gh-103763: Implement PEP 695 (#103764)

This implements PEP 695, Type Parameter Syntax. It adds support for:

- Generic functions (def func[T](): ...)
- Generic classes (class X[T](): ...)
- Type aliases (type X = ...)
- New scoping when the new syntax is used within a class body
- Compiler and interpreter changes to support the new syntax and scoping rules 

Co-authored-by: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
Co-authored-by: Eric Traut <eric@traut.com>
Co-authored-by: Larry Hastings <larry@hastings.org>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

1 files changed, 170 insertions, 421 deletions

diff --git a/Lib/typing.py b/Lib/typing.py
index 61aed09..8d132e2 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -23,6 +23,7 @@ from abc import abstractmethod, ABCMeta
 import collections
 from collections import defaultdict
 import collections.abc
+import copyreg
 import contextlib
 import functools
 import operator
@@ -32,12 +33,16 @@ import types
 import warnings
 from types import WrapperDescriptorType, MethodWrapperType, MethodDescriptorType, GenericAlias
 
-
-try:
-    from _typing import _idfunc
-except ImportError:
-    def _idfunc(_, x):
-        return x
+from _typing import (
+    _idfunc,
+    TypeVar,
+    ParamSpec,
+    TypeVarTuple,
+    ParamSpecArgs,
+    ParamSpecKwargs,
+    TypeAliasType,
+    Generic,
+)
 
 # Please keep __all__ alphabetized within each category.
 __all__ = [
@@ -149,6 +154,7 @@ __all__ = [
     'TYPE_CHECKING',
     'TypeAlias',
     'TypeGuard',
+    'TypeAliasType',
     'Unpack',
 ]
 
@@ -695,6 +701,15 @@ def Union(self, parameters):
         return _UnionGenericAlias(self, parameters, name="Optional")
     return _UnionGenericAlias(self, parameters)
 
+def _make_union(left, right):
+    """Used from the C implementation of TypeVar.
+
+    TypeVar.__or__ calls this instead of returning types.UnionType
+    because we want to allow unions between TypeVars and strings
+    (forward references.)
+    """
+    return Union[left, right]
+
 @_SpecialForm
 def Optional(self, parameters):
     """Optional type.
@@ -926,333 +941,162 @@ class _PickleUsingNameMixin:
         return self.__name__
 
 
-class _BoundVarianceMixin:
-    """Mixin giving __init__ bound and variance arguments.
-
-    This is used by TypeVar and ParamSpec, which both employ the notions of
-    a type 'bound' (restricting type arguments to be a subtype of some
-    specified type) and type 'variance' (determining subtype relations between
-    generic types).
-    """
-    def __init__(self, bound, covariant, contravariant):
-        """Used to setup TypeVars and ParamSpec's bound, covariant and
-        contravariant attributes.
-        """
-        if covariant and contravariant:
-            raise ValueError("Bivariant types are not supported.")
-        self.__covariant__ = bool(covariant)
-        self.__contravariant__ = bool(contravariant)
-        if bound:
-            self.__bound__ = _type_check(bound, "Bound must be a type.")
-        else:
-            self.__bound__ = None
-
-    def __or__(self, right):
-        return Union[self, right]
-
-    def __ror__(self, left):
-        return Union[left, self]
-
-    def __repr__(self):
-        if self.__covariant__:
-            prefix = '+'
-        elif self.__contravariant__:
-            prefix = '-'
-        else:
-            prefix = '~'
-        return prefix + self.__name__
-
-    def __mro_entries__(self, bases):
-        raise TypeError(f"Cannot subclass an instance of {type(self).__name__}")
-
-
-class TypeVar(_Final, _Immutable, _BoundVarianceMixin, _PickleUsingNameMixin,
-              _root=True):
-    """Type variable.
-
-    Usage::
-
-      T = TypeVar('T')  # Can be anything
-      A = TypeVar('A', str, bytes)  # Must be str or bytes
-
-    Type variables exist primarily for the benefit of static type
-    checkers.  They serve as the parameters for generic types as well
-    as for generic function definitions.  See class Generic for more
-    information on generic types.  Generic functions work as follows:
-
-      def repeat(x: T, n: int) -> List[T]:
-          '''Return a list containing n references to x.'''
-          return [x]*n
-
-      def longest(x: A, y: A) -> A:
-          '''Return the longest of two strings.'''
-          return x if len(x) >= len(y) else y
-
-    The latter example's signature is essentially the overloading
-    of (str, str) -> str and (bytes, bytes) -> bytes.  Also note
-    that if the arguments are instances of some subclass of str,
-    the return type is still plain str.
-
-    At runtime, isinstance(x, T) and issubclass(C, T) will raise TypeError.
-
-    Type variables defined with covariant=True or contravariant=True
-    can be used to declare covariant or contravariant generic types.
-    See PEP 484 for more details. By default generic types are invariant
-    in all type variables.
-
-    Type variables can be introspected. e.g.:
-
-      T.__name__ == 'T'
-      T.__constraints__ == ()
-      T.__covariant__ == False
-      T.__contravariant__ = False
-      A.__constraints__ == (str, bytes)
-
-    Note that only type variables defined in global scope can be pickled.
-    """
-
-    def __init__(self, name, *constraints, bound=None,
-                 covariant=False, contravariant=False):
-        self.__name__ = name
-        super().__init__(bound, covariant, contravariant)
-        if constraints and bound is not None:
-            raise TypeError("Constraints cannot be combined with bound=...")
-        if constraints and len(constraints) == 1:
-            raise TypeError("A single constraint is not allowed")
-        msg = "TypeVar(name, constraint, ...): constraints must be types."
-        self.__constraints__ = tuple(_type_check(t, msg) for t in constraints)
-        def_mod = _caller()
-        if def_mod != 'typing':
-            self.__module__ = def_mod
+def _typevar_subst(self, arg):
+    msg = "Parameters to generic types must be types."
+    arg = _type_check(arg, msg, is_argument=True)
+    if ((isinstance(arg, _GenericAlias) and arg.__origin__ is Unpack) or
+        (isinstance(arg, GenericAlias) and getattr(arg, '__unpacked__', False))):
+        raise TypeError(f"{arg} is not valid as type argument")
+    return arg
 
