python-peps/pep-0604.rst

PEP: 604
Title: Allow writing union types as ``X | Y``
Author: Philippe PRADOS <python@prados.fr>
Sponsor: Chris Angelico <rosuav@gmail.com>
BDFL-Delegate: Guido van Rossum <guido@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Aug-2019
Python-Version: 3.10


Abstract
========

This PEP proposes overloading the ``|`` operator on types to allow
writing ``Union[X, Y]`` as ``X | Y``, and allows it to appear in
``isinstance`` and ``issubclass`` calls.


Motivation
==========

PEP 484 and PEP 526 propose a generic syntax to add typing to variables,
parameters and function returns. PEP 585 proposes to `expose
parameters to generics at runtime
<https://www.python.org/dev/peps/pep-0585/#parameters-to-generics-are-available-at-runtime>`_.
Mypy [1]_ accepts a syntax which looks like::

    annotation: name_type
    name_type: NAME (args)?
    args: '[' paramslist ']'
    paramslist: annotation (',' annotation)* [',']

- To describe a disjunction (union type), the user must use ``Union[X, Y]``.

The verbosity of this syntax does not help the adoption.


Proposal
========

Inspired by Scala [2]_ and Pike [3]_, this proposal adds operator
``type.__or__()``.  With this new operator, it is possible to write
``int | str`` instead of ``Union[int, str]``. In addition to
annotations, the result of this expression would then be valid in
``isinstance()`` and ``issubclass()``::

    isinstance(5, int | str)
    issubclass(bool, int | float)

We will also be able to write ``t | None`` or ``None | t`` instead of
``Optional[t]``::

    isinstance(None, int | None)
    isinstance(42, None | int)


Examples
========

Here are some examples of what we can do with this feature.

::

    # Instead of
    # def f(list: List[Union[int, str]], param: Optional[int]) -> Union[float, str]
    def f(list: List[int | str], param: int | None) -> float | str:
        pass

    f([1, "abc"], None)

    assert str | int == Union[str,int]
    assert str | int | float == Union[str, int, float]

    assert isinstance("", int | str)
    assert issubclass(bool, int | float)

Once the Python language is extended, mypy [1]_ and other type checkers will
need to be updated to accept this new syntax.


Implementation
==============

A new built-in ``Union`` type must be implemented to hold the return
value of ``t1 | t2``, and it must be supported by ``isinstance()`` and
``issubclass()``. This type can be placed in the ``types`` module.
Interoperability between ``types.Union`` and ``typing.Union`` must be
provided.


Incompatible changes
====================

In some situations, some exceptions will not be raised as expected.

If a metaclass implements the ``__or__`` operator, it will override this::

    >>> class M(type):
    ... 	def __or__(self, other): return "Hello"
    ...
    >>> class C(metaclass=M): pass
    ...
    >>> C | int
    'Hello'
    >>> int | C
    typing.Union[int, __main__.C]
    >>> Union[C, int]
    typing.Union[__main__.C, int]


Objections and responses
========================

For more details about discussions, see links below:

- `Discussion in python-ideas <https://mail.python.org/archives/list/python-ideas@python.org/thread/FCTXGDT2NNKRJQ6CDEPWUXHVG2AAQZZY/>`_
- `Discussion in typing-sig <https://mail.python.org/archives/list/typing-sig@python.org/thread/D5HCB4NT4S3WSK33WI26WZSFEXCEMNHN/>`_

1. Add a new operator for ``Union[type1|type2]``?
-------------------------------------------------

PROS:

- This syntax can be more readable, and is similary to others languages (Scala, ...)
- At runtime, ``int|str`` might return a simple object in 3.9, rather than everything that
  you'd need to grab from importing ``typing``


CONS:

- Adding this operator introduce a dependency between ``typing`` and ``builtins``
- As breaking the backport (in that ``typing`` can easily be backported but core ``types`` can't)
- If Python itself doesn't have to be changed, we'd still need to implement it in mypy, Pyre, PyCharm,
  Pytype, and who knows what else (it's a minor change see "Reference Implementation"


2. Change only PEP 484 (Type hints) to accept the syntax ``type1 | type2`` ?
----------------------------------------------------------------------------

PEP 563 (Postponed Evaluation of Annotations) is enough to accept this proposition,
if we accept to not be compatible with the dynamic evaluation of annotations (``eval()``).

::

    >>> from __future__ import annotations
    >>> def foo() -> int | str: pass
    ...
    >>> eval(foo.__annotations__['return'])
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 1, in <module>
    TypeError: unsupported operand type(s) for |: 'type' and 'type'

3. Extend ``isinstance()`` and ``issubclass()`` to accept ``Union`` ?
---------------------------------------------------------------------

::

    isinstance(x, str | int) ==> "is x an instance of str or int"

PROS:

- If they were permitted, then instance checking could use an extremely clean-looking notation
- The implementation can use the tuple present in ``Union`` parameter, without create a new instance

CONS:

- Must migrate all the ``typing`` module in ``builtin``


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

- A proposed implementation for `cpython is here
  <https://github.com/python/cpython/pull/21515>`_.
- A proposed implementation for `mypy is here
  <https://github.com/pprados/mypy/tree/PEP604>`_.


References
==========

.. [1] mypy
   http://mypy-lang.org/
.. [2] Scala Union Types
   https://dotty.epfl.ch/docs/reference/new-types/union-types.html
.. [3] Pike
   http://pike.lysator.liu.se/docs/man/chapter_3.html#3.5


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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