PEP: 696 Title: Type Defaults for Type Parameters Author: James Hilton-Balfe Sponsor: Jelle Zijlstra Discussions-To: https://discuss.python.org/t/pep-696-type-defaults-for-typevarlikes/22569 Status: Final Type: Standards Track Topic: Typing Created: 14-Jul-2022 Python-Version: 3.13 Post-History: `22-Mar-2022 `__, `08-Jan-2023 `__, Resolution: https://discuss.python.org/t/pep-696-type-defaults-for-typevarlikes/22569/34 .. canonical-typing-spec:: :ref:`typing:type_parameter_defaults` and :external+py3.13:ref:`type-params` Abstract -------- This PEP introduces the concept of type defaults for type parameters, including ``TypeVar``, ``ParamSpec``, and ``TypeVarTuple``, which act as defaults for type parameters for which no type is specified. Default type argument support is available in some popular languages such as C++, TypeScript, and Rust. A survey of type parameter syntax in some common languages has been conducted by the author of :pep:`695` and can be found in its :pep:`Appendix A <695#appendix-a-survey-of-type-parameter-syntax>`. Motivation ---------- :: T = TypeVar("T", default=int) # This means that if no type is specified T = int @dataclass class Box(Generic[T]): value: T | None = None reveal_type(Box()) # type is Box[int] reveal_type(Box(value="Hello World!")) # type is Box[str] One place this `regularly comes up `__ is ``Generator``. I propose changing the *stub definition* to something like:: YieldT = TypeVar("YieldT") SendT = TypeVar("SendT", default=None) ReturnT = TypeVar("ReturnT", default=None) class Generator(Generic[YieldT, SendT, ReturnT]): ... Generator[int] == Generator[int, None] == Generator[int, None, None] This is also useful for a ``Generic`` that is commonly over one type. :: class Bot: ... BotT = TypeVar("BotT", bound=Bot, default=Bot) class Context(Generic[BotT]): bot: BotT class MyBot(Bot): ... reveal_type(Context().bot) # type is Bot # notice this is not Any which is what it would be currently reveal_type(Context[MyBot]().bot) # type is MyBot Not only does this improve typing for those who explicitly use it, it also helps non-typing users who rely on auto-complete to speed up their development. This design pattern is common in projects like: - `discord.py `__ — where the example above was taken from. - `NumPy `__ — the default for types like ``ndarray``'s ``dtype`` would be ``float64``. Currently it's ``Unknown`` or ``Any``. - `TensorFlow `__ — this could be used for Tensor similarly to ``numpy.ndarray`` and would be useful to simplify the definition of ``Layer``. Specification ------------- Default Ordering and Subscription Rules ''''''''''''''''''''''''''''''''''''''' The order for defaults should follow the standard function parameter rules, so a type parameter with no ``default`` cannot follow one with a ``default`` value. Doing so should ideally raise a ``TypeError`` in ``typing._GenericAlias``/``types.GenericAlias``, and a type checker should flag this as an error. :: DefaultStrT = TypeVar("DefaultStrT", default=str) DefaultIntT = TypeVar("DefaultIntT", default=int) DefaultBoolT = TypeVar("DefaultBoolT", default=bool) T = TypeVar("T") T2 = TypeVar("T2") class NonDefaultFollowsDefault(Generic[DefaultStrT, T]): ... # Invalid: non-default TypeVars cannot follow ones with defaults class NoNonDefaults(Generic[DefaultStrT, DefaultIntT]): ... ( NoNoneDefaults == NoNoneDefaults[str] == NoNoneDefaults[str, int] ) # All valid class OneDefault(Generic[T, DefaultBoolT]): ... OneDefault[float] == OneDefault[float, bool] # Valid reveal_type(OneDefault) # type is type[OneDefault[T, DefaultBoolT = bool]] reveal_type(OneDefault[float]()) # type is OneDefault[float, bool] class AllTheDefaults(Generic[T1, T2, DefaultStrT, DefaultIntT, DefaultBoolT]): ... reveal_type(AllTheDefaults) # type is type[AllTheDefaults[T1, T2, DefaultStrT = str, DefaultIntT = int, DefaultBoolT = bool]] reveal_type(AllTheDefaults[int, complex]()) # type