-    def __typing_subst__(self, arg):
-        msg = "Parameters to generic types must be types."
-        arg = _type_check(arg, msg, is_argument=True)
-        if ((isinstance(arg, _GenericAlias) and arg.__origin__ is Unpack) or
-            (isinstance(arg, GenericAlias) and getattr(arg, '__unpacked__', False))):
-            raise TypeError(f"{arg} is not valid as type argument")
-        return arg
 
+def _typevartuple_prepare_subst(self, alias, args):
+    params = alias.__parameters__
+    typevartuple_index = params.index(self)
+    for param in params[typevartuple_index + 1:]:
+        if isinstance(param, TypeVarTuple):
+            raise TypeError(f"More than one TypeVarTuple parameter in {alias}")
+
+    alen = len(args)
+    plen = len(params)
+    left = typevartuple_index
+    right = plen - typevartuple_index - 1
+    var_tuple_index = None
+    fillarg = None
+    for k, arg in enumerate(args):
+        if not isinstance(arg, type):
+            subargs = getattr(arg, '__typing_unpacked_tuple_args__', None)
+            if subargs and len(subargs) == 2 and subargs[-1] is ...:
+                if var_tuple_index is not None:
+                    raise TypeError("More than one unpacked arbitrary-length tuple argument")
+                var_tuple_index = k
+                fillarg = subargs[0]
+    if var_tuple_index is not None:
+        left = min(left, var_tuple_index)
+        right = min(right, alen - var_tuple_index - 1)
+    elif left + right > alen:
+        raise TypeError(f"Too few arguments for {alias};"
+                        f" actual {alen}, expected at least {plen-1}")
 
-class TypeVarTuple(_Final, _Immutable, _PickleUsingNameMixin, _root=True):
-    """Type variable tuple.
+    return (
+        *args[:left],
+        *([fillarg]*(typevartuple_index - left)),
+        tuple(args[left: alen - right]),
+        *([fillarg]*(plen - right - left - typevartuple_index - 1)),
+        *args[alen - right:],
+    )
 
-    Usage:
 
-      Ts = TypeVarTuple('Ts')  # Can be given any name
+def _paramspec_subst(self, arg):
+    if isinstance(arg, (list, tuple)):
+        arg = tuple(_type_check(a, "Expected a type.") for a in arg)
+    elif not _is_param_expr(arg):
+        raise TypeError(f"Expected a list of types, an ellipsis, "
+                        f"ParamSpec, or Concatenate. Got {arg}")
+    return arg
 
-    Just as a TypeVar (type variable) is a placeholder for a single type,
-    a TypeVarTuple is a placeholder for an *arbitrary* number of types. For
-    example, if we define a generic class using a TypeVarTuple:
 
-      class C(Generic[*Ts]): ...
+def _paramspec_prepare_subst(self, alias, args):
+    params = alias.__parameters__
+    i = params.index(self)
+    if i >= len(args):
+        raise TypeError(f"Too few arguments for {alias}")
+    # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
+    if len(params) == 1 and not _is_param_expr(args[0]):
+        assert i == 0
+        args = (args,)
+    # Convert lists to tuples to help other libraries cache the results.
+    elif isinstance(args[i], list):
+        args = (*args[:i], tuple(args[i]), *args[i+1:])
+    return args
 
-    Then we can parameterize that class with an arbitrary number of type
-    arguments:
 
-      C[int]       # Fine
-      C[int, str]  # Also fine
-      C[()]        # Even this is fine
+@_tp_cache
+def _generic_class_getitem(cls, params):
+    """Parameterizes a generic class.
 
