python-peps/peps/pep-0728.rst

PEP: 728
Title: TypedDict with Typed Extra Items
Author: Zixuan James Li <p359101898@gmail.com>
Sponsor: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Discussions-To: https://discuss.python.org/t/pep-728-typeddict-with-typed-extra-items/45443
Status: Draft
Type: Standards Track
Topic: Typing
Content-Type: text/x-rst
Created: 12-Sep-2023
Python-Version: 3.14
Post-History: `09-Feb-2024 <https://discuss.python.org/t/pep-728-typeddict-with-typed-extra-items/45443>`__,


Abstract
========

This PEP adds two class parameters, ``closed`` and ``extra_items``
to type the extra items on a :class:`~typing.TypedDict`. This addresses the
need to define closed TypedDict types or to type a subset of keys that might
appear in a ``dict`` while permitting additional items of a specified type.

Motivation
==========

A :py:class:`typing.TypedDict` type can annotate the value type of each known
item in a dictionary. However, due to :term:`typing:structural`
:term:`assignability <typing:assignable>`, a TypedDict can have extra items
that are not visible through its type. There is currently no way to restrict
the types of items that might be present in the TypedDict type's
:term:`consistent subtypes <typing:consistent subtype>`.

Disallowing Extra Items Explicitly
----------------------------------

The current behavior of TypedDict prevents users from defining a
TypedDict type when it is expected that the type contains no extra items.

Due to the possible presence of extra items, type checkers cannot infer more
precise return types for ``.items()`` and ``.values()`` on a TypedDict. This can
also be resolved by
`defining a closed TypedDict type <https://github.com/python/mypy/issues/7981>`__.

Another possible use case for this is a sound way to
`enable type narrowing <https://github.com/python/mypy/issues/9953>`__ with the
``in`` check::

    class Movie(TypedDict):
        name: str
        director: str
    
    class Book(TypedDict):
        name: str
        author: str

    def fun(entry: Movie | Book) -> None:
        if "author" in entry:
            reveal_type(entry)  # Revealed type is still 'Movie | Book'

Nothing prevents a ``dict`` that is assignable with ``Movie`` to have the
``author`` key, and under the current specification it would be incorrect for
the type checker to :term:`typing:narrow` its type.

Allowing Extra Items of a Certain Type
--------------------------------------

For supporting API interfaces or legacy codebase where only a subset of possible
keys are known, it would be useful to explicitly specify extra items of certain
value types.

However, the typing spec is more restrictive when checking the construction of a
TypedDict, `preventing users <https://github.com/python/mypy/issues/4617>`__
from doing this::

    class MovieBase(TypedDict):
        name: str

    def foo(movie: MovieBase) -> None:
        # movie can have extra items that are not visible through MovieBase
        ...

    movie: MovieBase = {"name": "Blade Runner", "year": 1982}  # Not OK
    foo({"name": "Blade Runner", "year": 1982})  # Not OK

While the restriction is enforced when constructing a TypedDict, due to
:term:`typing:structural` :term:`assignability <typing:assignable>`, the
TypedDict may have extra items that are not visible through its type.
For example::

    class Movie(MovieBase):
        year: int

    movie: Movie = {"name": "Blade Runner", "year": 1982}
    foo(movie)  # OK

It is not possible to acknowledge the existence of the extra items through
``in`` checks and access them without breaking type safety, even though they
might exist from some :term:`consistent subtypes <typing:consistent subtype>` of
``MovieBase``::

    def bar(movie: MovieBase) -> None:
        if "year" in movie:
            reveal_type(movie["year"])  # Error: TypedDict 'MovieBase' has no key 'year'

Some workarounds have already been implemented to allow
extra items, but none of them is ideal. For mypy,
``--disable-error-code=typeddict-unknown-key``
`suppresses type checking error <https://github.com/python/mypy/pull/14225>`__
specifically for unknown keys on TypedDict. This sacrifices type safety over
flexibility, and it does not offer a way to specify that the TypedDict type
expects additional keys whose value types are assignable with a certain type.

