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Python Enhancement Proposals

PEP 728 – TypedDict with Typed Extra Items

Author:
Zixuan James Li <p359101898 at gmail.com>
Sponsor:
Jelle Zijlstra <jelle.zijlstra at gmail.com>
Discussions-To:
Discourse thread
Status:
Draft
Type:
Standards Track
Topic:
Typing
Created:
12-Sep-2023
Python-Version:
3.13
Post-History:
09-Feb-2024

Table of Contents

Abstract

This PEP proposes a way to limit extra items for TypedDict using a closed argument and to type them with the special __extra_items__ key. This addresses the need to define closed TypedDict type or to type a subset of keys that might appear in a dict while permitting additional items of a specified type.

Motivation

A typing.TypedDict type can annotate the value type of each known item in a dictionary. However, due to structural subtyping, 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 structural subtypes.

Defining a Closed TypedDict Type

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

Another possible use case for this is a sound way to enable type narrowing 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 'Movie | Book'

Nothing prevents a dict that is structurally compatible with Movie to have the author key, and under the current specification it would be incorrect for the type checker to 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 expect additional keys of certain value types.

However, the typing spec is more restrictive on type checking the construction of a TypedDict, preventing users from doing this:

class MovieBase(TypedDict):
    name: str

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

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

While the restriction is enforced when constructing a TypedDict, due to structural subtyping, 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}
fun(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 arbitrary structural subtypes of MovieBase:

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

Some workarounds have already been implemented in response to the need to allow extra keys, but none of them is ideal. For mypy, --disable-error-code=typeddict-unknown-key suppresses type checking error 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 compatible 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 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 the new Unpack construct.

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 in TypeScript achieve this:

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 type consistency rules.

We propose that we add an argument closed to TypedDict. Similar to total, only a literal True or False value is allowed. When closed=True is used in the TypedDict type definition, we give the dunder attribute __extra_items__ a special meaning: extra items are allowed, and their types should be compatible with the value type of __extra_items__.

If closed=True is set, but there is no __extra_items__ key, the TypedDict is treated as if it contained an item __extra_items__: Never.

Note that __extra_items__ on the same TypedDict type definition will remain as a regular item if closed=True is not used.

Different from index signatures, the types of the known items do not need to be consistent with the value type of __extra_items__.

There are some advantages to this approach:

  • Inheritance works naturally. __extra_items__ defined on a TypedDict will also be available to its subclasses.
  • We can build on top of the type consistency rules defined in the typing spec. __extra_items__ can be treated as a pseudo-item in terms of type consistency.
  • 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 making __extra_items__ the union of known items.
  • We do not lose backwards compatibility as __extra_items__ still can be used as a regular key.

Specification

This specification is structured to parallel PEP 589 to highlight changes to the original TypedDict specification.

If closed=True is specified, extra items are treated as non-required items having the same type of __extra_items__ whose keys are allowed when determining supported and unsupported operations.

Using TypedDict Types

Assuming that closed=True is used in the TypedDict type definition.

For a TypedDict type that has the special __extra_items__ key, during construction, the value type of each unknown item is expected to be non-required and compatible with the value type of __extra_items__. For example:

class Movie(TypedDict, closed=True):
    name: str
    __extra_items__: bool

a: Movie = {"name": "Blade Runner", "novel_adaptation": True}  # OK
b: Movie = {
    "name": "Blade Runner",
    "year": 1982,  # Not OK. 'int' is incompatible with 'bool'
}

In this example, __extra_items__: bool does not mean that Movie has a required string key "__extra_items__" whose value type is bool. Instead, it specifies that keys 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}, closed=True)

Accessing extra keys is allowed. Type checkers must infer its value type from the value type of __extra_items__:

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

When a TypedDict type defines __extra_items__ without closed=True, closed defaults to False and the key is assumed to be a regular key:

class Movie(TypedDict):
    name: str
    __extra_items__: bool

a: Movie = {"name": "Blade Runner", "novel_adaptation": True}  # Not OK. Unexpected key 'novel_adaptation'
b: Movie = {
    "name": "Blade Runner",
    "__extra_items__": True,  # OK
}

For such non-closed TypedDict types, it is assumed that they allow non-required extra items of value type ReadOnly[object] during inheritance or type consistency checks. However, extra keys found during construction should still be rejected by the type checker.

closed is not inherited through subclassing:

class MovieBase(TypedDict, closed=True):
    name: str
    __extra_items__: ReadOnly[str | None]

class Movie(MovieBase):
    __extra_items__: str  # A regular key

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

Here, "__extra_items__" in a is a regular key defined on Movie where its value type is narrowed from ReadOnly[str | None] to str, "other_extra_key" in b is an extra key whose value type must be consistent with the value type of "__extra_items__" defined on MovieBase.