is AllTheDefaults[int, complex, str, int, bool] AllTheDefaults[int] # Invalid: expected 2 arguments to AllTheDefaults ( AllTheDefaults[int, complex] == AllTheDefaults[int, complex, str] == AllTheDefaults[int, complex, str, int] == AllTheDefaults[int, complex, str, int, bool] ) # All valid With the new Python 3.12 syntax for generics (introduced by :pep:`695`), this can be enforced at compile time:: type Alias[DefaultT = int, T] = tuple[DefaultT, T] # SyntaxError: non-default TypeVars cannot follow ones with defaults def generic_func[DefaultT = int, T](x: DefaultT, y: T) -> None: ... # SyntaxError: non-default TypeVars cannot follow ones with defaults class GenericClass[DefaultT = int, T]: ... # SyntaxError: non-default TypeVars cannot follow ones with defaults ``ParamSpec`` Defaults '''''''''''''''''''''' ``ParamSpec`` defaults are defined using the same syntax as ``TypeVar`` \ s but use a ``list`` of types or an ellipsis literal "``...``" or another in-scope ``ParamSpec`` (see `Scoping Rules`_). :: DefaultP = ParamSpec("DefaultP", default=[str, int]) class Foo(Generic[DefaultP]): ... reveal_type(Foo) # type is type[Foo[DefaultP = [str, int]]] reveal_type(Foo()) # type is Foo[[str, int]] reveal_type(Foo[[bool, bool]]()) # type is Foo[[bool, bool]] ``TypeVarTuple`` Defaults ''''''''''''''''''''''''' ``TypeVarTuple`` defaults are defined using the same syntax as ``TypeVar`` \ s but use an unpacked tuple of types instead of a single type or another in-scope ``TypeVarTuple`` (see `Scoping Rules`_). :: DefaultTs = TypeVarTuple("DefaultTs", default=Unpack[tuple[str, int]]) class Foo(Generic[*DefaultTs]): ... reveal_type(Foo) # type is type[Foo[DefaultTs = *tuple[str, int]]] reveal_type(Foo()) # type is Foo[str, int] reveal_type(Foo[int, bool]()) # type is Foo[int, bool] Using Another Type Parameter as ``default`` '''''''''''''''''''''''''''''''''''''''''''' This allows for a value to be used again when the type parameter to a generic is missing but another type parameter is specified. To use another type parameter as a default the ``default`` and the type parameter must be the same type (a ``TypeVar``'s default must be a ``TypeVar``, etc.). `This could be used on builtins.slice `__ where the ``start`` parameter should default to ``int``, ``stop`` default to the type of ``start`` and step default to ``int | None``. :: StartT = TypeVar("StartT", default=int) StopT = TypeVar("StopT", default=StartT) StepT = TypeVar("StepT", default=int | None) class slice(Generic[StartT, StopT, StepT]): ... reveal_type(slice) # type is type[slice[StartT = int, StopT = StartT, StepT = int | None]] reveal_type(slice()) # type is slice[int, int, int | None] reveal_type(slice[str]()) # type is slice[str, str, int | None] reveal_type(slice[str, bool, timedelta]()) # type is slice[str, bool, timedelta] T2 = TypeVar("T2", default=DefaultStrT) class Foo(Generic[DefaultStrT, T2]): def __init__(self, a: DefaultStrT, b: T2) -> None: ... reveal_type(Foo(1, "")) # type is Foo[int, str] Foo[int](1, "") # Invalid: Foo[int, str] cannot be assigned to self: Foo[int, int] in Foo.__init__ Foo[int]("", 1) # Invalid: Foo[str, int] cannot be assigned to self: Foo[int, int] in Foo.__init__ When using a type parameter as the default to another type parameter, the following rules apply, where ``T1`` is the default for ``T2``. Scoping Rules ~~~~~~~~~~~~~ ``T1`` must be used before ``T2`` in the parameter list of the generic. :: T2 = TypeVar("T2", default=T1) class Foo(Generic[T1, T2]): ... # Valid class Foo(Generic[T1]): class Bar(Generic[T2]): ... # Valid StartT = TypeVar("StartT", default="StopT") # Swapped defaults around from previous example StopT = TypeVar("StopT", default=int) class slice(Generic[StartT, StopT, StepT]): ... # ^^^^^^ Invalid: ordering does not allow StopT to be bound Using a type parameter from an outer scope as a default is not supported. Bound Rules ~~~~~~~~~~~ ``T1``'s