Support Additional Keys for ``Unpack``
--------------------------------------

:pep:`692` adds a way to precisely annotate the types of individual keyword
arguments represented by ``**kwargs`` using TypedDict with ``Unpack``. However,
because TypedDict cannot be defined to accept arbitrary extra items, it is not
possible to `allow additional keyword arguments
<https://discuss.python.org/t/pep-692-using-typeddict-for-more-precise-kwargs-typing/17314/87>`__
that are not known at the time the TypedDict is defined.

Given the usage of pre-:pep:`692` type annotation for ``**kwargs`` in existing
codebases, it will be valuable to accept and type extra items on TypedDict so
that the old typing behavior can be supported in combination with ``Unpack``.

Rationale
=========

A type that allows extra items of type ``str`` on a TypedDict can be loosely
described as the intersection between the TypedDict and ``Mapping[str, str]``.

`Index Signatures
<https://www.typescriptlang.org/docs/handbook/2/objects.html#index-signatures>`__
in TypeScript achieve this:

.. code-block:: typescript

    type Foo = {
        a: string
        [key: string]: string
    }

This proposal aims to support a similar feature without introducing general
intersection of types or syntax changes, offering a natural extension to the
existing assignability rules.

We propose to add a class parameter ``extra_items`` to TypedDict.
It accepts a :term:`typing:type expression` as the argument; when it is present,
extra items are allowed, and their value types must be assignable to the
type expression value.

An application of this is to disallow extra items. We propose to add a
``closed`` class parameter, which only accepts a literal ``True`` or ``False``
as the argument. It should be a runtime error when ``closed`` and
``extra_items`` are used at the same time.

Different from index signatures, the types of the known items do not need to be
assignable to the ``extra_items`` argument.

There are some advantages to this approach:

- We can build on top of the `assignability rules defined in the typing spec
  <https://typing.readthedocs.io/en/latest/spec/typeddict.html#assignability>`__,
  where ``extra_items`` can be treated as a pseudo-item.

- There is no need to introduce a grammar change to specify the type of the
  extra items.

- We can precisely type the extra items without requiring the value types of the
  known items to be :term:`typing:assignable` to ``extra_items``.

- We do not lose backwards compatibility as both ``extra_items`` and ``closed``
  are opt-in only features.

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

This specification is structured to parallel :pep:`589` to highlight changes to
the original TypedDict specification.

If ``extra_items`` is specified, extra items are treated as :ref:`non-required
<typing:required-notrequired>`
items matching the ``extra_items`` argument, whose keys are allowed when
determining `supported and unsupported operations
<https://typing.readthedocs.io/en/latest/spec/typeddict.html#supported-and-unsupported-operations>`__.

The ``extra_items`` Class Parameter
-----------------------------------

For a TypedDict type that specifies ``extra_items``, during construction, the
value type of each unknown item is expected to be non-required and assignable
to the ``extra_items`` argument. For example::

    class Movie(TypedDict, extra_items=bool):
        name: str
    
    a: Movie = {"name": "Blade Runner", "novel_adaptation": True}  # OK
    b: Movie = {
        "name": "Blade Runner",
        "year": 1982,  # Not OK. 'int' is not assignable to 'bool'
    }  

Here, ``extra_items=bool`` specifies that items other than ``'name'``
have a value type of ``bool`` and are non-required.

The alternative inline syntax is also supported::

    Movie = TypedDict("Movie", {"name": str}, extra_items=bool)

Accessing extra items is allowed. Type checkers must infer their value type from
the ``extra_items`` argument::

    def f(movie: Movie) -> None:
        reveal_type(movie["name"])              # Revealed type is 'str'
        reveal_type(movie["novel_adaptation"])  # Revealed type is 'bool'
    
``extra_items`` is inherited through subclassing::

    class MovieBase(TypedDict, extra_items=int | None):
        name: str
    
    class Movie(MovieBase):
        year: int

    a: Movie = {"name": "Blade Runner", "year": None}  # Not OK. 'None' is incompatible with 'int'
    b: Movie = {
        "name": "Blade Runner",
        "year": 1982,
        "other_extra_key": None,
    }  # OK

Here, ``'year'`` in ``a`` is an extra key defined on ``Movie`` whose value type
is ``int``. ``'other_extra_key'`` in ``b`` is another extra key whose value type
must be assignable to the value of ``extra_items`` defined on ``MovieBase``.

