasovskikh, Nathaniel Smith, and Guido van Rossum.

Version History

  • 2023-01-13
    • Clarify that the 4th step of the Module Resolution Order applies to any package with a py.typed marker file (and not just inline packages).
  • 2021-09-20
    • Clarify expectations and typechecker behavior for stub-only namespace packages
    • Clarify handling of single-file modules within namespace packages.
  • 2018-07-09
    • Add links to sample stub-only packages
  • 2018-06-19
    • Partial stub packages can look at typeshed as well as runtime packages
  • 2018-05-15
    • Add partial stub package spec.
  • 2018-04-09
    • Add reference to mypy implementation
    • Clarify stub package priority.
  • 2018-02-02
    • Change stub-only package suffix to be -stubs not _stubs.
    • Note that py.typed is not needed for stub-only packages.
    • Add note about pip and upgrading stub packages.
  • 2017-11-12
    • Rewritten to use existing tooling only
    • No need to indicate kind of type information in metadata
    • Name of marker file changed from .typeinfo to py.typed
  • 2017-11-10
    • Specification re-written to use package metadata instead of distribution metadata.
    • Removed stub-only packages and merged into third party packages spec.
    • Removed suggestion for typecheckers to consider checking runtime versions
    • Implementations updated to reflect PEP changes.
  • 2017-10-26
    • Added implementation references.
    • Added acknowledgements and version history.
  • 2017-10-06
    • Rewritten to use .distinfo/METADATA over a distutils specific command.
    • Clarify versioning of third party stub packages.
  • 2017-09-11
    • Added information about current solutions and typeshed.
    • Clarify rationale.



Last modified: 2024-06-05 15:44:49 GMT