PEP: 612 Title: Parameter Specification Variables Author: Mark Mendoza Sponsor: Guido van Rossum BDFL-Delegate: Guido van Rossum Discussions-To: typing-sig@python.org Status: Accepted Type: Standards Track Content-Type: text/x-rst Created: 18-Dec-2019 Python-Version: 3.10 Post-History: 18-Dec-2019, 13-Jul-2020 Parameter Specification Variables ================================= Abstract -------- There currently are two ways to specify the type of a callable, the ``Callable[[int, str], bool]`` syntax defined in :pep:`484`, and callback protocols from :pep:`PEP 544 <544#callback-protocols>`. Neither of these support forwarding the parameter types of one callable over to another callable, making it difficult to annotate function decorators. This PEP proposes ``typing.ParamSpec`` and ``typing.Concatenate`` to support expressing these kinds of relationships. Motivation ---------- The existing standards for annotating higher order functions don’t give us the tools to annotate the following common decorator pattern satisfactorily: .. code-block:: from typing import Awaitable, Callable, TypeVar R = TypeVar("R") def add_logging(f: Callable[..., R]) -> Callable[..., Awaitable[R]]: async def inner(*args: object, **kwargs: object) -> R: await log_to_database() return f(*args, **kwargs) return inner @add_logging def takes_int_str(x: int, y: str) -> int: return x + 7 await takes_int_str(1, "A") await takes_int_str("B", 2) # fails at runtime ``add_logging``\ , a decorator which logs before each entry into the decorated function, is an instance of the Python idiom of one function passing all arguments given to it over to another function. This is done through the combination of the ``*args`` and ``**kwargs`` features in both parameters and in arguments. When one defines a function (like ``inner``\ ) that takes ``(*args, **kwargs)`` and goes on to call another function with ``(*args, **kwargs)``, the wrapping function can only be safely called in all of the ways that the wrapped function could be safely called. To type this decorator, we’d like to be able to place a dependency between the parameters of the callable ``f`` and the parameters of the returned function. :pep:`484` supports dependencies between single types, as in ``def append(l: typing.List[T], e: T) -> typing.List[T]: ...``\ , but there is no existing way to do so with a complicated entity like the parameters of a function. Due to the limitations of the status quo, the ``add_logging`` example will type check but will fail at runtime. ``inner`` will pass the string “B” into ``takes_int_str``\, which will try to add 7 to it, triggering a type error. This was not caught by the type checker because the decorated ``takes_int_str`` was given the type ``Callable[..., Awaitable[int]]`` (an ellipsis in place of parameter types is specified to mean that we do no validation on arguments). Without the ability to define dependencies between the parameters of different callable types, there is no way, at present, to make ``add_logging`` compatible with all functions, while still preserving the enforcement of the parameters of the decorated function. With the addition of the ``ParamSpec`` variables proposed by this PEP, we can rewrite the previous example in a way that keeps the flexibility of the decorator and the parameter enforcement of the decorated function. .. code-block:: from typing import Awaitable, Callable, ParamSpec, TypeVar P = ParamSpec("P") R = TypeVar("R") def add_logging(f: Callable[P, R]) -> Callable[P, Awaitable[R]]: async def inner(*args: P.args, **kwargs: P.kwargs) -> R: await log_to_database() return f(*args, **kwargs) return inner @add_logging def takes_int_str(x: int, y: str) -> int: return x + 7 await takes_int_str(1, "A") # Accepted await takes_int_str("B", 2) # Correctly rejected by the type checker Another common decorator pattern that has previously been impossible to type is the practice of adding or removing arguments from the decorated function. For example: .. code-block:: class Request: ... def with_request(f: Callable[..., R]) -> Callable[..., R]: def inner(*args: object, **kwargs: object) -> R: return f(Request(), *args, **kwargs) return inner @with_request def takes_int_str(request: Request, x: int, y: