gh-148945: add support for bounds on type variable tuples and parameter specifications

Type parameter lists now accept a bound on type variable tuples and
parameter specifications, using the same colon syntax already available
for TypeVar:

    def call[*Ts: int, **P: [str]](*args: *Ts, f: Callable[P, int]): ...

A TypeVarTuple bound applies to each type the type variable tuple stands
for and is a star_expression, like its default. A ParamSpec bound is a
parameter list. As with TypeVar bounds, they are lazily evaluated in a
separate annotation scope and are exposed through __bound__ and the
evaluate_bound evaluate function.

The ast.TypeVarTuple and ast.ParamSpec nodes gain a 'bound' field.

Also normalizes ParamSpec('P').__bound__ to None; it was previously
types.NoneType, unlike TypeVar and TypeVarTuple.

Author: KotlinIsland

Doc/library/ast.rst

@@ -1914,22 +1914,27 @@ aliases.
    .. versionchanged:: 3.13
       Added the *default_value* parameter.
 
-.. class:: ParamSpec(name, default_value)
+.. class:: ParamSpec(name, bound, default_value)
 
    A :class:`typing.ParamSpec`. ``name`` is the name of the parameter specification.
-   ``default_value`` is the default value; if the :class:`!ParamSpec` has no default,
-   this attribute will be set to ``None``.
+   ``bound`` is the bound, if any; a parameter specification is bounded by a
+   parameter list, so the bound is usually a :class:`List`. ``default_value``
+   is the default value; if the :class:`!ParamSpec` has no bound or no default,
+   the corresponding attribute will be set to ``None``.
 
    .. doctest::
 
-        >>> print(ast.dump(ast.parse("type Alias[**P = [int, str]] = Callable[P, int]"), indent=4))
+        >>> print(ast.dump(ast.parse("type Alias[**P: [int] = [int, str]] = Callable[P, int]"), indent=4))
         Module(
             body=[
                 TypeAlias(
                     name=Name(id='Alias', ctx=Store()),
                     type_params=[
                         ParamSpec(
                             name='P',
+                            bound=List(
+                                elts=[
+                                    Name(id='int')]),
                             default_value=List(
                                 elts=[
                                     Name(id='int'),
@@ -1946,21 +1951,29 @@ aliases.
    .. versionchanged:: 3.13
       Added the *default_value* parameter.
 
-.. class:: TypeVarTuple(name, default_value)
+   .. versionchanged:: 3.16
+      Added the *bound* parameter.
+
+.. class:: TypeVarTuple(name, bound, default_value)
 
    A :class:`typing.TypeVarTuple`. ``name`` is the name of the type variable tuple.
-   ``default_value`` is the default value; if the :class:`!TypeVarTuple` has no
-   default, this attribute will be set to ``None``.
+   ``bound`` is the bound, if any, which applies to each type substituted for the
+   type variable tuple. ``default_value`` is the default value; if the
+   :class:`!TypeVarTuple` has no bound or no default, the corresponding attribute
+   will be set to ``None``.
 
    .. doctest::
 
-        >>> print(ast.dump(ast.parse("type Alias[*Ts = ()] = tuple[*Ts]"), indent=4))
+        >>> print(ast.dump(ast.parse("type Alias[*Ts: int = ()] = tuple[*Ts]"), indent=4))
         Module(
             body=[
                 TypeAlias(
                     name=Name(id='Alias', ctx=Store()),
                     type_params=[
-                        TypeVarTuple(name='Ts', default_value=Tuple())],
+                        TypeVarTuple(
+                            name='Ts',
+                            bound=Name(id='int'),
+                            default_value=Tuple())],
                     value=Subscript(
                         value=Name(id='tuple'),
                         slice=Tuple(
@@ -1973,6 +1986,9 @@ aliases.
    .. versionchanged:: 3.13
       Added the *default_value* parameter.
 
+   .. versionchanged:: 3.16
+      Added the *bound* parameter.
+
 Function and class definitions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

Doc/library/typing.rst

@@ -2109,6 +2109,30 @@ without the dedicated syntax, as documented below.
 
