PEP 728: Improve specification (#4166)
- Specify that extra_items and closed are also supported with the functional syntax. - Rewrite the rules for `closed=True` and inheritance. I attempted to make `closed=True` exactly equivalent to `extra_items=Never` in terms of inheritance. The semantics as specified in the previous version of the PEP felt harder to understand and less consistent. - Fix some incorrect comments regarding expected type checker errors. - Clarify section on assignability with Mapping - Add section on runtime behavior. I tried to make the intended runtime behavior simple to implement and understand. This makes the runtime simpler but may make life more complicated for tools consuming the metadata. - Fix typos and trailing whitespace
This commit is contained in:
Jelle Zijlstra
GitHub
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d98e3fa418
commit
484c43d0a7
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@ -48,7 +48,7 @@ Another possible use case for this is a sound way to
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class Movie(TypedDict):
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name: str
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director: str
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class Book(TypedDict):
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name: str
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author: str
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@ -194,12 +194,12 @@ to the ``extra_items`` argument. For example::
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class Movie(TypedDict, extra_items=bool):
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name: str
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a: Movie = {"name": "Blade Runner", "novel_adaptation": True} # OK
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b: Movie = {
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"name": "Blade Runner",
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"year": 1982, # Not OK. 'int' is not assignable to 'bool'
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}
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}
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Here, ``extra_items=bool`` specifies that items other than ``'name'``
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have a value type of ``bool`` and are non-required.
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@ -214,12 +214,12 @@ the ``extra_items`` argument::
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def f(movie: Movie) -> None:
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reveal_type(movie["name"]) # Revealed type is 'str'
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reveal_type(movie["novel_adaptation"]) # Revealed type is 'bool'
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``extra_items`` is inherited through subclassing::
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class MovieBase(TypedDict, extra_items=int | None):
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name: str
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class Movie(MovieBase):
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year: int
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@ -234,38 +234,46 @@ Here, ``'year'`` in ``a`` is an extra key defined on ``Movie`` whose value type
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is ``int``. ``'other_extra_key'`` in ``b`` is another extra key whose value type
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must be assignable to the value of ``extra_items`` defined on ``MovieBase``.
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``extra_items`` is also supported with the functional syntax::
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Movie = TypedDict("Movie", {"name": str}, extra_items=int | None)
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The ``closed`` Class Parameter
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------------------------------
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When ``closed=True`` is set, no extra items are allowed. This is a shorthand for
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When ``closed=True`` is set, no extra items are allowed. This is equivalent to
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``extra_items=Never``, because there can't be a value type that is assignable to
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:class:`~typing.Never`.
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:class:`~typing.Never`. It is a runtime error to use the ``closed`` and
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``extra_items`` parameters in the same TypedDict definition.
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Similar to ``total``, only a literal ``True`` or ``False`` is supported as the
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value of the ``closed`` argument; ``closed`` is ``False`` by default, which
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preserves the previous TypedDict behavior.
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value of the ``closed`` argument. Type checkers should reject any non-literal value.
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The value of ``closed`` is not inherited through subclassing, but the
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implicitly set ``extra_items=Never`` is. It should be an error to use the
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default ``closed=False`` when subclassing a closed TypedDict type::
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Passing ``closed=False`` explicitly requests the default TypedDict behavior,
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where arbitrary other keys may be present and subclasses may add arbitrary items.
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It is a type checker error to pass ``closed=False`` if a superclass has
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``closed=True`` or sets ``extra_items``.
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If ``closed`` is not provided, the behavior is inherited from the superclass.
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If the superclass is TypedDict itself or the superclass does not have ``closed=True``
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or the ``extra_items`` parameter, the previous TypedDict behavior is preserved:
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arbitrary extra items are allowed. If the superclass has ``closed=True``, the
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child class is also closed.
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class BaseMovie(TypedDict, closed=True):
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name: str
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class MovieA(BaseMovie): # Not OK. An explicit 'closed=True' is required
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class MovieA(BaseMovie): # OK, still closed
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pass
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class MovieB(BaseMovie, closed=True): # OK
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class MovieB(BaseMovie, closed=True): # OK, but redundant
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pass
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Setting both ``closed`` and ``extra_items`` when defining a TypedDict type
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should always be a runtime error::
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class MovieC(BaseMovie, closed=False): # Type checker error
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pass
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class Person(TypedDict, closed=False, extra_items=bool): # Not OK. 'closed' and 'extra_items' are incompatible
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name: str
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As a consequence of ``closed=True`` being equivalent to ``extra_items=Never``.