str) -> int: # use request return x + 7 takes_int_str(1, "A") takes_int_str("B", 2) # fails at runtime With the addition of the ``Concatenate`` operator from this PEP, we can even type this more complex decorator. .. code-block:: from typing import Concatenate def with_request(f: Callable[Concatenate[Request, P], R]) -> Callable[P, R]: def inner(*args: P.args, **kwargs: P.kwargs) -> R: return f(Request(), *args, **kwargs) return inner @with_request def takes_int_str(request: Request, x: int, y: str) -> int: # use request return x + 7 takes_int_str(1, "A") # Accepted takes_int_str("B", 2) # Correctly rejected by the type checker Specification ------------- ``ParamSpec`` Variables ^^^^^^^^^^^^^^^^^^^^^^^ Declaration ```````````` A parameter specification variable is defined in a similar manner to how a normal type variable is defined with ``typing.TypeVar``. .. code-block:: from typing import ParamSpec P = ParamSpec("P") # Accepted P = ParamSpec("WrongName") # Rejected because P =/= WrongName The runtime should accept ``bound``\ s and ``covariant`` and ``contravariant`` arguments in the declaration just as ``typing.TypeVar`` does, but for now we will defer the standardization of the semantics of those options to a later PEP. Valid use locations ``````````````````` Previously only a list of parameter arguments (``[A, B, C]``) or an ellipsis (signifying "undefined parameters") were acceptable as the first "argument" to ``typing.Callable`` . We now augment that with two new options: a parameter specification variable (``Callable[P, int]``\ ) or a concatenation on a parameter specification variable (``Callable[Concatenate[int, P], int]``\ ). .. code-block:: callable ::= Callable "[" parameters_expression, type_expression "]" parameters_expression ::= | "..." | "[" [ type_expression ("," type_expression)* ] "]" | parameter_specification_variable | concatenate "[" type_expression ("," type_expression)* "," parameter_specification_variable "]" where ``parameter_specification_variable`` is a ``typing.ParamSpec`` variable, declared in the manner as defined above, and ``concatenate`` is ``typing.Concatenate``. As before, ``parameters_expression``\ s by themselves are not acceptable in places where a type is expected .. code-block:: def foo(x: P) -> P: ... # Rejected def foo(x: Concatenate[int, P]) -> int: ... # Rejected def foo(x: typing.List[P]) -> None: ... # Rejected def foo(x: Callable[[int, str], P]) -> None: ... # Rejected User-Defined Generic Classes ```````````````````````````` Just as defining a class as inheriting from ``Generic[T]`` makes a class generic for a single parameter (when ``T`` is a ``TypeVar``\ ), defining a class as inheriting from ``Generic[P]`` makes a class generic on ``parameters_expression``\ s (when ``P`` is a ``ParamSpec``). .. code-block:: T = TypeVar("T") P_2 = ParamSpec("P_2") class X(Generic[T, P]): f: Callable[P, int] x: T def f(x: X[int, P_2]) -> str: ... # Accepted def f(x: X[int, Concatenate[int, P_2]]) -> str: ... # Accepted def f(x: X[int, [int, bool]]) -> str: ... # Accepted def f(x: X[int, ...]) -> str: ... # Accepted def f(x: X[int, int]) -> str: ... # Rejected By the rules defined above, spelling a concrete instance of a class generic with respect to only a single ``ParamSpec`` would require unsightly double brackets. For aesthetic purposes we allow these to be omitted. .. code-block:: class Z(Generic[P]): f: Callable[P, int] def f(x: Z[[int, str, bool]]) -> str: ... # Accepted def f(x: Z[int, str, bool]) -> str: ... # Equivalent # Both Z[[int, str, bool]] and Z[int, str, bool] express this: class Z_instantiated: f: Callable[[int, str, bool], int] Semantics ````````` The inference rules for the return type of a function invocation whose signature contains a ``ParamSpec`` variable are analogous to those around evaluating ones with ``TypeVar``\ s. .. code-block:: def changes_return_type_to_str(x: Callable[P, int]) -> Callable[P, str]: ... def returns_int(a: str, b: bool) -> int: ... f = changes_return_type_to_str(returns_int) # f should have the type: # (a: str, b: bool) -> str f("A", True) # Accepted f(a="A", b=True) # Accepted f("A", "A") # Rejected expects_str(f("A", True)) # Accepted expects_int(f("A", True)) # Rejected Just as with traditional ``TypeVars``\ , a user may include the same ``ParamSpec`` multiple times in the arguments of the same function, to indicate a dependency between multiple arguments. In these cases a type checker may choose to solve to a common behavioral supertype (i.e. a set of parameters for which all of the valid calls are valid in both of the subtypes), but is not obligated to do so. .. code-block:: P = ParamSpec("P") def foo(x: Callable[P, int], y: Callable[P, int]) -> Callable[P, bool]: ... def x_y(x: int, y: str) -> int: ... def y_x(y: int, x: str) -> int: ... foo(x_y, x_y) # Should return (x: int, y: str) -> bool foo(x_y, y_x) # Could return (__a: int, __b: str) -> bool # This works because both callables have types that are # behavioral subtypes of Callable[[int, str], int] def keyword_only_x(*, x: int) -> int: ... def keyword_only_y(*, y: int) -> int: ... foo(keyword_only_x, keyword_only_y) # Rejected The constructors of user-defined classes generic on ``ParamSpec``\ s should be evaluated in the same way. .. code-block:: U = TypeVar("U") class Y(Generic[U, P]): f: Callable[P, str] prop: U def __init__(self, f: Callable[P, str], prop: U) -> None: self.f = f self.prop = prop def a(q: int) -> str: ... Y(a, 1) # Should resolve to Y[(q: int), int] Y(a, 1).f # Should resolve to (q: int) -> str The semantics of ``Concatenate[X, Y, P]`` are that it represents the parameters represented by ``P`` with two positional-only parameters prepended. This means that we can use it to represent higher order functions that add, remove or transform a finite number of parameters of a callable. .. code-block:: def bar(x: int, *args: bool) -> int: ... def add(x: Callable[P, int]) -> Callable[Concatenate[str, P], bool]: ... add(bar) # Should return (__a: str, x: int, *args: bool) -> bool def remove(x: Callable[Concatenate[int, P], int]) -> Callable[P, bool]: ... remove(bar) # Should return (*args: bool) -> bool def transform( x: Callable[Concatenate[int, P], int] ) -> Callable[Concatenate[str, P], bool]: ... transform(bar) # Should return (__a: str, *args: bool) -> bool This also means that while any function that returns an ``R`` can satisfy ``typing.Callable[P, R]``, only functions that can be called positionally in their first position with a ``X`` can satisfy ``typing.Callable[Concatenate[X, P], R]``. .. code-block:: def expects_int_first(x: Callable[Concatenate[int, P], int]) -> None: ... @expects_int_first # Rejected def one(x: str) -> int: ... @expects_int_first # Rejected def two(*, x: int) -> int: ... @expects_int_first # Rejected def three(**kwargs: int) -> int: ... @expects_int_first # Accepted def four(*args: int) -> int: ... There are still some classes of decorators still not supported with these features: * those that add/remove/change a **variable** number of parameters (for example, ``functools.partial`` will remain untypable even after this PEP) * those that add/remove/change keyword-only parameters (See `Concatenating Keyword Parameters`_ for more details). The components of a ``ParamSpec`` ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ A ``ParamSpec`` captures both positional and keyword accessible parameters, but there unfortunately is no object in the runtime that captures both of these together. Instead, we are forced to separate them into ``*args`` and ``**kwargs``\ , respectively. This means we need to be able to split apart a single ``ParamSpec`` into these two components, and then bring them back together into a call. To do this, we introduce ``P.args`` to represent the tuple of positional arguments in a given call and ``P.kwargs`` to represent the corresponding ``Mapping`` of keywords to values. Valid use locations ``````````````````` These "properties" can only be used as the annotated types for ``*args`` and ``**kwargs``\ , accessed from a ParamSpec already in scope. .. code-block:: def puts_p_into_scope(f: Callable[P, int]) -> None: def inner(*args: P.args, **kwargs: P.kwargs) -> None: # Accepted pass def mixed_up(*args: P.kwargs, **kwargs: P.args) -> None: # Rejected pass def misplaced(x: P.args) -> None: # Rejected pass def out_of_scope(*args: P.args, **kwargs: P.kwargs) -> None: # Rejected pass Furthermore, because the default kind of parameter in Python (\ ``(x: int)``\ ) may be addressed both positionally and through its name, two valid invocations of a ``(*args: P.args, **kwargs: P.kwargs)`` function may give different partitions of the same set of parameters. Therefore, we need to make sure that these special types are only brought into the world together, and are used together, so that our usage is valid for all possible partitions. .. code-block:: def puts_p_into_scope(f: Callable[P, int]) -> None: stored_args: P.args # Rejected stored_kwargs: P.kwargs # Rejected def just_args(*args: P.args) -> None: # Rejected pass def just_kwargs(**kwargs: P.kwargs) -> None: # Rejected pass Semantics ````````` With those requirements met, we can now take advantage of the unique properties afforded to us by this set up: * Inside the function, ``args`` has the type ``P.args``\ , not ``Tuple[P.args, ...]`` as would be with a normal annotation (and likewise with the ``**kwargs``\ ) * This special case is necessary to encapsulate the heterogeneous contents of the ``args``/``kwargs`` of a given call, which cannot be expressed by an indefinite tuple/dictionary type. * A function of type ``Callable[P, R]`` can be called with ``(*args, **kwargs)`` if and only if ``args`` has the type ``P.args`` and ``kwargs`` has the type ``P.kwargs``\ , and that those types both originated from the same function declaration. * A function declared as ``def inner(*args: P.args, **kwargs: P.kwargs) -> X`` has type ``Callable[P, X]``. With these three properties, we now have the ability to fully type check parameter preserving decorators. .. code-block:: def decorator(f: Callable[P, int]) -> Callable[P, None]: def foo(*args: P.args, **kwargs: P.kwargs) -> None: f(*args, **kwargs) # Accepted, should resolve to int f(*kwargs, **args) # Rejected f(1, *args, **kwargs) # Rejected return foo # Accepted To extend this to include ``Concatenate``, we declare the following properties: * A function of type ``Callable[Concatenate[A, B, P], R]`` can only be called with ``(a, b, *args, **kwargs)`` when ``args`` and ``kwargs`` are the respective components of ``P``, ``a`` is of type ``A`` and ``b`` is of type ``B``. * A function declared as ``def inner(a: A, b: B, *args: P.args, **kwargs: P.kwargs) -> R`` has type ``Callable[Concatenate[A, B, P], R]``. Placing keyword-only parameters between the ``*args`` and ``**kwargs`` is forbidden. .. code-block:: def add(f: Callable[P, int]) -> Callable[Concatenate[str, P], None]: def foo(s: str, *args: P.args, **kwargs: P.kwargs) -> None: # Accepted pass def bar(*args: P.args, s: str, **kwargs: P.kwargs) -> None: # Rejected pass return foo # Accepted def remove(f: Callable[Concatenate[int, P], int]) -> Callable[P, None]: def foo(*args: P.args, **kwargs: P.kwargs) -> None: f(1, *args, **kwargs) # Accepted f(*args, 1, **kwargs) # Rejected f(*args, **kwargs) # Rejected return foo Note that the names of the parameters preceding the ``ParamSpec`` components are not mentioned in the resulting ``Concatenate``. This means that these parameters can not be addressed via a named argument: .. code-block:: def outer(f: Callable[P, None]) -> Callable[P, None]: def foo(x: int, *args: P.args, **kwargs: P.kwargs) -> None: f(*args, **kwargs) def bar(*args: P.args, **kwargs: P.kwargs) -> None: foo(1, *args, **kwargs) # Accepted foo(x=1, *args, **kwargs) # Rejected return bar .. _above: This is not an implementation convenience, but a soundness requirement. If we were to allow that second calling style, then the following snippet would be problematic. .. code-block:: @outer def problem(*, x: object) -> None: pass problem(x="uh-oh") Inside of ``bar``, we would get ``TypeError: foo() got multiple values for argument 'x'``. Requiring these concatenated arguments to be addressed positionally avoids this kind of problem, and simplifies the syntax for spelling these types. Note that this also why we have to reject signatures of the form ``(*args: P.args, s: str, **kwargs: P.kwargs)`` (See `Concatenating Keyword Parameters`_ for more details). If one of these prepended positional parameters contains a free ``ParamSpec``\ , we consider that variable in scope for the purposes of extracting the components of that ``ParamSpec``. That allows us to spell things like this: .. code-block:: def twice(f: Callable[P, int], *args: P.args, **kwargs: P.kwargs) -> int: return f(*args, **kwargs) + f(*args, **kwargs) The type of ``twice`` in the above example is ``Callable[Concatenate[Callable[P, int], P], int]``, where ``P`` is bound by the outer ``Callable``. This has the following semantics: .. code-block:: def a_int_b_str(a: int, b: str) -> int: pass twice(a_int_b_str, 1, "A") # Accepted twice(a_int_b_str, b="A", a=1) # Accepted twice(a_int_b_str, "A", 1) # Rejected Backwards Compatibility ----------------------- The only changes necessary to existing features in ``typing`` is allowing these ``ParamSpec`` and ``Concatenate`` objects to be the first parameter to ``Callable`` and to be a parameter to ``Generic``. Currently ``Callable`` expects a list of types there and ``Generic`` expects single types, so they are currently mutually exclusive. Otherwise, existing code that doesn't reference the new interfaces will be unaffected. Reference Implementation ------------------------ The `Pyre `_ type checker supports all of the behavior described above. A reference implementation of the runtime components needed for those uses is provided in the ``pyre_extensions`` module. A reference implementation for CPython can be found `here `_. Rejected Alternatives --------------------- Using List Variadics and Map Variadics ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ We considered just trying to make something like this with a callback protocol which was parameterized on a list-type variadic, and a map-type variadic like so: .. code-block:: R = typing.TypeVar(“R”) Tpositionals = ... Tkeywords = ... class BetterCallable(typing.Protocol[Tpositionals, Tkeywords, R]): def __call__(*args: Tpositionals, **kwargs: Tkeywords) -> R: ... However, there are some problems with trying to come up with a consistent solution for those type variables for a given callable. This problem comes up with even the simplest of callables: .. code-block:: def simple(x: int) -> None: ... simple <: BetterCallable[[int], [], None] simple <: BetterCallable[[], {“x”: int}, None] BetterCallable[[int], [], None] BetterCallable[[Ts], [Tmap], str]: def decorated(*args: Ts, **kwargs: Tmap) -> str: x = f(*args, **kwargs) return int_to_str(x) return decorated @decorator def foo(x: int) -> int: return x reveal_type(foo) # Option A: BetterCallable[[int], {}, str] # Option B: BetterCallable[[], {x: int}, str] foo(7) # fails under option B foo(x=7) # fails under option A The core problem here is that, by default, parameters in Python can either be called positionally or as a keyword argument. This means we really have three categories (positional-only, positional-or-keyword, keyword-only) we’re trying to jam into two categories. This is the same problem that we briefly mentioned when discussing ``.args`` and ``.kwargs``. Fundamentally, in order to capture two categories when there are some things that can be in either category, we need a higher level primitive (\ ``ParamSpec``\ ) to capture all three, and then split them out afterward. Defining ParametersOf ^^^^^^^^^^^^^^^^^^^^^^ Another proposal we considered was defining ``ParametersOf`` and ``ReturnType`` operators which would operate on a domain of a newly defined ``Function`` type. ``Function`` would be callable with, and only with ``ParametersOf[F]``. ``ParametersOf`` and ``ReturnType`` would only operate on type variables with precisely this bound. The combination of these three features could express everything that we can express with ``ParamSpecs``. .. code-block:: F = TypeVar("F", bound=Function) def no_change(f: F) -> F: def inner( *args: ParametersOf[F].args, **kwargs: ParametersOf[F].kwargs ) -> ReturnType[F]: return f(*args, **kwargs) return inner def wrapping(f: F) -> Callable[ParametersOf[F], List[ReturnType[F]]]: def inner( *args: ParametersOf[F].args, **kwargs: ParametersOf[F].kwargs ) -> List[ReturnType[F]]: return [f(*args, **kwargs)] return inner def unwrapping( f: Callable[ParametersOf[F], List[R]] ) -> Callable[ParametersOf[F], R]: def inner( *args: ParametersOf[F].args, **kwargs: ParametersOf[F].kwargs ) -> R: return f(*args, **kwargs)[0] return inner We decided to go with ``ParamSpec``\ s over this approach for several