       .. versionadded:: 3.15
 
+   .. attribute:: __bound__
+
+      The upper bound of each of the types the type variable tuple stands for,
+      if any.
+
+      .. versionchanged:: 3.16
+
+         For type variable tuples created through
+         :ref:`type parameter syntax <type-params>`, the bound is evaluated only
+         when the attribute is accessed, not when the type variable tuple is
+         created (see :ref:`lazy-evaluation`).
+
+   .. method:: evaluate_bound
+
+      An :term:`evaluate function` corresponding to the
+      :attr:`~TypeVarTuple.__bound__` attribute.
+      When called directly, this method supports only the :attr:`~annotationlib.Format.VALUE`
+      format, which is equivalent to accessing the :attr:`~TypeVarTuple.__bound__` attribute
+      directly, but the method object can be passed to
+      :func:`annotationlib.call_evaluate_function` to evaluate the value in a
+      different format.
+
+      .. versionadded:: 3.16
+
    .. attribute:: __default__
 
       The default value of the type variable tuple, or :data:`typing.NoDefault` if it
@@ -2135,11 +2159,6 @@ without the dedicated syntax, as documented below.
 
       .. versionadded:: 3.13
 
-   Type variable tuples created with ``covariant=True`` or
-   ``contravariant=True`` can be used to declare covariant or contravariant
-   generic types.  The ``bound`` argument is also accepted, similar to
-   :class:`TypeVar`, but its actual semantics are yet to be decided.
-
    .. versionadded:: 3.11
 
    .. versionchanged:: 3.12
@@ -2156,6 +2175,12 @@ without the dedicated syntax, as documented below.
       Added support for the ``bound``, ``covariant``, ``contravariant``, and
       ``infer_variance`` parameters.
 
+   .. versionchanged:: 3.16
+
+      Type variable tuple bounds can now be declared using the
+      :ref:`type parameter <type-params>` syntax, and are
+      :ref:`lazily evaluated <lazy-evaluation>`.
+
 .. class:: ParamSpec(name, *, bound=None, covariant=False, contravariant=False, infer_variance=False, default=typing.NoDefault)
 
    Parameter specification variable.  A specialized version of
@@ -2239,6 +2264,34 @@ without the dedicated syntax, as documented below.
 
       .. versionadded:: 3.12
 
+   .. attribute:: __bound__
+
+      The upper bound of the parameter specification, if any. Because a
+      parameter specification stands for the parameters of a callable, its
+      bound is a parameter list, such as ``[int, str]``.
+
+      .. versionchanged:: 3.16
+
+         For parameter specifications created through
+         :ref:`type parameter syntax <type-params>`, the bound is evaluated only
+         when the attribute is accessed, not when the parameter specification is
+         created (see :ref:`lazy-evaluation`).
+
+         Previously, :attr:`!__bound__` was :class:`types.NoneType` rather than
+         ``None`` when no bound was given.
+
+   .. method:: evaluate_bound
+
+      An :term:`evaluate function` corresponding to the
+      :attr:`~ParamSpec.__bound__` attribute.
+      When called directly, this method supports only the :attr:`~annotationlib.Format.VALUE`
+      format, which is equivalent to accessing the :attr:`~ParamSpec.__bound__` attribute
+      directly, but the method object can be passed to
+      :func:`annotationlib.call_evaluate_function` to evaluate the value in a
+      different format.
+
+      .. versionadded:: 3.16
+
    .. attribute:: __default__
 
       The default value of the parameter specification, or :data:`typing.NoDefault` if it
@@ -2267,8 +2320,7 @@ without the dedicated syntax, as documented below.
 
    Parameter specification variables created with ``covariant=True`` or
    ``contravariant=True`` can be used to declare covariant or contravariant
-   generic types.  The ``bound`` argument is also accepted, similar to
-   :class:`TypeVar`.  However the actual semantics of these keywords are yet to
+   generic types.  However the actual semantics of these keywords are yet to
    be decided.
 
    .. versionadded:: 3.10
@@ -2282,6 +2334,12 @@ without the dedicated syntax, as documented below.
 
       Support for default values was added.
 
+   .. versionchanged:: 3.16
+
+      Parameter specification bounds can now be declared using the
+      :ref:`type parameter <type-params>` syntax, and are
+      :ref:`lazily evaluated <lazy-evaluation>`.
+
    .. note::
       Only parameter specification variables defined in global scope can
       be pickled.

Doc/reference/compound_stmts.rst

@@ -1768,8 +1768,8 @@ Type parameter lists
    type_params: "[" `type_param` ("," `type_param`)* "]"
    type_param: `typevar` | `typevartuple` | `paramspec`
    typevar: `identifier` (":" `expression`)? ("=" `expression`)?
-   typevartuple: "*" `identifier` ("=" `expression`)?
-   paramspec: "**" `identifier` ("=" `expression`)?
+   typevartuple: "*" `identifier` (":" `starred_expression`)? ("=" `starred_expression`)?
+   paramspec: "**" `identifier` (":" `expression`)? ("=" `expression`)?
 