The ``closed`` Class Parameter
------------------------------

When ``closed=True`` is set, no extra items are allowed. This is a shorthand for
``extra_items=Never``, because there can't be a value type that is assignable to
:class:`~typing.Never`.

Similar to ``total``, only a literal ``True`` or ``False`` is supported as the
value of the ``closed`` argument; ``closed`` is ``False`` by default, which
preserves the previous TypedDict behavior.

The value of ``closed`` is not inherited through subclassing, but the
implicitly set ``extra_items=Never`` is. It should be an error to use the
default ``closed=False`` when subclassing a closed TypedDict type::

    class BaseMovie(TypedDict, closed=True):
        name: str

    class MovieA(BaseMovie):  # Not OK. An explicit 'closed=True' is required
        pass

    class MovieB(BaseMovie, closed=True):  # OK
        pass

Setting both ``closed`` and ``extra_items`` when defining a TypedDict type
should always be a runtime error::

    class Person(TypedDict, closed=False, extra_items=bool):  # Not OK. 'closed' and 'extra_items' are incompatible
        name: str

As a consequence of ``closed=True`` being equivalent to ``extra_items=Never``.
The same rules that apply to ``extra_items=Never`` should also apply to
``closed=True``. It is possible to use ``closed=True`` when subclassing if the
``extra_items`` argument is a read-only type::

    class Movie(TypedDict, extra_items=ReadOnly[str]):
        pass

    class MovieClosed(Movie, closed=True):  # OK
        pass

    class MovieNever(Movie, extra_items=Never):  # Not OK. 'closed=True' is preferred
        pass

This will be further discussed in
:ref:`a later section <pep728-inheritance-read-only>`.

When neither ``extra_items`` nor ``closed=True`` is specified, the TypedDict
is assumed to allow non-required extra items of value type ``ReadOnly[object]``
during inheritance or assignability checks. This preserves the existing behavior
of TypedDict.

Interaction with Totality
-------------------------

It is an error to use ``Required[]`` or ``NotRequired[]`` with ``extra_items``.
``total=False`` and ``total=True`` have no effect on ``extra_items`` itself.

The extra items are non-required, regardless of the `totality
<https://typing.readthedocs.io/en/latest/spec/typeddict.html#totality>`__ of the
TypedDict. `Operations
<https://typing.readthedocs.io/en/latest/spec/typeddict.html#supported-and-unsupported-operations>`__
that are available to ``NotRequired`` items should also be available to the
extra items::

    class Movie(TypedDict, extra_items=int):
        name: str

    def f(movie: Movie) -> None:
        del movie["name"]  # Not OK. The value type of 'name' is 'Required[int]'
        del movie["year"]  # OK. The value type of 'year' is 'NotRequired[int]'

Interaction with ``Unpack``
---------------------------

For type checking purposes, ``Unpack[SomeTypedDict]`` with extra items should be
treated as its equivalent in regular parameters, and the existing rules for
function parameters still apply::

    class Movie(TypedDict, extra_items=int):
        name: str
    
    def f(**kwargs: Unpack[Movie]) -> None: ...

    # Should be equivalent to:
    def f(*, name: str, **kwargs: int) -> None: ...

Interaction with Read-only Items
--------------------------------

When the ``extra_items`` argument is annotated with the ``ReadOnly[]``
:term:`typing:type qualifier`, the extra items on the TypedDict have the
properties of read-only items. This interacts with inheritance rules specified
in :ref:`Read-only Items <typing:readonly>`.

Notably, if the TypedDict type specifies ``extra_items`` to be read-only,
subclasses of the TypedDict type may redeclare ``extra_items``.

Because a non-closed TypedDict type implicitly allows non-required extra items
of value type ``ReadOnly[object]``, its subclass can override the
``extra_items`` argument with more specific types.

More details are discussed in the later sections.

Inheritance
-----------

``extra_items`` is inherited in a similar way as a regular ``key: value_type``
item. As with the other keys, the `inheritance rules
<https://typing.readthedocs.io/en/latest/spec/typeddict.html#inheritance>`__
and :ref:`Read-only Items <typing:readonly>` inheritance rules apply.