bound must be a subtype of ``T2``'s bound. :: T1 = TypeVar("T1", bound=int) TypeVar("Ok", default=T1, bound=float) # Valid TypeVar("AlsoOk", default=T1, bound=int) # Valid TypeVar("Invalid", default=T1, bound=str) # Invalid: int is not a subtype of str Constraint Rules ~~~~~~~~~~~~~~~~ The constraints of ``T2`` must be a superset of the constraints of ``T1``. :: T1 = TypeVar("T1", bound=int) TypeVar("Invalid", float, str, default=T1) # Invalid: upper bound int is incompatible with constraints float or str T1 = TypeVar("T1", int, str) TypeVar("AlsoOk", int, str, bool, default=T1) # Valid TypeVar("AlsoInvalid", bool, complex, default=T1) # Invalid: {bool, complex} is not a superset of {int, str} Type Parameters as Parameters to Generics ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Type parameters are valid as parameters to generics inside of a ``default`` when the first parameter is in scope as determined by the `previous section `_. :: T = TypeVar("T") ListDefaultT = TypeVar("ListDefaultT", default=list[T]) class Bar(Generic[T, ListDefaultT]): def __init__(self, x: T, y: ListDefaultT): ... reveal_type(Bar) # type is type[Bar[T, ListDefaultT = list[T]]] reveal_type(Bar[int]) # type is type[Bar[int, list[int]]] reveal_type(Bar[int]()) # type is Bar[int, list[int]] reveal_type(Bar[int, list[str]]()) # type is Bar[int, list[str]] reveal_type(Bar[int, str]()) # type is Bar[int, str] Specialisation Rules ~~~~~~~~~~~~~~~~~~~~ Type parameters currently cannot be further subscripted. This might change if `Higher Kinded TypeVars `__ are implemented. ``Generic`` ``TypeAlias``\ es ''''''''''''''''''''''''''''' ``Generic`` ``TypeAlias``\ es should be able to be further subscripted following normal subscription rules. If a type parameter has a default that hasn't been overridden it should be treated like it was substituted into the ``TypeAlias``. However, it can be specialised further down the line. :: class SomethingWithNoDefaults(Generic[T, T2]): ... MyAlias: TypeAlias = SomethingWithNoDefaults[int, DefaultStrT] # Valid reveal_type(MyAlias) # type is type[SomethingWithNoDefaults[int, DefaultStrT]] reveal_type(MyAlias[bool]()) # type is SomethingWithNoDefaults[int, bool] MyAlias[bool, int] # Invalid: too many arguments passed to MyAlias Subclassing ''''''''''' Subclasses of ``Generic``\ s with type parameters that have defaults behave similarly to ``Generic`` ``TypeAlias``\ es. That is, subclasses can be further subscripted following normal subscription rules, non-overridden defaults should be substituted in, and type parameters with such defaults can be further specialised down the line. :: class SubclassMe(Generic[T, DefaultStrT]): x: DefaultStrT class Bar(SubclassMe[int, DefaultStrT]): ... reveal_type(Bar) # type is type[Bar[DefaultStrT = str]] reveal_type(Bar()) # type is Bar[str] reveal_type(Bar[bool]()) # type is Bar[bool] class Foo(SubclassMe[float]): ... reveal_type(Foo().x) # type is str Foo[str] # Invalid: Foo cannot be further subscripted class Baz(Generic[DefaultIntT, DefaultStrT]): ... class Spam(Baz): ... reveal_type(Spam()) # type is Using ``bound`` and ``default`` ''''''''''''''''''''''''''''''' If both ``bound`` and ``default`` are passed ``default`` must be a subtype of ``bound``. Otherwise the type checker should generate an error. :: TypeVar("Ok", bound=float, default=int) # Valid TypeVar("Invalid", bound=str, default=int) # Invalid: the bound and default are incompatible Constraints ''''''''''' For constrained ``TypeVar``\ s, the default needs to be one of the constraints. A type checker should generate an error even if it is a subtype of one of the constraints. :: TypeVar("Ok", float, str, default=float) # Valid TypeVar("Invalid", float, str, default=int) # Invalid: expected one of float or str got int .. _696-function-defaults: Function Defaults ''''''''''''''''' In generic functions, type checkers may use a type parameter's