Interaction with Totality

It is an error to use Required[] or NotRequired[] with the special __extra_items__ item. total=False and total=True have no effect on __extra_items__ itself.

The extra items are non-required, regardless of the totality of the TypedDict. Operations that are available to NotRequired items should also be available to the extra items:

class Movie(TypedDict, closed=True):
    name: str
    __extra_items__: int

def f(movie: Movie) -> None:
    del movie["name"]  # Not OK
    del movie["year"]  # OK

Interaction with Unpack

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

class Movie(TypedDict, closed=True):
    name: str
    __extra_items__: int

def f(**kwargs: Unpack[Movie]) -> None: ...

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

Interaction with PEP 705

When the special __extra_items__ item is annotated with ReadOnly[], the extra items on the TypedDict have the properties of read-only items. This interacts with inheritance rules specified in PEP 705.

Notably, if the TypedDict type declares __extra_items__ to be read-only, a subclass of the TypedDict type may redeclare __extra_items__’s value type or additional non-extra items’ value type.

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

More details are discussed in the later sections.

Inheritance

When the TypedDict type is defined as closed=False (the default), __extra_items__ should behave and be inherited the same way a regular key would. A regular __extra_items__ key can coexist with the special __extra_items__ and both should be inherited when subclassing.

We assume that closed=True whenever __extra_items__ is mentioned for the rest of this section.

__extra_items__ is inherited the same way as a regular key: value_type item. As with the other keys, the same rules from the typing spec and PEP 705 apply. We interpret the existing rules in the context of __extra_items__.

We need to reinterpret the following rule to define how __extra_items__ interacts with it:

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

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

class Parent(TypedDict, closed=True):
    __extra_items__: int | None

class Child(Parent, closed=True):
    __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:

  • If __extra_items__ is read-only
    • The item can be either required or non-required
    • The item’s value type is consistent with T
  • If __extra_items__ is not read-only
    • The item is non-required
    • The item’s value type is consistent with T
    • T is consistent with the item’s value type
  • If __extra_items__ is not redeclared, the subclass inherits it as-is.

For example:

class MovieBase(TypedDict, closed=True):
    name: str
    __extra_items__: int | None

class AdaptedMovie(MovieBase):  # Not OK. 'bool' is not consistent with '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 consistent with 'int'
    year: NotRequired[int]

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

Due to this nature, an important side effect allows us to define a TypedDict type that disallows additional items:

class MovieFinal(TypedDict, closed=True):
    name: str
    __extra_items__: Never

Here, annotating __extra_items__ with typing.Never specifies that there can be no other keys in MovieFinal other than the known ones. Because of its potential common use, this is equivalent to:

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

where we implicitly assume the __extra_items__: Never field by default if only closed=True is specified.

Type Consistency

In addition to the set S of keys of the explicitly defined items, a TypedDict type that has the item __extra_items__: T 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 consistent with 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 consistent with T
    • T is consistent with the key’s value type
    • The key is not in S.

For type checking purposes, let __extra_items__ be a non-required pseudo-item to be included whenever “for each … item/key” is stated in the existing type consistency rules from PEP 705, and we modify it as follows:

A TypedDict type A is consistent with TypedDict B if A is structurally compatible with B. This is true if and only if all of the following are satisfied:
  • For each item in B, A has the corresponding key, unless the item in B is read-only, not required, and of top value type (ReadOnly[NotRequired[object]]). [Edit: Otherwise, if the corresponding key with the same name cannot be found in ``A``, “__extra_items__” is considered the corresponding key.]
  • For each item in B, if A has the corresponding key [Edit: or “__extra_items__”], the corresponding value type in A is consistent with the value type in B.
  • For each non-read-only item in B, its value type is consistent with the corresponding value type in A. [Edit: if the corresponding key with the same name cannot be found in ``A``, “__extra_items__” is considered the corresponding key.]
  • For each required key in B, the corresponding key is required in A. For each non-required key in B, if the item is not read-only in B, the corresponding key is not required in A. [Edit: if the corresponding key with the same name cannot be found in ``A``, “__extra_items__” is considered to be non-required as the corresponding key.]

The following examples illustrate these checks in action.

__extra_items__ puts various restrictions on additional items for type consistency checks:

class Movie(TypedDict, closed=True):
    name: str
    __extra_items__: int | None

class MovieDetails(TypedDict, closed=True):
    name: str
    year: NotRequired[int]
    __extra_items__: int | None

details: MovieDetails = {"name": "Kill Bill Vol. 1", "year": 2003}
movie: Movie = details  # Not OK. While 'int' is consistent with 'int | None',
                        # 'int | None' is not consistent with 'int'

class MovieWithYear(TypedDict, closed=True):
    name: str
    year: int | None
    __extra_items__: 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 the rule “For each required key in B, the corresponding key is required in A”.