We need to reinterpret these rules to define how ``extra_items`` interacts with
them.

    * Changing a field type of a parent TypedDict class in a subclass is not allowed.

First, it is not allowed to change the value of ``extra_items`` in a subclass
unless it is declared to be ``ReadOnly`` in the superclass::

    class Parent(TypedDict, extra_items=int | None):
        pass
    
    class Child(Parent, extra_items=int): # Not OK. Like any other TypedDict item, extra_items's type cannot be changed

Second, ``extra_items=T`` effectively defines the value type of any unnamed
items accepted to the TypedDict and marks them as non-required. Thus, the above
restriction applies to any additional items defined in a subclass. For each item
added in a subclass, all of the following conditions should apply:

.. _pep728-inheritance-read-only:

- If ``extra_items`` is read-only

  - The item can be either required or non-required

  - The item's value type is :term:`typing:assignable` to ``T``

- If ``extra_items`` is not read-only

  - The item is non-required

  - The item's value type is :term:`typing:consistent` with ``T``

- If ``extra_items`` is not overriden, the subclass inherits it as-is.

For example::

    class MovieBase(TypedDict, extra_items=int | None):
        name: str
    
    class AdaptedMovie(MovieBase):  # Not OK. 'bool' is not assignable to 'int | None'
        adapted_from_novel: bool
 
    class MovieRequiredYear(MovieBase):  # Not OK. Required key 'year' is not known to 'Parent'
        year: int | None

    class MovieNotRequiredYear(MovieBase):  # Not OK. 'int | None' is not assignable to 'int'
        year: NotRequired[int]

    class MovieWithYear(MovieBase):  # OK
        year: NotRequired[int | None]

    class BookBase(TypedDict, extra_items=ReadOnly[int | str]):
        title: str

    class Book(BookBase, extra_items=str):  # OK
        year: int  # OK

An important side effect of the inheritance rules is that we can define a
TypedDict type that disallows additional items::

    class MovieClosed(TypedDict, extra_items=Never):
        name: str

Here, passing the value :class:`~typing.Never` to ``extra_items`` specifies that
there can be no other keys in ``MovieFinal`` other than the known ones.
Because of its potential common use, there is a preferred alternative::

    class MovieClosed(TypedDict, closed=True):
        name: str

where we implicitly assume that ``extra_items=Never``.

Assignability
-------------

Let ``S`` be the set of keys of the explicitly defined items on a TypedDict
type. If it specifies ``extra_items=T``, the TypedDict type is considered to
have an infinite set of items that all satisfy the following conditions.

- If ``extra_items`` is read-only:

  - The key's value type is :term:`typing:assignable` to ``T``.

  - The key is not in ``S``.

- If ``extra_items`` is not read-only:

  - The key is non-required.

  - The key's value type is :term:`typing:consistent` with ``T``.

  - The key is not in ``S``.

For type checking purposes, let ``extra_items`` be a non-required pseudo-item
when checking for assignability according to rules defined in the
:ref:`Read-only Items <typing:readonly>` section, with a new rule added in bold
text as follows:

    A TypedDict type ``B`` is :term:`typing:assignable` to a TypedDict type
    ``A`` if ``B`` is :term:`structurally <typing:structural>` assignable to
    ``A``. This is true if and only if all of the following are satisfied:

    * **[If no key with the same name can be found in ``B``, the 'extra_items'
      argument is considered the value type of the corresponding key.]**

    * For each item in ``A``, ``B`` has the corresponding key, unless the item in
      ``A`` is read-only, not required, and of top value type
      (``ReadOnly[NotRequired[object]]``).

    * For each item in ``A``, if ``B`` has the corresponding key, the corresponding
      value type in ``B`` is assignable to the value type in ``A``.

    * For each non-read-only item in ``A``, its value type is assignable to the
      corresponding value type in ``B``, and the corresponding key is not read-only
      in ``B``.

    * For each required key in ``A``, the corresponding key is required in ``B``.

    * For each non-required key in ``A``, if the item is not read-only in ``A``,
      the corresponding key is not required in ``B``.

The following examples illustrate these checks in action.