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The same rules that apply to ``extra_items=Never`` should also apply to
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As a consequence of ``closed=True`` being equivalent to ``extra_items=Never``,
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the same rules that apply to ``extra_items=Never`` also apply to
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``closed=True``. It is possible to use ``closed=True`` when subclassing if the
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``extra_items`` argument is a read-only type::
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@ -275,7 +283,7 @@ The same rules that apply to ``extra_items=Never`` should also apply to
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class MovieClosed(Movie, closed=True): # OK
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pass
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class MovieNever(Movie, extra_items=Never): # Not OK. 'closed=True' is preferred
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class MovieNever(Movie, extra_items=Never): # OK, but 'closed=True' is preferred
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pass
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This will be further discussed in
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@ -286,6 +294,10 @@ is assumed to allow non-required extra items of value type ``ReadOnly[object]``
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during inheritance or assignability checks. This preserves the existing behavior
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of TypedDict.
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``closed`` is also supported with the functional syntax::
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Movie = TypedDict("Movie", {"name": str}, closed=True)
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Interaction with Totality
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-------------------------
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@ -315,7 +327,7 @@ function parameters still apply::
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class Movie(TypedDict, extra_items=int):
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name: str
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def f(**kwargs: Unpack[Movie]) -> None: ...
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# Should be equivalent to:
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@ -356,7 +368,7 @@ unless it is declared to be ``ReadOnly`` in the superclass::
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class Parent(TypedDict, extra_items=int | None):
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pass
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class Child(Parent, extra_items=int): # Not OK. Like any other TypedDict item, extra_items's type cannot be changed
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Second, ``extra_items=T`` effectively defines the value type of any unnamed
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@ -378,20 +390,17 @@ added in a subclass, all of the following conditions should apply:
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- The item's value type is :term:`typing:consistent` with ``T``
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- If ``extra_items`` is not overriden, the subclass inherits it as-is.
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- If ``extra_items`` is not overridden, the subclass inherits it as-is.
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For example::
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class MovieBase(TypedDict, extra_items=int | None):
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name: str
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class AdaptedMovie(MovieBase): # Not OK. 'bool' is not assignable to 'int | None'
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adapted_from_novel: bool
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class MovieRequiredYear(MovieBase): # Not OK. Required key 'year' is not known to 'Parent'
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class MovieRequiredYear(MovieBase): # Not OK. Required key 'year' is not known to 'MovieBase'
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year: int | None
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class MovieNotRequiredYear(MovieBase): # Not OK. 'int | None' is not assignable to 'int'
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class MovieNotRequiredYear(MovieBase): # Not OK. 'int | None' is not consistent with 'int'
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year: NotRequired[int]
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class MovieWithYear(MovieBase): # OK
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@ -478,7 +487,7 @@ checks::
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class MovieDetails(TypedDict, extra_items=int | None):
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name: str
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year: NotRequired[int]
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details: MovieDetails = {"name": "Kill Bill Vol. 1", "year": 2003}
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movie: Movie = details # Not OK. While 'int' is assignable to 'int | None',
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# 'int | None' is not assignable to 'int'
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@ -502,7 +511,7 @@ possible for an item to have a :term:`narrower <typing:narrow>` type than the
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class Movie(TypedDict, extra_items=ReadOnly[str | int]):
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name: str
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class MovieDetails(TypedDict, extra_items=int):
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name: str
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year: NotRequired[int]
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@ -522,19 +531,19 @@ enforced::
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class MovieExtraStr(TypedDict, extra_items=str):
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name: str
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extra_int: MovieExtraInt = {"name": "No Country for Old Men", "year": 2007}
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extra_str: MovieExtraStr = {"name": "No Country for Old Men", "description": ""}
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extra_int = extra_str # Not OK. 'str' is not assignable to extra items type 'int'
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extra_str = extra_int # Not OK. 'int' is not assignable to extra items type 'str'
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A non-closed TypedDict type implicitly allows non-required extra keys of value
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type ``ReadOnly[object]``. Applying the assignability rules between this type
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and a closed TypedDict type is allowed::
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class MovieNotClosed(TypedDict):
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name: str
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extra_int: MovieExtraInt = {"name": "No Country for Old Men", "year": 2007}
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not_closed: MovieNotClosed = {"name": "No Country for Old Men"}
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extra_int = not_closed # Not OK.
