The new syntax Required[...] and NotRequired[...] for marking individual
keys of a TypedDict has now been formally proposed as a draft PEP.
Please see the latest text at the bottom of this email, or online at:
https://www.python.org/dev/peps/pep-0655/
Comments welcome. :)
Meanwhile I'll proceed to implement this syntax in mypy.
--
David Foster | Seattle, WA, USA
Contributor to TypedDict support for mypy
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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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