When __extra_items__ is defined to be read-only in a TypedDict type, it is possible for an item to have a narrower type than __extra_items__’s value type:

class Movie(TypedDict, closed=True):
    name: str
    __extra_items__: ReadOnly[str | int]

class MovieDetails(TypedDict, closed=True):
    name: str
    year: NotRequired[int]
    __extra_items__: int

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

This behaves the same way as PEP 705 specified if year: ReadOnly[str | int] is an item defined in Movie.

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

class MovieExtraInt(TypedDict, closed=True):
    name: str
    __extra_items__: int

class MovieExtraStr(TypedDict, closed=True):
    name: str
    __extra_items__: 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 inconsistent with 'int' for item '__extra_items__'
extra_str = extra_int  # Not OK. 'int' is inconsistent with 'str' for item '__extra_items__'

A non-closed TypedDict type implicitly allows non-required extra keys of value type ReadOnly[object]. This allows to apply the type consistency rules between this type and a closed TypedDict type:

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. 'ReadOnly[object]' implicitly on 'MovieNotClosed' is not consistent with 'int' for item '__extra_items__'
not_closed = extra_int  # OK

Interaction with Mapping[KT, VT]

A TypedDict type can be consistent with Mapping[KT, VT] types other than Mapping[str, object] as long as the union of value types on the TypedDict type is consistent with VT. It is an extension of this rule from the typing spec:

  • A TypedDict with all int values is not consistent with Mapping[str, int], since there may be additional non-int values not visible through the type, due to structural subtyping. These can be accessed using the values() and items() methods in Mapping

For example:

class MovieExtraStr(TypedDict, closed=True):
    name: str
    __extra_items__: 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 consistent with 'int'
int_str_mapping: Mapping[str, int | str] = extra_int  # OK

Furthermore, 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 structural subtypes, we can determine if the TypedDict type and its structural subtypes will ever have any required key during static analysis.

If there is no required key, the TypedDict type is consistent with dict[KT, VT] and vice versa if all items on the TypedDict type satisfy the following conditions:

  • VT is consistent with the value type of the item
  • The value type of the item is consistent with VT

For example:

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

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

def f(x: IntDict) -> None:
    v: dict[str, int] = x  # OK
    v.clear()  # OK

not_required_num: IntDictWithNum = {"num": 1, "bar": 2}
regular_dict: dict[str, int] = not_required_num  # OK
f(not_required_num)  # 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]

How to Teach this

The choice of "__extra_items__" and the requirement of closed=True whenever it is used as a special key intended to make it more understandable to new users even without former knowledge of this feature.

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

Backwards Compatibility

Because __extra_items__ remains as a regular key if closed=True is not specified, no existing codebase will break due to this change.

If the proposal is accepted, none of __required_keys__, __optional_keys__, __readonly_keys__ and __mutable_keys__ should include "__extra_items__" defined on the same TypedDict type when closed=True is specified.

Note that closed as a keyword argument does not collide with the keyword arguments alternative to define keys with the functional syntax that allows things like TD = TypedDict("TD", foo=str, bar=int), because it is scheduled to be 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.

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 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 structural subtyping. closed=True plays a similar role in the current proposal.

Supporting TypedDict(extra=type)

While this design is potentially viable, there are several partially addressable concerns to consider. Adding everything up, it is slightly less favorable than the current proposal.

  • Usability of forward reference As in the functional syntax, using a quoted type or a type alias will be required when SomeType is a forward reference. This is already a requirement for the functional syntax, so implementations can potentially reuse that piece of logic, but this is still extra work that the closed=True proposal doesn’t have.
  • Concerns about using type as a value Whatever is not allowed as the value type in the functional syntax should not be allowed as the argument for extra either. While type checkers might be able to reuse this check, it still needs to be somehow special-cased for the class-based syntax.
  • How to teach Notably, the extra=type often gets brought up due to it being an intuitive solution for the use case, so it is potentially simpler to learn than the less obvious solution. However, the more common used case only requires closed=True, and the other drawbacks mentioned earlier outweigh what is need to teach the usage of the special key.

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 consistent with __extra_items__’s type, and __extra_items__’s type is not necessarily consistent with the value types of all known items.

This differs from TypeScript’s Index Signatures syntax, which requires all properties’ types to match the string index’s type. For example:

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, 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

pyanalyze has experimental support for a similar feature.

Reference implementation for this specific proposal, however, is not currently available.

Acknowledgments

Thanks to Jelle Zijlstra for sponsoring this PEP and providing review feedback, Eric Traut who proposed the original design this PEP iterates on, and Alice Purcell for offering their perspective as the author of PEP 705.


Source: https://github.com/python/peps/blob/main/peps/pep-0728.rst

Last modified: 2024-02-17 22:31:57 GMT