 :ref:`Functions <def>` (including :ref:`coroutines <async def>`),
 :ref:`classes <class>` and :ref:`type aliases <type>` may
@@ -1832,8 +1832,19 @@ but only when the value is explicitly accessed through the attributes ``__bound_
 and ``__constraints__``. To accomplish this, the bounds or constraints are
 evaluated in a separate :ref:`annotation scope <annotation-scopes>`.
 
-:data:`typing.TypeVarTuple`\ s and :data:`typing.ParamSpec`\ s cannot have bounds
-or constraints.
+:data:`typing.TypeVarTuple`\ s and :data:`typing.ParamSpec`\ s can also declare a
+bound with a colon (``:``) followed by an expression, but they cannot declare
+constraints. For a :data:`!typing.TypeVarTuple`, the bound applies to each of the
+types it stands for (e.g. in ``*Ts: int``, every type substituted for ``Ts`` must
+be a subtype of :class:`int`). For a :data:`!typing.ParamSpec`, the bound is a
+parameter list that the substituted parameters must be compatible with (e.g.
+``**P: [int]``). As with :data:`!typing.TypeVar`, these bounds are lazily
+evaluated in a separate :ref:`annotation scope <annotation-scopes>` and are not
+enforced at runtime.
+
+.. versionchanged:: 3.16
+   Added support for bounds on :data:`!typing.TypeVarTuple`\ s and
+   :data:`!typing.ParamSpec`\ s.
 
 All three flavors of type parameters can also have a *default value*, which is used
 when the type parameter is not explicitly provided. This is added by appending
@@ -1853,7 +1864,9 @@ The following example indicates the full set of allowed type parameter declarati
       TypeVarWithBound: int,
       TypeVarWithConstraints: (str, bytes),
       *SimpleTypeVarTuple = (int, float),
+      *TypeVarTupleWithBound: int,
       **SimpleParamSpec = (str, bytearray),
+      **ParamSpecWithBound: [int],
    ](
       a: SimpleTypeVar,
       b: TypeVarWithDefault,

Doc/whatsnew/3.16.rst

@@ -75,6 +75,20 @@ New features
 Other language changes
 ======================
 
+* :ref:`Type parameter lists <type-params>` now accept bounds on type variable
+  tuples and parameter specifications, using the same syntax already available
+  for :class:`~typing.TypeVar`::
+
+     def call[*Ts: int, **P: [str]](*args: *Ts, f: Callable[P, int]) -> None: ...
+
+  A :class:`~typing.TypeVarTuple` bound applies to each type the type variable
+  tuple stands for, while a :class:`~typing.ParamSpec` bound is a parameter
+  list. Like other type parameter bounds, they are
+  :ref:`lazily evaluated <lazy-evaluation>` and are available through the
+  ``__bound__`` attribute and the ``evaluate_bound``
+  :term:`evaluate function`.
+  (Contributed by KotlinIsland in :gh:`148945`.)
+
 * :meth:`memoryview.cast` now allows casting a multidimensional
   F-contiguous view to a one-dimensional view.
   (Contributed by Jaemin Park in :gh:`91484`.)

Grammar/python.gram

@@ -694,11 +694,14 @@ type_param_seq[asdl_type_param_seq*]: a[asdl_type_param_seq*]=','.type_param+ ['
 
 type_param[type_param_ty] (memo):
     | a=NAME b=[type_param_bound] c=[type_param_default] { _PyAST_TypeVar(a->v.Name.id, b, c, EXTRA) }
-    | invalid_type_param
-    | '*' a=NAME b=[type_param_starred_default] { _PyAST_TypeVarTuple(a->v.Name.id, b, EXTRA) }
-    | '**' a=NAME b=[type_param_default] { _PyAST_ParamSpec(a->v.Name.id, b, EXTRA) }
+    | '*' a=NAME b=[type_param_starred_bound] c=[type_param_starred_default] { _PyAST_TypeVarTuple(a->v.Name.id, b, c, EXTRA) }
+    | '**' a=NAME b=[type_param_paramspec_bound] c=[type_param_default] { _PyAST_ParamSpec(a->v.Name.id, b, c, EXTRA) }
 
 type_param_bound[expr_ty]: ':' e=expression { e }
+type_param_starred_bound[expr_ty]: ':' e=star_expression {
+    CHECK_VERSION(expr_ty, 16, "Type variable tuple bounds are", e) }
+type_param_paramspec_bound[expr_ty]: ':' e=expression {
+    CHECK_VERSION(expr_ty, 16, "Parameter specification bounds are", e) }
 type_param_default[expr_ty]: '=' e=expression {
     CHECK_VERSION(expr_ty, 13, "Type parameter defaults are", e) }
 type_param_starred_default[expr_ty]: '=' e=star_expression {
@@ -1249,18 +1252,6 @@ invalid_legacy_expression:
         _PyPegen_check_legacy_stmt(p, a) ? RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b,
             "Missing parentheses in call to '%U'. Did you mean %U(...)?", a->v.Name.id, a->v.Name.id) : NULL}
 
