python-peps/pep-0655.rst

PEP: 655
Title: Marking individual TypedDict items as required or potentially-missing
Author: David Foster <david at dafoster.net>
Sponsor: Guido van Rossum <guido at python.org>
Discussions-To: typing-sig at python.org
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Requires: 604
Created: 30-Jan-2021
Python-Version: 3.10
Post-History: 31-Jan-2021, 11-Feb-2021, 20-Feb-2021, 26-Feb-2021


Abstract
========

`PEP 589 <https://www.python.org/dev/peps/pep-0589/>`__ defines syntax
for declaring a TypedDict with all required keys and syntax for defining
a TypedDict with `all potentially-missing
keys <https://www.python.org/dev/peps/pep-0589/#totality>`__ however it
does not provide any syntax to declare some keys as required and others
as potentially-missing. This PEP introduces two new syntaxes:
``Required[...]`` which can be used on individual items of a
TypedDict to mark them as required, and
``NotRequired[...]`` which can be used on individual items
to mark them as potentially-missing.


Motivation
==========

It is not uncommon to want to define a TypedDict with some keys that are
required and others that are potentially-missing. Currently the only way
to define such a TypedDict is to declare one TypedDict with one value
for ``total`` and then inherit it from another TypedDict with a
different value for ``total``:

::

   class _MovieBase(TypedDict):  # implicitly total=True
       title: str

   class Movie(_MovieBase, total=False):
       year: int

Having to declare two different TypedDict types for this purpose is
cumbersome.


Rationale
=========

One might think it unusual to propose syntax that prioritizes marking
*required* keys rather than syntax for *potentially-missing* keys, as is
customary in other languages like TypeScript:

::

   interface Movie {
       title: string;
       year?: number;  // ? marks potentially-missing keys
   }

The difficulty is that the best word for marking a potentially-missing
key, ``Optional[...]``, is already used in Python for a completely
different purpose: marking values that could be either of a particular
type or ``None``. In particular the following does not work:

::

   class Movie(TypedDict):
       ...
       year: Optional[int]  # means int|None, not potentially-missing!

Attempting to use any synonym of “optional” to mark potentially-missing
keys (like ``Missing[...]``) would be too similar to ``Optional[...]``
and be easy to confuse with it.

Thus it was decided to focus on positive-form phrasing for required keys
instead, which is straightforward to spell as ``Required[...]``.

Nevertheless it is common for folks wanting to extend a regular
(``total=True``) TypedDict to only want to add a small number of
potentially-missing keys, which necessitates a way to mark keys that are
*not* required and potentially-missing, and so we also allow the
``NotRequired[...]`` form for that case.


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

The ``typing.Required`` type qualifier is used to indicate that a
variable declared in a TypedDict definition is a required key:

::

   class Movie(TypedDict, total=False):
       title: Required[str]
       year: int

Additionally the ``typing.NotRequired`` type qualifier is used to
indicate that a variable declared in a TypedDict definition is a
potentially-missing key:

::

   class Movie(TypedDict):  # implicitly total=True
       title: str
       year: NotRequired[int]

It is an error to use ``Required[...]`` or ``NotRequired[...]`` in any
location that is not an item of a TypedDict.

It is valid to use ``Required[...]`` and ``NotRequired[...]`` even for
items where it is redundant, to enable additional explicitness if desired:

::

   class Movie(TypedDict):
       title: Required[str]  # redundant
       year: NotRequired[int]


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

No backward incompatible changes are made by this PEP.


How to Teach This
=================

To define a TypedDict where most keys are required and some are
potentially-missing, define a single TypedDict as normal
and mark those few keys that are potentially-missing with ``NotRequired[...]``.

To define a TypedDict where most keys are potentially-missing and a few are
required, define a ``total=False`` TypedDict
and mark those few keys that are required with ``Required[...]``.