``extra_items`` puts various restrictions on additional items for assignability
checks::

    class Movie(TypedDict, extra_items=int | None):
        name: str

    class MovieDetails(TypedDict, extra_items=int | None):
        name: str
        year: NotRequired[int]
    
    details: MovieDetails = {"name": "Kill Bill Vol. 1", "year": 2003}
    movie: Movie = details  # Not OK. While 'int' is assignable to 'int | None',
                            # 'int | None' is not assignable to 'int'

    class MovieWithYear(TypedDict, extra_items=int | None):
        name: str
        year: int | None

    details: MovieWithYear = {"name": "Kill Bill Vol. 1", "year": 2003}
    movie: Movie = details  # Not OK. 'year' is not required in 'Movie',
                            # so it shouldn't be required in 'MovieWithYear' either

Because ``'year'`` is absent in ``Movie``, ``extra_items`` is considered the
corresponding key. ``'year'`` being required violates this rule:

    * For each required key in ``A``, the corresponding key is required in ``B``.

When ``extra_items`` is specified to be read-only on a TypedDict type, it is
possible for an item to have a :term:`narrower <typing:narrow>` type than the
``extra_items`` argument::

    class Movie(TypedDict, extra_items=ReadOnly[str | int]):
        name: str
    
    class MovieDetails(TypedDict, extra_items=int):
        name: str
        year: NotRequired[int]

    details: MovieDetails = {"name": "Kill Bill Vol. 2", "year": 2004}
    movie: Movie = details  # OK. 'int' is assignable to 'str | int'.

This behaves the same way as if ``year: ReadOnly[str | int]`` is an item
explicitly defined in ``Movie``.

``extra_items`` as a pseudo-item follows the same rules that other items have,
so when both TypedDicts types specify ``extra_items``, this check is naturally
enforced::

    class MovieExtraInt(TypedDict, extra_items=int):
        name: str

    class MovieExtraStr(TypedDict, extra_items=str):
        name: str
    
    extra_int: MovieExtraInt = {"name": "No Country for Old Men", "year": 2007}
    extra_str: MovieExtraStr = {"name": "No Country for Old Men", "description": ""}
    extra_int = extra_str  # Not OK. 'str' is not assignable to extra items type 'int'
    extra_str = extra_int  # Not OK. 'int' is not assignable to extra items type 'str'
    
A non-closed TypedDict type implicitly allows non-required extra keys of value
type ``ReadOnly[object]``. Applying the assignability rules between this type
and a closed TypedDict type is allowed::

    class MovieNotClosed(TypedDict):
        name: str
    
    extra_int: MovieExtraInt = {"name": "No Country for Old Men", "year": 2007}
    not_closed: MovieNotClosed = {"name": "No Country for Old Men"}
    extra_int = not_closed  # Not OK.
                            # 'extra_items=ReadOnly[object]' implicitly on 'MovieNotClosed'
                            # is not assignable to with 'extra_items=int'
    not_closed = extra_int  # OK

Interaction with Constructors
-----------------------------

TypedDicts that allow extra items of type ``T`` also allow arbitrary keyword
arguments of this type when constructed by calling the class object::

    class NonClosedMovie(TypedDict):
        name: str

    NonClosedMovie(name="No Country for Old Men")  # OK
    NonClosedMovie(name="No Country for Old Men", year=2007)  # Not OK. Unrecognized item

    class ExtraMovie(TypedDict, extra_items=int):
        name: str

    ExtraMovie(name="No Country for Old Men")  # OK
    ExtraMovie(name="No Country for Old Men", year=2007)  # OK
    ExtraMovie(
        name="No Country for Old Men",
        language="English",
    )  # Not OK. Wrong type for extra item 'language'

    # This implies 'extra_items=Never',
    # so extra keyword arguments would produce an error
    class ClosedMovie(TypedDict, closed=True):
        name: str

    ClosedMovie(name="No Country for Old Men")  # OK
    ClosedMovie(
        name="No Country for Old Men",
        year=2007,
    )  # Not OK. Extra items not allowed

Interaction with Mapping[KT, VT]
--------------------------------

A TypedDict type can be assignable to ``Mapping[KT, VT]`` types other than
``Mapping[str, object]`` as long as all value types of the items on the
TypedDict type is :term:`typing:assignable` to ``VT``. This is an extension of this
assignability rule from the `typing spec
<https://typing.readthedocs.io/en/latest/spec/typeddict.html#assignability>`__:

