python-peps/pep-0645.rst

PEP: 645
Title: Allow writing optional types as ``x?``
Author: Maggie Moss <maggiebmoss@gmail.com>
Sponsor: Guido van Rossum <guido@python.org>
Status: Draft
Type: Process
Content-Type: text/x-rst
Created: 25-Aug-2020


Abstract
========
This PEP proposes adding a ``?`` operator for types to allow writing ``int?`` in place of ``Optional[int]``.


Motivation
==========
Types have become a valuable and powerful part of the Python language. However, many type annotations are verbose and add
considerable friction to using type annotations. By improving the typing syntax, adding types to Python code becomes simpler
and improves the development experience for Python users.

In a similar vein, a PEP to introduce short hand syntax for `Union types <https://www.python.org/dev/peps/pep-0604/>`_ [1]_ has
been approved and implemented.


Rationale
=========

Types in Python can be quite verbose, this can be a hindrance when working towards type adoption. Making types more ergonomic,
as was done with the Union type in PEP 604 (e.g., int | str), would reduce the effort needed to add types to new and existing Python code.
The Optional annotation is used frequently in both partially and fully typed Python code bases. In a small sampling of `5 well-typed open
source projects, on average 7% of annotations
<https://gist.github.com/MaggieMoss/fd8dfe002b2702fae243dbf81a62624e>`_ [2] included at least one optional type. This indicates
that updating the syntax has the potential to make types more concise, reduce code length and improve readability.

Simplifying the syntax for optionals has been `discussed previously <https://github.com/python/typing/issues/429>`_ [3] within the typing community.
The consensus during these conversations has been that ``?`` is the preferred operator. There is no native support for unary ``?`` in Python and this will
need to be added to the runtime.

Adding the ? sigil to the Python grammar has been proposed previously in `PEP 505 <https://www.python.org/dev/peps/pep-0505/>`_ [4], which is currently in a deferred state.
PEP 505 proposes a:

  - "None coalescing" binary operator ``??``

  - "None-aware attribute access" operator ``?.`` ("maybe dot")

  - "None-aware indexing" operator ``?[]`` ("maybe subscript")


Should PEP 505 be approved in the future, it would not interfere with the typing specific ``?`` proposed in this PEP. As well,
since all uses of the ``?`` would be conceptually related, it would not be confusing in terms of learning Python or a hindrance to quick visual comprehension.

The proposed syntax, with the postfix operator, mimics the optional syntax found in other typed languages, like C#, TypeScript and Swift.
The widespread adoption and popularity of these languages means that Python developers are likely already familiar with this syntax.::

    // Optional in Swift
    var example: String?

    // Optional in C#
    string? example;

Adding this syntax would also follow the often used pattern of using builtin types as annotations. For example, `list`, `dict` and `None`. This would allow more annotations to be
added to Python code without importing from `typing`.


Specification
=============

The new optional syntax should be accepted for function, variable, attribute and parameter annotations.

::

    # instead of
    # def foo(x: Optional[int], y: Optional[str], z: Optional[list[int]): ...
    def foo(x: int?, y: str?, x: list[int]?): ...

    # def bar(x: list[typing.Optional[int]]): ...
    def bar(x: list[int?]): ...

The new optional syntax should be equivalent to the existing typing.Optional syntax

::

    typing.Optional[int] == int?

The new optional syntax should have the same identity as the existing typing.Optional syntax.

::

    typing.Optional[int] is int?


It should also be equivalent to a Union with None.

::

    # old syntax
    int? == typing.Union[int, None]

    # new syntax
    int? == int | None

Since the new Union syntax specified in PEP 604 is supported in ``isinstance`` and ``issubclass``, the new optional syntax should be supported in both ``isinstance`` and ``issubclass``,

::

    isinstance(1, int?) # true
    issubclass(Child, Super?) # true

A new dunder method will need to be implemented to allow the `?` operator to be overloaded for other functionality.


Backwards Compatibility
=======================

``?`` is currently unused in Python syntax, therefore this PEP is fully backwards compatible.

Reference Implementation
========================

A reference implementation can be found `here <https://github.com/python/cpython/compare/master...MaggieMoss:new-optional-syntax-postfix>`_ [5].

Rejected Ideas
==============

Discussed alternatives were

* The ``~`` operator was considered in place of ``?``.
* A prefix operator (``?int``).


References
==========

.. [1] PEP 604
    (https://www.python.org/dev/peps/pep-0604/)
.. [2] Use of Optional Annotations in Open Source Python projects
    (https://gist.github.com/MaggieMoss/fd8dfe002b2702fae243dbf81a62624e)
.. [3] Github Issue Discussion of Optional syntax
    (https://github.com/python/typing/issues/429)
.. [4] PEP 505
    (https://www.python.org/dev/peps/pep-0505/)
.. [5] Reference Implementation
    (https://github.com/python/cpython/compare/master...MaggieMoss:new-optional-syntax-postfix)

Copyright
=========

This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.

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