If some items accept ``None`` in addition to a regular value, it is
recommended that the ``TYPE|None`` syntax be preferred over
``Optional[TYPE]`` for marking such item values, to avoid using
``Required[...]`` or ``NotRequired[...]`` alongside ``Optional[...]``
within the same TypedDict definition:

Yes:

::

   from __future__ import annotations  # for Python 3.7-3.9

   class Dog(TypedDict):
       name: str
       owner: NotRequired[str|None]

Avoid (unless Python 3.5-3.6):

::

   class Dog(TypedDict):
       name: str
       # ick; avoid using both Optional and NotRequired
       owner: NotRequired[Optional[str]]


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

The goal is to be able to make the following statement:

    The `mypy <http://www.mypy-lang.org/>`__ type checker supports
    ``Required`` and ``NotRequired``. A reference implementation of the
    runtime component is provided in the
    `typing_extensions <https://github.com/python/typing/tree/master/typing_extensions>`__
    module.

The mypy implementation is currently still being worked on.


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

Special syntax around the *key* of a TypedDict item
---------------------------------------------------

::

   class MyThing(TypedDict):
       opt1?: str  # may not exist, but if exists, value is string
       opt2: Optional[str]  # always exists, but may have null value

or:

::

   class MyThing(TypedDict):
       Optional[opt1]: str  # may not exist, but if exists, value is string
       opt2: Optional[str]  # always exists, but may have null value

These syntaxes would require Python grammar changes and it is not
believed that marking TypedDict items as required or potentially-missing
would meet the high bar required to make such grammar changes.

Also, “let’s just not put funny syntax before the colon.” [1]_


Marking required or potentially-missing keys with an operator
-------------------------------------------------------------

We could use unary ``+`` as shorthand to mark a required key, unary
``-`` to mark a potentially-missing key, or unary ``~`` to mark a key
with opposite-of-normal totality:

::

   class MyThing(TypedDict, total=False):
       req1: +int    # + means a required key, or Required[...]
       opt1: str
       req2: +float

   class MyThing(TypedDict):
       req1: int
       opt1: -str    # - means a potentially-missing key, or NotRequired[...]
       req2: float

   class MyThing(TypedDict):
       req1: int
       opt1: ~str    # ~ means a opposite-of-normal-totality key
       req2: float

Such operators could be implemented on ``type`` via the ``__pos__``,
``__neg__`` and ``__invert__`` special methods without modifying the
grammar.

It was decided that it would be prudent to introduce longform syntax
(i.e. ``Required[...]`` and ``NotRequired[...]``) before introducing
any shortform syntax. Future PEPs may reconsider introducing this
or other shortform syntax options.


Marking absence of a value with a special constant
--------------------------------------------------

We could introduce a new type-level constant which signals the absence
of a value when used as a union member, similar to JavaScript’s
``undefined`` type, perhaps called ``Missing``:

::

   class MyThing(TypedDict):
       req1: int
       opt1: str|Missing
       req2: float

Such a ``Missing`` constant could also be used for other scenarios such
as the type of a variable which is only conditionally defined:

::

   class MyClass:
       attr: int|Missing
       
       def __init__(self, set_attr: bool) -> None:
           if set_attr:
               self.attr = 10

::

   def foo(set_attr: bool) -> None:
       if set_attr:
           attr = 10
       reveal_type(attr)  # int|Missing

Misalignment with how unions apply to values
''''''''''''''''''''''''''''''''''''''''''''

However this use of ``...|Missing``, equivalent to
``Union[..., Missing]``, doesn’t align well with what a union normally
means: ``Union[...]`` always describes the type of a *value* that is
present. By contrast missingness or non-totality is a property of a
*variable* instead. Current precedent for marking properties of a
variable include ``Final[...]`` and ``ClassVar[...]``, which the
proposal for ``Required[...]`` is aligned with.