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@ -578,17 +587,13 @@ arguments of this type when constructed by calling the class object::
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Interaction with Mapping[KT, VT]
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--------------------------------
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A TypedDict type can be assignable to ``Mapping[KT, VT]`` types other than
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``Mapping[str, object]`` as long as all value types of the items on the
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TypedDict type is :term:`typing:assignable` to ``VT``. This is an extension of this
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A TypedDict type is :term:`typing:assignable` to a type of the form ``Mapping[str, VT]``
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when all value types of the items in the TypedDict
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are assignable to ``VT``. For the purpose of this rule, a
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TypedDict that does not have ``extra_items=`` or ``closed=`` set is considered
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to have an item with a value of type ``object``. This extends the current
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assignability rule from the `typing spec
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<https://typing.readthedocs.io/en/latest/spec/typeddict.html#assignability>`__:
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* A TypedDict with all ``int`` values is not :term:`typing:assignable` to
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``Mapping[str, int]``, since there may be additional non-``int`` values
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not visible through the type, due to :term:`typing:structural`
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assignability. These can be accessed using the ``values()`` and
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``items()`` methods in ``Mapping``,
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<https://typing.readthedocs.io/en/latest/spec/typeddict.html#assignability>`__.
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For example::
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@ -598,6 +603,10 @@ For example::
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extra_str: MovieExtraStr = {"name": "Blade Runner", "summary": ""}
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str_mapping: Mapping[str, str] = extra_str # OK
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class MovieExtraInt(TypedDict, extra_items=int):
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name: str
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extra_int: MovieExtraInt = {"name": "Blade Runner", "year": 1982}
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int_mapping: Mapping[str, int] = extra_int # Not OK. 'int | str' is not assignable with 'int'
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int_str_mapping: Mapping[str, int | str] = extra_int # OK
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@ -611,7 +620,7 @@ and ``items()`` on such TypedDict types::
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Interaction with dict[KT, VT]
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-----------------------------
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Note that because the presence of ``extra_items`` on a closed TypedDict type
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Because the presence of ``extra_items`` on a closed TypedDict type
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prohibits additional required keys in its :term:`typing:structural`
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:term:`typing:subtypes <subtype>`, we can determine if the TypedDict type and
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its structural subtypes will ever have any required key during static analysis.
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@ -636,8 +645,8 @@ For example::
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def f(x: IntDict) -> None:
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v: dict[str, int] = x # OK
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v.clear() # OK
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not_required_num_dict: IntDictWithNum = {"num": 1, "bar": 2}
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not_required_num_dict: IntDictWithNum = {"num": 1, "bar": 2}
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regular_dict: dict[str, int] = not_required_num_dict # OK
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f(not_required_num_dict) # OK
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@ -652,10 +661,28 @@ because such dict can be a subtype of dict::
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class CustomDict(dict[str, int]):
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pass
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not_a_regular_dict: CustomDict = {"num": 1}
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int_dict: IntDict = not_a_regular_dict # Not OK
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Runtime behavior
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----------------
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At runtime, it is an error to pass both the ``closed`` and ``extra_items``
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arguments in the same TypedDict definition, whether using the class syntax or
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the functional syntax. For simplicity, the runtime does not check other invalid
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combinations involving inheritance.
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For introspection, the ``closed`` and ``extra_items`` arguments are mapped to
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two new attributes on the resulting TypedDict object: ``__closed__`` and
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``__extra_items__``. These attributes reflect exactly what was passed to the
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TypedDict constructor, without considering superclasses.
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If ``closed`` is not passed, the value of ``__closed__`` is None. If ``extra_items``
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is not passed, the value of ``__extra_items__`` is the new sentinel object
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``typing.NoExtraItems``. (It cannot be ``None``, because ``extra_items=None`` is a
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valid definition that indicates all extra items must be ``None``.)
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How to Teach This
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=================
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@ -673,7 +700,7 @@ Because ``extra_items`` is an opt-in feature, no existing codebase will break
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due to this change.
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Note that ``closed`` and ``extra_items`` as keyword arguments do not collide
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with othere keys when using something like
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with other keys when using something like
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``TD = TypedDict("TD", foo=str, bar=int)``, because this syntax has already
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been removed in Python 3.13.
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