-    For more details, see PEP 646.
+    At least, parameterizing a generic class is the *main* thing this method
+    does. For example, for some generic class `Foo`, this is called when we
+    do `Foo[int]` - there, with `cls=Foo` and `params=int`.
 
-    Note that only TypeVarTuples defined in global scope can be pickled.
+    However, note that this method is also called when defining generic
+    classes in the first place with `class Foo(Generic[T]): ...`.
     """
+    if not isinstance(params, tuple):
+        params = (params,)
 
-    def __init__(self, name):
-        self.__name__ = name
-
-        # Used for pickling.
-        def_mod = _caller()
-        if def_mod != 'typing':
-            self.__module__ = def_mod
-
-    def __iter__(self):
-        yield Unpack[self]
-
-    def __repr__(self):
-        return self.__name__
+    params = tuple(_type_convert(p) for p in params)
+    is_generic_or_protocol = cls in (Generic, Protocol)
 
-    def __typing_subst__(self, arg):
-        raise TypeError("Substitution of bare TypeVarTuple is not supported")
+    if is_generic_or_protocol:
+        # Generic and Protocol can only be subscripted with unique type variables.
+        if not params:
+            raise TypeError(
+                f"Parameter list to {cls.__qualname__}[...] cannot be empty"
+            )
+        if not all(_is_typevar_like(p) for p in params):
+            raise TypeError(
+                f"Parameters to {cls.__name__}[...] must all be type variables "
+                f"or parameter specification variables.")
+        if len(set(params)) != len(params):
+            raise TypeError(
+                f"Parameters to {cls.__name__}[...] must all be unique")
+    else:
+        # Subscripting a regular Generic subclass.
+        for param in cls.__parameters__:
+            prepare = getattr(param, '__typing_prepare_subst__', None)
+            if prepare is not None:
+                params = prepare(cls, params)
+        _check_generic(cls, params, len(cls.__parameters__))
 
