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Contributing to typeshed

Welcome! typeshed is a community project that aims to work for a wide range of Python users and Python codebases. If you're trying a type checker on your Python code, your experience and what you can contribute are important to the project's success.

The contribution process at a glance

  1. Prepare your environment.
  2. Find out where to make your changes.
  3. Prepare your changes:
    • Small fixes and additions can be submitted directly as pull requests, but contact us before starting significant work.
    • Create your stubs, considering what to include and conforming to the coding style.
  4. Format and check your stubs.
  5. Optionally run the tests.
  6. Submit your changes by opening a pull request.

You can expect a reply within a few days, but please be patient when it takes a bit longer. For more details, read below.

Preparing the environment

To reformat the code, check for common problems, and run the tests, you need to prepare a virtual environment with the necessary libraries installed. To do this, run:

$ python3 -m venv .venv3
$ source .venv3/bin/activate
(.venv3)$ pip install -U pip
(.venv3)$ pip install -r requirements-tests-py3.txt

To automatically check your code before committing, copy the file pre-commit to .git/hooks/pre-commit.

Where to make changes

Standard library stubs

The stdlib directory contains stubs for modules in the Python 3 standard library — which includes pure Python modules, dynamically loaded extension modules, hard-linked extension modules, and the builtins. The VERSIONS file lists the versions of Python where the module is available.

Stubs for Python 2 are available in the stdlib/@python2 subdirectory. Modules that are only available for Python 2 are not listed in VERSIONS.

Third-party library stubs

We accept stubs for third-party packages into typeshed as long as:

  • the package is publicly available on the Python Package Index;
  • the package supports any Python version supported by typeshed; and
  • the package does not ship with its own stubs or type annotations.

Stubs for third-party packages go into stubs. Each subdirectory there represents a PyPI distribution, and contains the following:

  • METADATA.toml, describing the package. See below for details.
  • Stubs (i.e. *.pyi files) for packages and modules that are shipped in the source distribution.
  • If the stubs are either Python 2-only, or if the Python 2 and Python 3 stubs are separate, the Python 2 stubs are put in a @python2 subdirectory. Stubs outside @python2 are always used with Python 3, and also with Python 2 if python2 = true is set in METADATA.toml (see below).
  • (Rarely) some docs specific to a given type stub package in README file.

The fastest way to generate new stubs is to use stubgen, a tool shipped with mypy. Please make sure to use the latest version. The generated stubs usually need some trimming of imports. You also need to run black and isort manually on the generated stubs (see below).

When a third party stub is added or modified, an updated version of the corresponding distribution will be automatically uploaded to PyPI within a few hours. Each time this happens the least significant version level is incremented. For example, if stubs/foo/METADATA.toml has version = "x.y" the package on PyPI will be updated from types-foo-x.y.n to types-foo-x.y.n+1.

Note: In its current implementation, typeshed cannot contain stubs for multiple versions of the same third-party library. Prefer to generate stubs for the latest version released on PyPI at the time of your stubbing.

The METADATA.toml file

The metadata file describes the stubs package using the TOML file format. Currently, the following keys are supported:

  • version: The latest version of the library that the stubs support. Note that only two most significant version levels are supported (i.e. only single dot). When the stubs are updated to a newer version of the library, the version of the stub should be bumped (note that previous versions are still available on PyPI). Some legacy stubs are marked with version 0.1, indicating that their supported version is unknown and needs to be updated.
  • python2 (default: false): If set to true, the top-level stubs support both Python 2 and Python 3.
  • requires (optional): A list of other stub packages or packages with type information that are imported by the stubs in this package. Only packages generated by typeshed or required by the upstream package are allowed to be listed here, for security reasons.
  • extra_description (optional): Can be used to add a custom description to the package's long description. It should be a multi-line string in Markdown format.
  • obsolete_since (optional): This field is part of our process for removing obsolete third-party libraries. It contains the first version of the corresponding library that ships its own py.typed file.

The format of all METADATA.toml files can be checked by running python3 ./tests/check_consistent.py.

