typeshed/CONTRIBUTING.md

# 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. Find out [where to make your changes](#where-to-make-changes).
2. [Prepare your changes](#preparing-changes):
    * Small fixes and additions can be submitted directly as pull requests,
      but [contact us](README.md#discussion) before starting significant work.
    * Create your stubs, considering [what to include](#what-to-include) and
      conforming to the [coding style](#stub-file-coding-style).
    * Reformat your stubs with `black` and `isort`.
3. If you want, set up your environment to be able to [run tests](#running-the-tests).
    This can be useful for big pull requests or fixing specific errors, but
    usually is not needed, because the tests run automatically on GitHub Actions
    for all pull requests.
4. [Submit your changes](#submitting-changes) by opening a pull request.
5. You can expect a reply within a few days, but please be patient when
   it takes a bit longer.

For more details, read below.


## Where to make changes

### Standard library stubs

The `stdlib` directory contains stubs for modules in the
Python 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.

The structure of the `VERSIONS` file is as follows:
- Blank lines and lines starting with `#` are ignored.
- Lines contain the name of a top-level module, followed by a colon,
  a space, and a version range (for example: `symbol: 2.7-3.9`).

Version ranges may be of the form "X.Y-A.B" or "X.Y-". The
first form means that a module was introduced in version X.Y and last
available in version A.B. The second form means that the module was
introduced in version X.Y and is still available in the latest
version of Python.

Python versions before 2.7 are ignored, so any module that was already
present in 2.7 will have "2.7" as its minimum version. Version ranges
for unsupported versions of Python 3 (currently 3.5 and lower) are
generally accurate but we do not guarantee their correctness.

The `stdlib/@python2` subdirectory contains Python 2-only stubs,
both for modules that must be kept different for Python 2 and 3, like
`builtins.pyi`, and for modules that only existed in Python 2, like
`ConfigParser.pyi`. The latter group of modules are not listed in
`VERSIONS`.

Note that if a package is present in `@python2`, any stub in the main
`stdlib` directory should be ignored when looking for Python 2 stubs. For
example, typeshed contains files `stdlib/@python2/collections.pyi` and
`stdlib/collections/abc.pyi`. A client looking for stubs for
`collections.abc` in Python 2 should not pick up the latter file, but
instead report that the module does not exist.

### Third-party library stubs

Modules that are not shipped with Python but have a type description in Python
go into `stubs`. Each subdirectory there represents a PyPI distribution, and
contains the following:
* `METADATA.toml` that specifies oldest version of the source library for
  which the stubs are applicable, supported Python versions (Python 3 defaults
  to `True`, Python 2 defaults to `False`), and dependency on other type stub
  packages.
* Stubs (i.e. `*.pyi` files) for packages and modules that are shipped in the
  source distribution. Similar to standard library, if the Python 2 version of
  the stubs must be kept *separate*, it can be put in a `@python` subdirectory.
* (Rarely) some docs specific to a given type stub package in `README` file.

When a third party stub is
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 oldest version of the library for which the stubs are still
applicable (i.e. reflect the actual runtime behaviour) can be indicated
in `METADATA.toml` as `version = "x.y"`. Note that only two most significant
version levels are supported (i.e. only single dot). When a significant change
is made in the library, the version of the stub should be bumped (note that
previous versions are still available on PyPI).

## 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](https://www.python.org/dev/peps/pep-3107/))
are used to describe the signature of each function or method.

See [PEP 484](http://www.python.org/dev/peps/pep-0484/) for the exact
syntax of the stub files and [below](#stub-file-coding-style) for the
coding style used in typeshed.

### 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:

```python
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()`:

