PEP 827: Split whether a param has a default out from "quals"

peps/pep-0827.rst

@@ -8,7 +8,7 @@ Status: Draft
 Type: Standards Track
 Topic: Typing
 Created: 27-Feb-2026
-Python-Version: 3.15
+Python-Version: 3.16
 Post-History: 02-Mar-2026
 
 
@@ -376,18 +376,37 @@ this PEP.
 
 We introduce a ``Param`` type that contains all the information about a function param::
 
-    class Param[N: str | None, T, Q: ParamQuals = typing.Never]:
+    class Param[
+        N: str | None,
+        T,
+        K: ParamKind = typing.Never,
+        D = typing.Never,
+    ]:
         pass
 
-    ParamQuals = typing.Literal["*", "**", "default", "keyword"]
+    ParamKind = typing.Literal["*", "**", "keyword", "positional"]
 
-    type PosParam[N: str | None, T] = Param[N, T, Literal["positional"]]
-    type PosDefaultParam[N: str | None, T] = Param[N, T, Literal["positional", "default"]]
-    type DefaultParam[N: str, T] = Param[N, T, Literal["default"]]
+    type PosParam[T] = Param[None, T, Literal["positional"]]
+    type PosDefaultParam[T] = Param[None, T, Literal["positional"], T]
+    type DefaultParam[N: str, T] = Param[N, T, typing.Never, T]
     type NamedParam[N: str, T] = Param[N, T, Literal["keyword"]]
-    type NamedDefaultParam[N: str, T] = Param[N, T, Literal["keyword", "default"]]
-    type ArgsParam[T] = Param[Literal[None], T, Literal["*"]]
-    type KwargsParam[T] = Param[Literal[None], T, Literal["**"]]
+    type NamedDefaultParam[N: str, T] = Param[N, T, Literal["keyword"], T]
+    type ArgsParam[T] = Param[None, T, Literal["*"]]
+    type KwargsParam[T] = Param[None, T, Literal["**"]]
+
+
+The argument ``K``, of type ``ParamKind``, represents the parameter
+kind of the parameter. The default "positional or named" kind of
+argument is represented as ``Never``. It is an error to create
+``Callable`` with a ``Param`` containing multiple kinds unioned
+together.
+
+The argument ``D`` carries the type of the parameter's default, if one
+exists, and is ``Never`` otherwise. When the default value is a
+literal (e.g. ``None``, an int, a string, an enum member), ``D`` may
+be a ``Literal`` carrying that value. (Having it be such a ``Literal``
+has no effect other than to make it available to introspection and
+potentially for diagnostics.)
 
 We also introduce a ``Params`` type that wraps a sequence of ``Param``
 types, serving as the first argument to ``Callable``::
@@ -419,17 +438,18 @@ as::
         Params[
             Param[Literal["a"], int, Literal["positional"]],
             Param[Literal["b"], int],
-            Param[Literal["c"], int, Literal["default"]],
+            Param[Literal["c"], int, typing.Never, Literal[0]],
             Param[None, int, Literal["*"]],
             Param[Literal["d"], int, Literal["keyword"]],
-            Param[Literal["e"], int, Literal["default", "keyword"]],
+            Param[Literal["e"], int, Literal["keyword"], Literal[0]],
             Param[None, int, Literal["**"]],
         ],
         int,
     ]
 
 
-or, using the type abbreviations we provide::
+or, using the type abbreviations we provide (though this version will not track
+specific values for the defaults)::
 
     Callable[
         Params[
@@ -793,8 +813,9 @@ Callable inspection and creation
 ``Callable`` types always have their arguments exposed in the extended
 Callable format discussed above.
 
-The names, type, and qualifiers share associated type names with
-``Member`` (``.name``, ``.type``, and ``.quals``).
+The name and type associated type names with ``Member`` (``.name`` and
+``.type``).  ``Param`` also has a ``.kind`` associated type, which
+exposes ``K`` and a ``.default`` associated type, which exposes ``D``.
 
 .. _pep827-generic-callable:
 
@@ -1101,13 +1122,10 @@ based on iterating over all attributes.
                         p.name,
                         p.type,
                         # All arguments are keyword-only
-                        # It takes a default if a default is specified in the class
-                        Literal["keyword"]
-                        if typing.IsAssignable[
-                            GetDefault[p.init],
-                            Never,
-                        ]
-                        else Literal["keyword", "default"],
+                        Literal["keyword"],
+                        # GetDefault is Never when there's no default, so use it
+                        # directly as D.
+                        GetDefault[p.init],
                     ]
                     for p in typing.Iter[typing.Attrs[T]]
                 ],
@@ -1343,7 +1361,7 @@ functions with explicit generic annotations. For old-style generics,
 we'll probably have to try to evaluate it and then raise an error when
 we encounter a variable.)
 
-With our real syntax, this look likes::
+With our real syntax, this looks like::
 
     type Foo = NewProtocol[
         Member[
@@ -1888,10 +1906,6 @@ Open Issues
   would be to mirror ``inspect.Signature`` more directly, and have an enum
   with names like ``ParamKind.POSITIONAL_OR_KEYWORD``. Would that be better?
 
-  A related potential change would be to fully separate the kind from whether
-  there is a default, and have whether there is a default represented in
-  an ``init`` field, like we do for class member initializers with ``Member``.
-
 * :ref:`Members <pep827-members>`: Should ``Members`` return all
   methods, even those without annotations? We excluded them out of the
   desire for some consistency with attributes, but it would not be