Preparing Changes

Before you begin

If your change will be a significant amount of work to write, we highly recommend starting by opening an issue laying out what you want to do. That lets a conversation happen early in case other contributors disagree with what you'd like to do or have ideas that will help you do it.

Format

Each Python module is represented by a .pyi "stub file". This is a syntactically valid Python file, although it usually cannot be run by Python 3 (since forward references don't require string quotes). All the methods are empty.

Python function annotations (PEP 3107) are used to describe the signature of each function or method.

See PEP 484 for the exact syntax of the stub files and below for the coding style used in typeshed.

Supported type system features

Since PEP 484 was accepted, there have been many other PEPs that added new features to the Python type system. In general, new features can be used in typeshed as soon as the PEP has been accepted and implemented and most type checkers support the new feature.

Accepted features that cannot yet be used in typeshed include:

  • PEP 570 (positional-only arguments): see #4972, use argument names prefixed with __ instead
  • PEP 613 (TypeAlias): see #4913

The following features are partially supported:

  • PEP 585 (builtin generics): see #4820, mostly supported but bugs remain for a few specific cases
  • PEP 612 (ParamSpec): see #4827, supported in some contexts but requires # type: ignore comments

Supported features include:

Features from the typing module that are not present in all supported Python 3 versions must be imported from typing_extensions instead in typeshed stubs. This currently affects:

  • Final and @final (new in Python 3.8)
  • Literal (new in Python 3.8)
  • SupportsIndex (new in Python 3.8)
  • TypedDict (new in Python 3.8)
  • TypeGuard (new in Python 3.10)

An exception is Protocol: although it was added in Python 3.8, it can be used in stubs regardless of Python version.

What to include

Stubs should include the complete interface (classes, functions, constants, etc.) of the module they cover, but it is not always clear exactly what is part of the interface.

The following should always be included:

  • All objects listed in the module's documentation.
  • All objects included in __all__ (if present).

Other objects may be included if they are being used in practice or if they are not prefixed with an underscore. This means that typeshed will generally accept contributions that add missing objects, even if they are undocumented. Undocumented objects should be marked with a comment of the form # undocumented. Example:

def list2cmdline(seq: Sequence[str]) -> str: ...  # undocumented

We accept such undocumented objects because omitting objects can confuse users. Users who see an error like "module X has no attribute Y" will not know whether the error appeared because their code had a bug or because the stub is wrong. Although it may also be helpful for a type checker to point out usage of private objects, we usually prefer false negatives (no errors for wrong code) over false positives (type errors for correct code). In addition, even for private objects a type checker can be helpful in pointing out that an incorrect type was used.

What to do when a project's documentation and implementation disagree

Type stubs are meant to be external type annotations for a given library. While they are useful documentation in its own merit, they augment the project's concrete implementation, not the project's documentation. Whenever you find them disagreeing, model the type information after the actual implementation and file an issue on the project's tracker to fix their documentation.

Stub versioning

You can use checks like if sys.version_info >= (3, 8): to denote new functionality introduced in a given Python version or solve type differences. When doing so, only use one-tuples or two-tuples. Because of this, if a given functionality was introduced in, say, Python 3.7.4, your check:

  • should be expressed as if sys.version_info >= (3, 7):
  • should NOT be expressed as if sys.version_info >= (3, 7, 4):
  • should NOT be expressed as if sys.version_info >= (3, 8):

When your stub contains if statements for different Python versions, always put the code for the most recent Python version first.

Incomplete stubs

We accept partial stubs, especially for larger packages. These need to follow the following guidelines:

  • Included functions and methods must list all arguments, but the arguments can be left unannotated. Do not use Any to mark unannotated arguments or return values.
  • Partial classes must include a __getattr__() method marked with an # incomplete comment (see example below).
  • Partial modules (i.e. modules that are missing some or all classes, functions, or attributes) must include a top-level __getattr__() function marked with an # incomplete comment (see example below).
  • Partial packages (i.e. packages that are missing one or more sub-modules) must have a __init__.pyi stub that is marked as incomplete (see above). A better alternative is to create empty stubs for all sub-modules and mark them as incomplete individually.