-invalid_type_param:
-    | '*' a=NAME colon=':' e=expression {
-            RAISE_SYNTAX_ERROR_STARTING_FROM(colon, e->kind == Tuple_kind
-                ? "cannot use constraints with TypeVarTuple"
-                : "cannot use bound with TypeVarTuple")
-        }
-    | '**' a=NAME colon=':' e=expression {
-            RAISE_SYNTAX_ERROR_STARTING_FROM(colon, e->kind == Tuple_kind
-                ? "cannot use constraints with ParamSpec"
-                : "cannot use bound with ParamSpec")
-        }
-
 invalid_expression:
    |  STRING a=(!STRING expression_without_invalid)+ STRING {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE( PyPegen_first_item(a, expr_ty), PyPegen_last_item(a, expr_ty),

Include/internal/pycore_ast.h

@@ -31,8 +31,10 @@
 #define INTRINSIC_SET_FUNCTION_TYPE_PARAMS       4
 #define INTRINSIC_SET_TYPEPARAM_DEFAULT          5
 #define INTRINSIC_ADD_CONDITIONAL_ANNOTATION     6
+#define INTRINSIC_TYPEVARTUPLE_WITH_BOUND        7
+#define INTRINSIC_PARAMSPEC_WITH_BOUND           8
 
-#define MAX_INTRINSIC_2                          6
+#define MAX_INTRINSIC_2                          8
 
 typedef PyObject *(*intrinsic_func1)(PyThreadState* tstate, PyObject *value);
 typedef PyObject *(*intrinsic_func2)(PyThreadState* tstate, PyObject *value1, PyObject *value2);

Include/internal/pycore_intrinsics.h

@@ -303,6 +303,7 @@ Known values:
     Python 3.16a1 3703 (Replace DELETE_GLOBAL with PUSH_NULL; STORE_GLOBAL)
     Python 3.16a1 3704 (Replace DELETE_ATTR with PUSH_NULL; STORE_ATTR)
     Python 3.16a1 3705 (Add INTRINSIC_ADD_CONDITIONAL_ANNOTATION)
+    Python 3.16a1 3706 (Add INTRINSIC_TYPEVARTUPLE_WITH_BOUND and INTRINSIC_PARAMSPEC_WITH_BOUND)
 
     Python 3.17 will start with 3750
 
@@ -312,7 +313,7 @@ Known values:
 
 */
 
-#define PYC_MAGIC_NUMBER 3705
+#define PYC_MAGIC_NUMBER 3706
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \

Include/internal/pycore_magic_number.h

@@ -10,7 +10,9 @@ extern "C" {
 
 extern PyObject *_Py_make_typevar(PyObject *, PyObject *, PyObject *);
 extern PyObject *_Py_make_paramspec(PyThreadState *, PyObject *);
+extern PyObject *_Py_make_paramspec_with_bound(PyObject *, PyObject *);
 extern PyObject *_Py_make_typevartuple(PyThreadState *, PyObject *);
+extern PyObject *_Py_make_typevartuple_with_bound(PyObject *, PyObject *);
 extern PyObject *_Py_make_typealias(PyThreadState *, PyObject *);
 extern PyObject *_Py_subscript_generic(PyThreadState *, PyObject *);
 extern PyObject *_Py_set_typeparam_default(PyThreadState *, PyObject *, PyObject *);

Include/internal/pycore_typevarobject.h

@@ -453,12 +453,18 @@ def visit_TypeVar(self, node):
 
     def visit_TypeVarTuple(self, node):
         self.write("*" + node.name)
+        if node.bound:
+            self.write(": ")
+            self.traverse(node.bound)
         if node.default_value:
             self.write(" = ")
             self.traverse(node.default_value)
 
     def visit_ParamSpec(self, node):
         self.write("**" + node.name)
+        if node.bound:
+            self.write(": ")
+            self.traverse(node.bound)
         if node.default_value:
             self.write(" = ")
             self.traverse(node.default_value)