PEP 827: Wrap the new-style extended callables in a Params argument (#4866)

This Params argument can be thought of as kind of being the bound for
ParamSpec, and makes it easy to distinguish when the extended system
is being used.

Author: msullivan

peps/pep-0827.rst

@@ -389,7 +389,17 @@ We introduce a ``Param`` type that contains all the information about a function
     type ArgsParam[T] = Param[Literal[None], T, Literal["*"]]
     type KwargsParam[T] = Param[Literal[None], T, Literal["**"]]
 
-And then, we can represent the type of a function like::
+We also introduce a ``Params`` type that wraps a sequence of ``Param``
+types, serving as the first argument to ``Callable``::
+
+    class Params[*Ps]:
+        pass
+
+The presence of ``Params`` as the first argument to ``Callable``
+distinguishes the extended callable format from the standard format.
+``Params`` also serves as a natural bound for ``ParamSpec``.
+
+We can then represent the type of a function like::
 
     def func(
         a: int,
@@ -406,7 +416,7 @@ And then, we can represent the type of a function like::
 as::
 
     Callable[
-        [
+        Params[
             Param[Literal["a"], int, Literal["positional"]],
             Param[Literal["b"], int],
             Param[Literal["c"], int, Literal["default"]],
@@ -422,7 +432,7 @@ as::
 or, using the type abbreviations we provide::
 
     Callable[
-        [
+        Params[
             PosParam[Literal["a"], int],
             Param[Literal["b"], int],
             DefaultParam[Literal["c"], int],
@@ -1257,6 +1267,10 @@ We present implementations of a selection of them::
         ]
     ]
 
+    # KeyOf[T]
+    # Constructs a union of the names of every member of T.
+    type KeyOf[T] = Union[*[p.name for p in typing.Iter[typing.Members[T]]]]
+
     # Exclude<T, U>
     # Constructs a type by excluding from T all union members assignable to U.
     type Exclude[T, U] = Union[
@@ -1884,10 +1898,6 @@ Open Issues
   ``readonly`` had been added as a parameter to ``TypedDict`` we would
   use that, but it wasn't.
 
-* :ref:`Extended Callables <pep827-extended-callables-prereq>`: Should the extended
-  argument list be wrapped in a ``typing.Parameters[*Params]`` type (that
-  will also kind of serve as a bound for ``ParamSpec``)?
-
 * :ref:`Extended Callables <pep827-extended-callables-prereq>`: Currently the
   qualifiers are short strings for code brevity, but an alternate approach
   would be to mirror ``inspect.Signature`` more directly, and have an enum