Example of a partial module with a partial class Foo and a partially annotated function bar():

def __getattr__(name: str) -> Any: ...  # incomplete

class Foo:
    def __getattr__(self, name: str) -> Any: ...  # incomplete
    x: int
    y: str

def bar(x: str, y, *, z=...): ...

Using stubgen

Mypy includes a tool called stubgen that auto-generates stubs for Python and C modules using static analysis, Sphinx docs, and runtime introspection. It can be used to get a starting point for your stubs. Note that this generator is currently unable to determine most argument and return types and omits them or uses Any in their place. Fill out manually the types that you know.

Stub file coding style

Syntax example

The below is an excerpt from the types for the datetime module.

MAXYEAR: int
MINYEAR: int

class date:
    def __new__(cls: Type[_S], year: int, month: int, day: int) -> _S: ...
    @classmethod
    def fromtimestamp(cls: Type[_S], __timestamp: float) -> _S: ...
    @classmethod
    def today(cls: Type[_S]) -> _S: ...
    @classmethod
    def fromordinal(cls: Type[_S], n: int) -> _S: ...
    @property
    def year(self) -> int: ...
    def replace(self, year: int = ..., month: int = ..., day: int = ...) -> date: ...
    def ctime(self) -> str: ...
    def weekday(self) -> int: ...

Conventions

Stub files are like Python files and you should generally expect them to look the same. Your tools should be able to successfully treat them as regular Python files. However, there are a few important differences you should know about.

Style conventions for stub files are different from PEP 8. The general rule is that they should be as concise as possible. Specifically:

  • all function bodies should be empty;
  • prefer ... over pass;
  • prefer ... on the same line as the class/function signature;
  • avoid vertical whitespace between consecutive module-level functions, names, or methods and fields within a single class;
  • use a single blank line between top-level class definitions, or none if the classes are very small;
  • do not use docstrings;
  • use variable annotations instead of type comments, even for stubs that target older versions of Python.

Stub files should only contain information necessary for the type checker, and leave out unnecessary detail:

  • for arguments with a default, use ... instead of the actual default;
  • for arguments that default to None, use Foo | None explicitly (see below for details);
  • use float instead of int | float.

Some further tips for good type hints:

  • use built-in generics (list, dict, tuple, set), instead of importing them from typing, except in type aliases, in base classes, and for arbitrary length tuples (Tuple[int, ...]);
  • use X | Y instead of Union[X, Y] and X | None, instead of Optional[X], except when it is not possible due to mypy bugs (type aliases and base classes);
  • in Python 3 stubs, import collections (Mapping, Iterable, etc.) from collections.abc instead of typing;
  • avoid invariant collection types (list, dict) in argument positions, in favor of covariant types like Mapping or Sequence;
  • avoid union return types: python/mypy#1693;
  • in Python 2, whenever possible, use unicode if that's the only possible type, and Text if it can be either unicode or bytes;
  • use platform checks like if sys.platform == 'win32' to denote platform-dependent APIs.

Imports in stubs are considered private (not part of the exported API) unless:

  • they use the form from library import name as name (sic, using explicit as even if the name stays the same); or
  • they use the form from library import * which means all names from that library are exported.

When adding type hints, avoid using the Any type when possible. Reserve the use of Any for when:

  • the correct type cannot be expressed in the current type system; and
  • to avoid union returns (see above).

Note that Any is not the correct type to use if you want to indicate that some function can accept literally anything: in those cases use object instead.

Stub files support forward references natively. In other words, the order of class declarations and type aliases does not matter in a stub file. You can also use the name of the class within its own body. Focus on making your stubs clear to the reader. Avoid using string literals in type annotations.

Type variables and aliases you introduce purely for legibility reasons should be prefixed with an underscore to make it obvious to the reader they are not part of the stubbed API.