Misalignment with how unions are subdivided
'''''''''''''''''''''''''''''''''''''''''''

Furthermore the use of ``Union[..., Missing]`` doesn’t align with the
usual ways that union values are broken down: Normally you can eliminate
components of a union type using ``isinstance`` checks:

::

   class Packet:
       data: Union[str, bytes]

   def send_data(packet: Packet) -> None:
       if isinstance(packet.data, str):
           reveal_type(packet.data)  # str
           packet_bytes = packet.data.encode('utf-8')
       else:
           reveal_type(packet.data)  # bytes
           packet_bytes = packet.data
       socket.send(packet_bytes)

However if we were to allow ``Union[..., Missing]`` you’d either have to
eliminate the ``Missing`` case with ``hasattr`` for object attributes:

::

   class Packet:
       data: Union[str, Missing]

   def send_data(packet: Packet) -> None:
       if hasattr(packet, 'data'):
           reveal_type(packet.data)  # str
           packet_bytes = packet.data.encode('utf-8')
       else:
           reveal_type(packet.data)  # Missing? error?
           packet_bytes = b''
       socket.send(packet_bytes)

or a check against ``locals()`` for local variables:

::

   def send_data(packet_data: Optional[str]) -> None:
       packet_bytes: Union[str, Missing]
       if packet_data is not None:
           packet_bytes = packet.data.encode('utf-8')
       
       if 'packet_bytes' in locals():
           reveal_type(packet_bytes)  # bytes
           socket.send(packet_bytes)
       else:
           reveal_type(packet_bytes)  # Missing? error?

or a check via other means, such as against ``globals()`` for global
variables:

::

   warning: Union[str, Missing]
   import sys
   if sys.version_info < (3, 6):
       warning = 'Your version of Python is unsupported!'

   if 'warning' in globals():
       reveal_type(warning)  # str
       print(warning)
   else:
       reveal_type(warning)  # Missing? error?

Weird and inconsistent. ``Missing`` is not really a value at all; it’s
an absence of definition and such an absence should be treated
specially.

Difficult to implement
''''''''''''''''''''''

Eric Traut from the Pyright type checker team has stated that
implementing a ``Union[..., Missing]``-style syntax would be
difficult. [2]_

Introduces a second null-like value into Python
'''''''''''''''''''''''''''''''''''''''''''''''

Defining a new ``Missing`` type-level constant would be very close to
introducing a new ``Missing`` value-level constant at runtime, creating
a second null-like runtime value in addition to ``None``. Having two
different null-like constants in Python (``None`` and ``Missing``) would
be confusing. Many newcomers to JavaScript already have difficulty
distinguishing between its analogous constants ``null`` and
``undefined``.


Replace Optional with Nullable. Repurpose Optional to mean “optional item”.
---------------------------------------------------------------------------

``Optional[...]`` is too ubiquitous to deprecate. Although use of it
*may* fade over time in favor of the ``T|None`` syntax specified by `PEP
604 <https://www.python.org/dev/peps/pep-0604/>`__.


Change Optional to mean “optional item” in certain contexts instead of “nullable”
---------------------------------------------------------------------------------

Consider the use of a special flag on a TypedDict definition to alter
the interpretation of ``Optional`` inside the TypedDict to mean
“optional item” rather than its usual meaning of “nullable”:

::

   class MyThing(TypedDict, optional_as_missing=True):
       req1: int
       opt1: Optional[str]

or:

::

   class MyThing(TypedDict, optional_as_nullable=False):
       req1: int
       opt1: Optional[str]

This would add more confusion for users because it would mean that in
*some* contexts the meaning of ``Optional[...]`` is different than in
other contexts, and it would be easy to overlook the flag.


Various synonyms for “potentially-missing item”
-----------------------------------------------

-  Omittable – too easy to confuse with optional
-  OptionalItem, OptionalKey – two words; too easy to confuse with
   optional
-  MayExist, MissingOk – two words
-  Droppable – too similar to Rust’s ``Drop``, which means something
   different
-  Potential – too vague
-  Open – sounds like applies to an entire structure rather then to an
   item
-  Excludable
-  Checked


References
==========

.. [1] https://mail.python.org/archives/list/typing-sig@python.org/message/4I3GPIWDUKV6GUCHDMORGUGRE4F4SXGR/

.. [2] https://mail.python.org/archives/list/typing-sig@python.org/message/S2VJSVG6WCIWPBZ54BOJPG56KXVSLZK6/


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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