Thanks, David, for working on this. Some questions:
Q1. Though the intent is to use in TypedDict, is it necessary to preclude its use elsewhere by making its use outside of TypedDict an error?
Q2. Can the PEP explicitly address how Required[...] and NotRequired[...] will manifest in TypedDict state? Namely, its effect on __required_keys__ and __optional_keys__?
Q3. How will get_type_hints(...), get_options(...) and get_args(...) behave with TypedDicts (and their __annotations__ values) where keys are explicitly marked Required[...] or NotRequired[...]?
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:
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
========
for declaring a TypedDict with all required keys and syntax for defining
a TypedDict with `all potentially-missing
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:
``Required`` and ``NotRequired``. A reference implementation of the
runtime component is provided in the
`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
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]
.. [2]
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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