reasons: * The footprint of this change would be larger, as we would need two new operators, and a new type, while ``ParamSpec`` just introduces a new variable. * Python typing has so far has avoided supporting operators, whether user-defined or built-in, in favor of destructuring. Accordingly, ``ParamSpec`` based signatures look much more like existing Python. * The lack of user-defined operators makes common patterns hard to spell. ``unwrapping`` is odd to read because ``F`` is not actually referring to any callable. It’s just being used as a container for the parameters we wish to propagate. It would read better if we could define an operator ``RemoveList[List[X]] = X`` and then ``unwrapping`` could take ``F`` and return ``Callable[ParametersOf[F], RemoveList[ReturnType[F]]]``. Without that, we unfortunately get into a situation where we have to use a ``Function``-variable as an improvised ``ParamSpec``, in that we never actually bind the return type. In summary, between these two equivalently powerful syntaxes, ``ParamSpec`` fits much more naturally into the status quo. .. _Concatenating Keyword Parameters: Concatenating Keyword Parameters ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In principle the idea of concatenation as a means to modify a finite number of positional parameters could be expanded to include keyword parameters. .. code-block:: def add_n(f: Callable[P, R]) -> Callable[Concatenate[("n", int), P], R]: def inner(*args: P.args, n: int, **kwargs: P.kwargs) -> R: # use n return f(*args, **kwargs) return inner However, the key distinction is that while prepending positional-only parameters to a valid callable type always yields another valid callable type, the same cannot be said for adding keyword-only parameters. As alluded to above_ , the issue is name collisions. The parameters ``Concatenate[("n", int), P]`` are only valid when ``P`` itself does not already have a parameter named ``n``\ . .. code-block:: def innocent_wrapper(f: Callable[P, R]) -> Callable[P, R]: def inner(*args: P.args, **kwargs: P.kwargs) -> R: added = add_n(f) return added(*args, n=1, **kwargs) return inner @innocent_wrapper def problem(n: int) -> None: pass Calling ``problem(2)`` works fine, but calling ``problem(n=2)`` leads to a ``TypeError: problem() got multiple values for argument 'n'`` from the call to ``added`` inside of ``innocent_wrapper``\ . This kind of situation could be avoided, and this kind of decorator could be typed if we could reify the constraint that a set of parameters **not** contain a certain name, with something like: .. code-block:: P_without_n = ParamSpec("P_without_n", banned_names=["n"]) def add_n( f: Callable[P_without_n, R] ) -> Callable[Concatenate[("n", int), P_without_n], R]: ... The call to ``add_n`` inside of ``innocent_wrapper`` could then be rejected since the callable was not guaranteed not to already have a parameter named ``n``\ . However, enforcing these constraints would require enough additional implementation work that we judged this extension to be out of scope of this PEP. Fortunately the design of ``ParamSpec``\ s are such that we can return to this idea later if there is sufficient demand. Naming this a ``ParameterSpecification`` ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ We decided that ParameterSpecification was a little too long-winded for use here, and that this style of abbreviated name made it look more like TypeVar. Naming this an ``ArgSpec`` ^^^^^^^^^^^^^^^^^^^^^^^^^^ We think that calling this a ParamSpec is more correct than referring to it as an ArgSpec, since callables have parameters, which are distinct from the arguments which are passed to them in a given call site. A given binding for a ParamSpec is a set of function parameters, not a call-site’s arguments. Acknowledgements ---------------- Thanks to all of the members of the Pyre team for their comments on early drafts of this PEP, and for their help with the reference implementation. Thanks are also due to the whole Python typing community for their early feedback on this idea at a Python typing meetup, leading directly to the much more compact ``.args``\ /\ ``.kwargs`` syntax. Copyright --------- This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.