default when the type parameter cannot be solved to anything. We leave the semantics of this usage unspecified, as ensuring the ``default`` is returned in every code path where the type parameter can go unsolved may be too hard to implement. Type checkers are free to either disallow this case or experiment with implementing support. :: T = TypeVar('T', default=int) def func(x: int | set[T]) -> T: ... reveal_type(func(0)) # a type checker may reveal T's default of int here Defaults following ``TypeVarTuple`` ''''''''''''''''''''''''''''''''''' A ``TypeVar`` that immediately follows a ``TypeVarTuple`` is not allowed to have a default, because it would be ambiguous whether a type argument should be bound to the ``TypeVarTuple`` or the defaulted ``TypeVar``. :: Ts = TypeVarTuple("Ts") T = TypeVar("T", default=bool) class Foo(Generic[Ts, T]): ... # Type checker error # Could be reasonably interpreted as either Ts = (int, str, float), T = bool # or Ts = (int, str), T = float Foo[int, str, float] With the Python 3.12 built-in generic syntax, this case should raise a SyntaxError. However, it is allowed to have a ``ParamSpec`` with a default following a ``TypeVarTuple`` with a default, as there can be no ambiguity between a type argument for the ``ParamSpec`` and one for the ``TypeVarTuple``. :: Ts = TypeVarTuple("Ts") P = ParamSpec("P", default=[float, bool]) class Foo(Generic[Ts, P]): ... # Valid Foo[int, str] # Ts = (int, str), P = [float, bool] Foo[int, str, [bytes]] # Ts = (int, str), P = [bytes] Subtyping ''''''''' Type parameter defaults do not affect the subtyping rules for generic classes. In particular, defaults can be ignored when considering whether a class is compatible with a generic protocol. ``TypeVarTuple``\ s as Defaults ''''''''''''''''''''''''''''''' Using a ``TypeVarTuple`` as a default is not supported because: - `Scoping Rules`_ does not allow usage of type parameters from outer scopes. - Multiple ``TypeVarTuple``\ s cannot appear in the type parameter list for a single object, as specified in :pep:`646#multiple-type-variable-tuples-not-allowed`. These reasons leave no current valid location where a ``TypeVarTuple`` could be used as the default of another ``TypeVarTuple``. Binding rules ------------- Type parameter defaults should be bound by attribute access (including call and subscript). :: class Foo[T = int]: def meth(self) -> Self: return self reveal_type(Foo.meth) # type is (self: Foo[int]) -> Foo[int] Implementation -------------- At runtime, this would involve the following changes to the ``typing`` module. - The classes ``TypeVar``, ``ParamSpec``, and ``TypeVarTuple`` should expose the type passed to ``default``. This would be available as a ``__default__`` attribute, which would be ``None`` if no argument is passed and ``NoneType`` if ``default=None``. The following changes would be required to both ``GenericAlias``\ es: - logic to determine the defaults required for a subscription. - ideally, logic to determine if subscription (like ``Generic[T, DefaultT]``) would be valid. The grammar for type parameter lists would need to be updated to allow defaults; see below. A reference implementation of the runtime changes can be found at https://github.com/Gobot1234/cpython/tree/pep-696 A reference implementation of the type checker can be found at https://github.com/Gobot1234/mypy/tree/TypeVar-defaults Pyright currently supports this functionality. Grammar changes ''''''''''''''' The syntax added in :pep:`695` will be extended to introduce a way to specify defaults for type parameters using the "=" operator inside of the square brackets like so: :: # TypeVars class Foo[T = str]: ... # ParamSpecs class Baz[**P = [int, str]]: ... # TypeVarTuples class Qux[*Ts = *tuple[int, bool]]: ... # TypeAliases type Foo[T, U = str] = Bar[T, U] type Baz[**P = [int, str]] = Spam[**P] type Qux[*Ts = *tuple[str]] = Ham[*Ts] type Rab[U, T = str] = Bar[T, U] :ref:`Similarly to the bound for a type parameter <695-scoping-behavior>`, defaults should be lazily evaluated, with the same scoping rules to avoid the unnecessary usage of quotes around them. This functionality was included in the initial draft of :pep:`695` but was removed due to scope creep. The following changes would be made to the grammar:: type_param: | a=NAME b=[type_param_bound] d=[type_param_default] | a=NAME c=[type_param_constraint] d=[type_param_default] | '*' a=NAME d=[type_param_default] | '**' a=NAME d=[type_param_default] type_param_default: | '=' e=expression | '=' e=starred_expression The compiler would enforce that type parameters without defaults cannot follow type parameters with defaults and that ``TypeVar``\ s with defaults cannot immediately follow ``TypeVarTuple``\ s. Rejected Alternatives --------------------- Allowing the Type Parameters Defaults to Be Passed to ``type.__new__``'s ``**kwargs`` ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' :: T = TypeVar("T") @dataclass class Box(Generic[T], T=int): value: T | None = None While this is much easier to read and follows a similar rationale to the ``TypeVar`` `unary syntax `__, it would not be backwards compatible as ``T`` might already be passed to a metaclass/superclass or support classes that don't subclass ``Generic`` at runtime. Ideally, if :pep:`637` wasn't rejected, the following would be acceptable:: T = TypeVar("T") @dataclass class Box(Generic[T = int]): value: T | None = None Allowing Non-defaults to Follow Defaults '''''''''''''''''''''''''''''''''''''''' :: YieldT = TypeVar("YieldT", default=Any) SendT = TypeVar("SendT", default=Any) ReturnT = TypeVar("ReturnT") class Coroutine(Generic[YieldT, SendT, ReturnT]): ... Coroutine[int] == Coroutine[Any, Any, int] Allowing non-defaults to follow defaults would alleviate the issues with returning types like ``Coroutine`` from functions where the most used type argument is the last (the return). Allowing non-defaults to follow defaults is too confusing and potentially ambiguous, even if only the above two forms were valid. Changing the argument order now would also break a lot of codebases. This is also solvable in most cases using a ``TypeAlias``. :: Coro: TypeAlias = Coroutine[Any, Any, T] Coro[int] == Coroutine[Any, Any, int] Having ``default`` Implicitly Be ``bound`` '''''''''''''''''''''''''''''''''''''''''' In an earlier version of this PEP, the ``default`` was implicitly set to ``bound`` if no value was passed for ``default``. This while convenient, could have a type parameter with no default follow a type parameter with a default. Consider: :: T = TypeVar("T", bound=int) # default is implicitly int U = TypeVar("U") class Foo(Generic[T, U]): ... # would expand to T = TypeVar("T", bound=int, default=int) U = TypeVar("U") class Foo(Generic[T, U]): ... This would have also been a breaking change for a small number of cases where the code relied on ``Any`` being the implicit default. Allowing Type Parameters With Defaults To Be Used in Function Signatures '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' A previous version of this PEP allowed ``TypeVarLike``\s with defaults to be used in function signatures. This was removed for the reasons described in `Function Defaults`_. Hopefully, this can be added in the future if a way to get the runtime value of a type parameter is added. Allowing Type Parameters from Outer Scopes in ``default`` '''''''''''''''''''''''''''''''''''''''''''''''''''''''''' This was deemed too niche a feature to be worth the added complexity. If any cases arise where this is needed, it can be added in a future PEP. Acknowledgements ---------------- Thanks to the following people for their feedback on the PEP: Eric Traut, Jelle Zijlstra, Joshua Butt, Danny Yamamoto, Kaylynn Morgan and Jakub Kuczys Copyright --------- This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.