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May 17, 2021
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Add section on supported type-system features #5479

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Add section on supported type-system features
JelleZijlstra May 17, 2021
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emphasis
JelleZijlstra May 17, 2021
3386a5f
say that you need to use `typing_extension`
JelleZijlstra May 17, 2021
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list other supported features
JelleZijlstra May 17, 2021
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bug bug
JelleZijlstra May 17, 2021
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42 changes: 42 additions & 0 deletions CONTRIBUTING.md
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Expand Up @@ -126,6 +126,48 @@ 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.

### 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](https://www.python.org/dev/peps/pep-0570/) (positional-only
arguments): see [#4972](https://github.com/python/typeshed/issues/4972),
use argument names prefixed with `__` instead
- [PEP 585](https://www.python.org/dev/peps/pep-0585/) (builtin
generics): see [#4820](https://github.com/python/typeshed/issues/4820),
mostly supported but bugs remain for a few specific cases
- [PEP 604](https://www.python.org/dev/peps/pep-0604/) (Union
pipe operator): see [#4819](https://github.com/python/typeshed/issues/4819)
- [PEP 612](https://www.python.org/dev/peps/pep-0612/) (ParamSpec):
see [#4827](https://github.com/python/typeshed/issues/4827)
- [PEP 613](https://www.python.org/dev/peps/pep-0613/) (TypeAlias):
see [#4913](https://github.com/python/typeshed/issues/4913)

Supported features include:
- [PEP 544](https://www.python.org/dev/peps/pep-0544/) (Protocol)
- [PEP 586](https://www.python.org/dev/peps/pep-0586/) (Literal)
- [PEP 591](https://www.python.org/dev/peps/pep-0591/) (Final/@final)
- [PEP 589](https://www.python.org/dev/peps/pep-0589/) (TypedDict)
- [PEP 647](https://www.python.org/dev/peps/pep-0647/) (TypeGuard):
see [#5406](https://github.com/python/typeshed/issues/5406)

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,
Expand Down