    * A TypedDict with all ``int`` values is not :term:`typing:assignable` to
      ``Mapping[str, int]``, since there may be additional non-``int`` values
      not visible through the type, due to :term:`typing:structural`
      assignability. These can be accessed using the ``values()`` and
      ``items()`` methods in ``Mapping``,

For example::

    class MovieExtraStr(TypedDict, extra_items=str):
        name: str

    extra_str: MovieExtraStr = {"name": "Blade Runner", "summary": ""}
    str_mapping: Mapping[str, str] = extra_str  # OK

    int_mapping: Mapping[str, int] = extra_int  # Not OK. 'int | str' is not assignable with 'int'
    int_str_mapping: Mapping[str, int | str] = extra_int  # OK

Type checkers should be able to infer the precise return types of ``values()``
and ``items()`` on such TypedDict types::

    def fun(movie: MovieExtraStr) -> None:
        reveal_type(movie.items())  # Revealed type is 'dict_items[str, str]'
        reveal_type(movie.values())  # Revealed type is 'dict_values[str, str]'

Interaction with dict[KT, VT]
-----------------------------

Note that because the presence of ``extra_items`` on a closed TypedDict type
prohibits additional required keys in its :term:`typing:structural`
:term:`typing:subtypes <subtype>`, we can determine if the TypedDict type and
its structural subtypes will ever have any required key during static analysis.

The TypedDict type is :term:`typing:assignable` to ``dict[str, VT]`` if all
items on the TypedDict type satisfy the following conditions:

- The value type of the item is :term:`typing:consistent` with ``VT``.

- The item is not read-only.

- The item is not required.

For example::

    class IntDict(TypedDict, extra_items=int):
        pass

    class IntDictWithNum(IntDict):
        num: NotRequired[int]

    def f(x: IntDict) -> None:
        v: dict[str, int] = x  # OK
        v.clear()  # OK
    
    not_required_num_dict: IntDictWithNum = {"num": 1, "bar": 2} 
    regular_dict: dict[str, int] = not_required_num_dict  # OK
    f(not_required_num_dict)  # OK

In this case, methods that are previously unavailable on a TypedDict are allowed::

    not_required_num.clear()  # OK

    reveal_type(not_required_num.popitem())  # OK. Revealed type is tuple[str, int]

However, ``dict[str, VT]`` is not necessarily assignable to a TypedDict type,
because such dict can be a subtype of dict::

    class CustomDict(dict[str, int]):
        pass
    
    not_a_regular_dict: CustomDict = {"num": 1}
    int_dict: IntDict = not_a_regular_dict  # Not OK

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

The choice of the spelling ``"extra_items"`` is intended to make this
feature more understandable to new users compared to shorter alternatives like
``"extra"``.

Details of this should be documented in both the typing spec and the
:mod:`typing` documentation.

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

Because ``extra_items`` is an opt-in feature, no existing codebase will break
due to this change.

Note that ``closed`` and ``extra_items`` as keyword arguments do not collide
with othere keys when using something like
``TD = TypedDict("TD", foo=str, bar=int)``, because this syntax has already
been removed in Python 3.13.

Because this is a type-checking feature, it can be made available to older
versions as long as the type checker supports it.

Open Issues
===========

Use a Special ``__extra_items__`` Key with the ``closed`` Class Parameter
-------------------------------------------------------------------------

In an earlier revision of this proposal, we discussed an approach that would
utilize ``__extra_items__``'s value type to specify the type of extra items
accepted, like so::

    class IntDict(TypedDict, closed=True):
        __extra_items__: int

where ``closed=True`` is required for ``__extra_items__`` to be treated
specially, to avoid key collision.

Some members of the community concern about the elegance of the syntax.
Practiaclly, the key collision with a regular key can be mitigated with
workarounds, but since using a reserved key is central to this proposal,
there are limited ways forward to address the concerns.

Support a New Syntax of Specifying Keys
---------------------------------------

By introducing a new syntax that allows specifying string keys, we could
deprecate the functional syntax of defining TypedDict types and address the
key conflict issues if we decide to reserve a special key to type extra items.