When adding type annotations for context manager classes, annotate the return type of __exit__ as bool only if the context manager sometimes suppresses exceptions -- if it sometimes returns True at runtime. If the context manager never suppresses exceptions, have the return type be either None or bool | None. If you are not sure whether exceptions are suppressed or not or if the context manager is meant to be subclassed, pick bool | None. See python/mypy#7214 for more details.

__enter__ methods and other methods that return instances of the current class should be annotated with the _typeshed.Self type variable (example).

A few guidelines for protocol names below. In cases that don't fall into any of those categories, use your best judgement.

  • Use plain names for protocols that represent a clear concept (e.g. Iterator, Container).
  • Use SupportsX for protocols that provide callable methods (e.g. SupportsInt, SupportsRead, SupportsReadSeek).
  • Use HasX for protocols that have readable and/or writable attributes or getter/setter methods (e.g. HasItems, HasFileno).

Formatting stubs

Stubs should be reformatted with the formatters black and isort before submission. They should also be checked for common problems by using flake8 and the flake8 plugins flake8-pyi and flake8-bugbear. All of these packages have been installed if you followed the setup instructions above.

To format and check your stubs, run the following commands:

(.venv3)$ black stdlib stubs
(.venv3)$ isort stdlib stubs
(.venv3)$ flake8

Running the tests

The tests are automatically run on every PR and push to the repo. Therefore you don't need to run them locally, unless you want to run them before making a pull request or you want to debug some problem without creating several small commits.

For more information about our available tests, see tests/README.md.

Submitting Changes

Even more excellent than a good bug report is a fix for a bug, or the implementation of a much-needed stub. We'd love to have your contributions.

We use the usual GitHub pull-request flow, which may be familiar to you if you've contributed to other projects on GitHub. For the mechanics, see Mypy's git and GitHub workflow help page, or GitHub's own documentation.

Anyone interested in type stubs may review your code. One of the maintainers will merge your pull request when they think it's ready. For every pull request, we aim to promptly either merge it or say why it's not yet ready; if you go a few days without a reply, please feel free to ping the thread by adding a new comment.

To get your pull request merged sooner, you should explain why you are making the change. For example, you can point to a code sample that is processed incorrectly by a type checker. It is also helpful to add links to online documentation or to the implementation of the code you are changing.

Also, do not squash your commits or use git commit --amend after you have submitted a pull request, as this erases context during review. We will squash commits when the pull request is merged. This way, your pull request will appear as a single commit in our git history, even if it consisted of several smaller commits.

Third-party library removal policy

Third-party packages are generally removed from typeshed when one of the following criteria is met:

  • The upstream package ships a py.typed file for at least 6-12 months, or
  • the package does not support any of the Python versions supported by typeshed.

If a package ships its own py.typed file, please follow these steps:

  1. Open an issue with the earliest month of removal in the subject.
  2. A maintainer will add the "removal" label.
  3. Open a PR that sets the obsolete_since field in the METADATA.toml file to the first version of the package that shipped py.typed.

Maintainer guidelines

The process for preparing and submitting changes also applies to maintainers. This ensures high quality contributions and keeps everybody on the same page. Avoid direct pushes to the repository.

When reviewing pull requests, follow these guidelines:

  • Typing is hard. Try to be helpful and explain issues with the PR, especially to new contributors.
  • When reviewing auto-generated stubs, just scan for red flags and obvious errors. Leave possible manual improvements for separate PRs.
  • When reviewing large, hand-crafted PRs, you only need to look for red flags and general issues, and do a few spot checks.
  • Review smaller, hand-crafted PRs thoroughly.

When merging pull requests, follow these guidelines:

  • Always wait for tests to pass before merging PRs.
  • Use "Squash and merge" to merge PRs.
  • Make sure the commit message is meaningful. For example, remove irrelevant intermediate commit messages.
  • The commit message for third-party stubs is used to generate the changelog. It should be valid Markdown, be comprehensive, read like a changelog entry, and assume that the reader has no access to the diff.
  • Delete branches for merged PRs (by maintainers pushing to the main repo).