```python
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](https://mypy.readthedocs.io/en/latest/stubgen.html)
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.

```python
MAXYEAR: int
MINYEAR: int

class date:
    def __init__(self, year: int, month: int, day: int) -> None: ...
    @classmethod
    def fromtimestamp(cls, timestamp: float) -> date: ...
    @classmethod
    def today(cls) -> date: ...
    @classmethod
    def fromordinal(cls, ordinal: int) -> date: ...
    @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.

Stubs should be reformatted with the formatters
[black](https://github.com/psf/black) and
[isort](https://github.com/PyCQA/isort) before submission.
These formatters are included in typeshed's `requirements-tests-py3.txt` file.
A sample `pre-commit` file is included in the typeshed repository.  Copy it
to `.git/hooks` and adjust the path to your virtual environment's `bin`
directory to automatically reformat stubs before commit.

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 `Optional[]` explicitly
  (see below for details);
* use `float` instead of `Union[int, float]`.

Some further tips for good type hints:
* use built-in generics (`list`, `dict`, `tuple`, `set`), instead
  of importing them from `typing`, **except** for arbitrary length tuples
  (`Tuple[int, ...]`) (see
  [python/mypy#9980](https://github.com/python/mypy/issues/9980));
* 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: https://github.com/python/mypy/issues/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 `Optional[bool]`. If you
are not sure whether exceptions are suppressed or not or if the
context manager is meant to be subclassed, pick `Optional[bool]`.
See https://github.com/python/mypy/issues/7214 for more details.

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`).

## 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.

There are several tests:
- `tests/mypy_test.py`
tests typeshed with [mypy](https://github.com/python/mypy/)
- `tests/pytype_test.py` tests typeshed with
[pytype](https://github.com/google/pytype/).
- `tests/pyright_test.py` tests typeshed with
[pyright](https://github.com/microsoft/pyright).
- `tests/mypy_test_suite.py` runs a subset of mypy's test suite using this version of
typeshed.
- `tests/check_consistent.py` checks certain files in typeshed remain
consistent with each other.
- `tests/stubtest_test.py` checks stubs against the objects at runtime.
- `flake8` enforces a style guide.

### Setup

Run:
```
$ python3 -m venv .venv3
$ source .venv3/bin/activate
(.venv3)$ pip install -U pip
(.venv3)$ pip install -r requirements-tests-py3.txt
```
This will install mypy (you need the latest master branch from GitHub),
typed-ast, flake8 (and plugins), pytype, black and isort.

If you want to run the pyright tests, you need to have
[Node.js](https://nodejs.org/) installed.

### mypy\_test.py

This test requires Python 3.6 or higher; Python 3.6.1 or higher is recommended.
Run using:
```
(.venv3)$ python3 tests/mypy_test.py
```

This test is shallow — it verifies that all stubs can be
imported but doesn't check whether stubs match their implementation
(in the Python standard library or a third-party package). It has an exclude list of
modules that are not tested at all, which also lives in the tests directory.

You can restrict mypy tests to a single version by passing `-p2` or `-p3.9`:
```bash
(.venv3)$ python3 tests/mypy_test.py -p3.9
```

### pytype\_test.py

This test requires Python 2.7 and Python 3.6. Pytype will
find these automatically if they're in `PATH`.
Run using:
```
(.venv3)$ python3 tests/pytype_test.py
```

This test works similarly to `mypy_test.py`, except it uses `pytype`.

### pyright\_test.py

This test requires Node.js to be installed. It is currently not part of the CI,
but it uses the same pyright version and configuration as the CI.
```
(.venv3)$ python3 tests/pyright_test.py                 # Check all files
(.venv3)$ python3 tests/pyright_test.py stdlib/sys.pyi  # Check one file
```

### mypy\_test\_suite.py

This test requires Python 3.5 or higher; Python 3.6.1 or higher is recommended.
Run using:
```
(.venv3)$ python3 tests/mypy_test_suite.py
```

This test runs mypy's own test suite using the typeshed code in your repo. This
will sometimes catch issues with incorrectly typed stubs, but is much slower
than the other tests.

### check\_consistent.py

Run using:
```
python3 tests/check_consistent.py
```

### stubtest\_test.py

This test requires Python 3.6 or higher.
Run using
```
(.venv3)$ python3 tests/stubtest_test.py
```

This test compares the stdlib stubs against the objects at runtime. Because of
this, the output depends on which version of Python and on what kind of system
it is run.
Thus the easiest way to run this test is via Github Actions on your fork;
if you run it locally, it'll likely complain about system-specific
differences (in e.g, `socket`) that the type system cannot capture.
If you need a specific version of Python to repro a CI failure,
[pyenv](https://github.com/pyenv/pyenv) can help.

Due to its dynamic nature, you may run into false positives. In this case, you
can add to the whitelists for each affected Python version in
`tests/stubtest_whitelists`. Please file issues for stubtest false positives
at [mypy](https://github.com/python/mypy/issues).

To run stubtest against third party stubs, it's easiest to use stubtest
directly, with
```
(.venv3)$ python3 -m mypy.stubtest \
  --custom-typeshed-dir <path-to-typeshed> \
  <third-party-module>
```
stubtest can also help you find things missing from the stubs.


### flake8

flake8 requires Python 3.6 or higher. Run using:
```
(.venv3)$ flake8
```

Note typeshed uses the `flake8-pyi` and `flake8-bugbear` plugins.

## 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](https://github.com/python/mypy/wiki/Using-Git-And-GitHub),
or [GitHub's own documentation](https://help.github.com/articles/using-pull-requests/).

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.

## 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.

Maintainers should follow these rules when processing pull requests:

* Always wait for tests to pass before merging PRs.
* Use "[Squash and merge](https://github.com/blog/2141-squash-your-commits)" to merge PRs.
* Delete branches for merged PRs (by maintainers pushing to the main repo).
* Make sure commit messages to master are meaningful. For example, remove irrelevant
  intermediate commit messages.
* If stubs for a new library are submitted, notify the library's maintainers.

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.