For example::

    class Foo(TypedDict):
        name: str  # Regular item
        _: bool    # Type of extra items
        __items__ = {
            "_": int,   # Literal "_" as a key
            "class": str,  # Keyword as a key
            "tricky.name?": float,  # Arbitrary str key
        }

This was proposed `here by Jukka
<https://discuss.python.org/t/pep-728-typeddict-with-typed-extra-items/45443/115>`__.
The ``'_'`` key is chosen for not needing to invent a new name, and its
similarity with the match statement.

This will allow us to deprecate the functional syntax of defining TypedDict
types altogether, but there are some disadvantages. `For example
<https://github.com/python/peps/pull/4066#discussion_r1806986861>`__:

- It's less apparent to a reader that ``_: bool`` makes the TypedDict
  special, relative to adding a class argument like ``extra_items=bool``.

- It's backwards incompatible with existing TypedDicts using the
  ``_: bool`` key. While such users have a way to get around the issue,
  it's still a problem for them if they upgrade Python (or
  typing-extensions).

- The types don't appear in an annotation context, so their evaluation will
  not be deferred.

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

Allowing Extra Items without Specifying the Type
------------------------------------------------

``extra=True`` was originally proposed for defining a TypedDict that accepts
extra items regardless of the type, like how ``total=True`` works::

    class ExtraDict(TypedDict, extra=True):
        pass

Because it did not offer a way to specify the type of the extra items, the type
checkers will need to assume that the type of the extra items is ``Any``, which
compromises type safety. Furthermore, the current behavior of TypedDict already
allows untyped extra items to be present in runtime, due to
:term:`typing:structural` :term:`assignability <typing:assignable>`.
``closed=True`` plays a similar role in the current proposal.

Support Extra Items with Intersection
-------------------------------------

Supporting intersections in Python's type system requires a lot of careful
consideration, and it can take a long time for the community to reach a
consensus on a reasonable design.

Ideally, extra items in TypedDict should not be blocked by work on
intersections, nor does it necessarily need to be supported through
intersections.

Moreover, the intersection between ``Mapping[...]`` and ``TypedDict`` is not
equivalent to a TypedDict type with the proposed ``extra_items`` special
item, as the value type of all known items in ``TypedDict`` needs to satisfy the
is-subtype-of relation with the value type of ``Mapping[...]``.

Requiring Type Compatibility of the Known Items with ``extra_items``
------------------------------------------------------------------------

``extra_items`` restricts the value type for keys that are *unknown* to the
TypedDict type. So the value type of any *known* item is not necessarily
assignable to ``extra_items``, and ``extra_items`` is
not necessarily assignable to the value types of all known items.

This differs from TypeScript's `Index Signatures
<https://www.typescriptlang.org/docs/handbook/2/objects.html#index-signatures>`__
syntax, which requires all properties' types to match the string index's type.
For example:

.. code-block:: typescript

    interface MovieWithExtraNumber {
        name: string // Property 'name' of type 'string' is not assignable to 'string' index type 'number'.
        [index: string]: number
    }

    interface MovieWithExtraNumberOrString {
        name: string // OK
        [index: string]: number | string
    }

This is a known limitation discussed in `TypeScript's issue tracker
<https://github.com/microsoft/TypeScript/issues/17867>`__,
where it is suggested that there should be a way to exclude the defined keys
from the index signature so that it is possible to define a type like
``MovieWithExtraNumber``.

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

An earlier revision of proposal is supported in `pyright 1.1.352
<https://github.com/microsoft/pyright/releases/tag/1.1.352>`_, and `pyanalyze
0.12.0 <https://github.com/quora/pyanalyze/releases/tag/v0.12.0>`_.

Acknowledgments
===============

Thanks to Jelle Zijlstra for sponsoring this PEP and providing review feedback,
Eric Traut who `proposed the original design
<https://mail.python.org/archives/list/typing-sig@python.org/message/3Z72OQWVTOVS6UYUUCCII2UZN56PV5II/>`__
this PEP iterates on, and Alice Purcell for offering their perspective as the
author of :pep:`705`.

Copyright
=========

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