-    def __typing_prepare_subst__(self, alias, args):
-        params = alias.__parameters__
-        typevartuple_index = params.index(self)
-        for param in params[typevartuple_index + 1:]:
+        new_args = []
+        for param, new_arg in zip(cls.__parameters__, params):
             if isinstance(param, TypeVarTuple):
-                raise TypeError(f"More than one TypeVarTuple parameter in {alias}")
-
-        alen = len(args)
-        plen = len(params)
-        left = typevartuple_index
-        right = plen - typevartuple_index - 1
-        var_tuple_index = None
-        fillarg = None
-        for k, arg in enumerate(args):
-            if not isinstance(arg, type):
-                subargs = getattr(arg, '__typing_unpacked_tuple_args__', None)
-                if subargs and len(subargs) == 2 and subargs[-1] is ...:
-                    if var_tuple_index is not None:
-                        raise TypeError("More than one unpacked arbitrary-length tuple argument")
-                    var_tuple_index = k
-                    fillarg = subargs[0]
-        if var_tuple_index is not None:
-            left = min(left, var_tuple_index)
-            right = min(right, alen - var_tuple_index - 1)
-        elif left + right > alen:
-            raise TypeError(f"Too few arguments for {alias};"
-                            f" actual {alen}, expected at least {plen-1}")
-
-        return (
-            *args[:left],
-            *([fillarg]*(typevartuple_index - left)),
-            tuple(args[left: alen - right]),
-            *([fillarg]*(plen - right - left - typevartuple_index - 1)),
-            *args[alen - right:],
-        )
-
-    def __mro_entries__(self, bases):
-        raise TypeError(f"Cannot subclass an instance of {type(self).__name__}")
-
-
-class ParamSpecArgs(_Final, _Immutable, _root=True):
-    """The args for a ParamSpec object.
-
-    Given a ParamSpec object P, P.args is an instance of ParamSpecArgs.
-
-    ParamSpecArgs objects have a reference back to their ParamSpec:
-
-       P.args.__origin__ is P
-
-    This type is meant for runtime introspection and has no special meaning to
-    static type checkers.
-    """
-    def __init__(self, origin):
-        self.__origin__ = origin
-
-    def __repr__(self):
-        return f"{self.__origin__.__name__}.args"
-
-    def __eq__(self, other):
-        if not isinstance(other, ParamSpecArgs):
-            return NotImplemented
-        return self.__origin__ == other.__origin__
-
-    def __mro_entries__(self, bases):
-        raise TypeError(f"Cannot subclass an instance of {type(self).__name__}")
-
-
-class ParamSpecKwargs(_Final, _Immutable, _root=True):
-    """The kwargs for a ParamSpec object.
-
-    Given a ParamSpec object P, P.kwargs is an instance of ParamSpecKwargs.
-
-    ParamSpecKwargs objects have a reference back to their ParamSpec:
-
-       P.kwargs.__origin__ is P
-
-    This type is meant for runtime introspection and has no special meaning to
-    static type checkers.
-    """
-    def __init__(self, origin):
-        self.__origin__ = origin
-
-    def __repr__(self):
-        return f"{self.__origin__.__name__}.kwargs"
-
-    def __eq__(self, other):
-        if not isinstance(other, ParamSpecKwargs):
-            return NotImplemented
-        return self.__origin__ == other.__origin__
-
-    def __mro_entries__(self, bases):
-        raise TypeError(f"Cannot subclass an instance of {type(self).__name__}")
-
-
-class ParamSpec(_Final, _Immutable, _BoundVarianceMixin, _PickleUsingNameMixin,
-                _root=True):
-    """Parameter specification variable.
-
-    Usage::
-
-       P = ParamSpec('P')
-
-    Parameter specification variables exist primarily for the benefit of static
-    type checkers.  They are used to forward the parameter types of one
-    callable to another callable, a pattern commonly found in higher order
-    functions and decorators.  They are only valid when used in ``Concatenate``,
-    or as the first argument to ``Callable``, or as parameters for user-defined
-    Generics.  See class Generic for more information on generic types.  An
-    example for annotating a decorator::
-
-       T = TypeVar('T')
-       P = ParamSpec('P')
-
-       def add_logging(f: Callable[P, T]) -> Callable[P, T]:
-           '''A type-safe decorator to add logging to a function.'''
-           def inner(*args: P.args, **kwargs: P.kwargs) -> T:
-               logging.info(f'{f.__name__} was called')
-               return f(*args, **kwargs)
-           return inner
-
-       @add_logging
-       def add_two(x: float, y: float) -> float:
-           '''Add two numbers together.'''
-           return x + y
-
-    Parameter specification variables defined with covariant=True or
-    contravariant=True can be used to declare covariant or contravariant
-    generic types.  These keyword arguments are valid, but their actual semantics
-    are yet to be decided.  See PEP 612 for details.
-
-    Parameter specification variables can be introspected. e.g.:
-
-       P.__name__ == 'P'
-       P.__bound__ == None
-       P.__covariant__ == False
-       P.__contravariant__ == False
-
-    Note that only parameter specification variables defined in global scope can
-    be pickled.
-    """
+                new_args.extend(new_arg)
+            else:
+                new_args.append(new_arg)
+        params = tuple(new_args)
 
-    @property
-    def args(self):
-        return ParamSpecArgs(self)
+    return _GenericAlias(cls, params)
 
-    @property
-    def kwargs(self):
-        return ParamSpecKwargs(self)
-
-    def __init__(self, name, *, bound=None, covariant=False, contravariant=False):
-        self.__name__ = name
-        super().__init__(bound, covariant, contravariant)
-        def_mod = _caller()
-        if def_mod != 'typing':
-            self.__module__ = def_mod
 
-    def __typing_subst__(self, arg):
-        if isinstance(arg, (list, tuple)):
-            arg = tuple(_type_check(a, "Expected a type.") for a in arg)
-        elif not _is_param_expr(arg):
-            raise TypeError(f"Expected a list of types, an ellipsis, "
-                            f"ParamSpec, or Concatenate. Got {arg}")
-        return arg
+def _generic_init_subclass(cls, *args, **kwargs):
+    super(Generic, cls).__init_subclass__(*args, **kwargs)
+    tvars = []
+    if '__orig_bases__' in cls.__dict__:
+        error = Generic in cls.__orig_bases__
+    else:
+        error = (Generic in cls.__bases__ and
+                    cls.__name__ != 'Protocol' and
+                    type(cls) != _TypedDictMeta)
+    if error:
+        raise TypeError("Cannot inherit from plain Generic")
+    if '__orig_bases__' in cls.__dict__:
+        tvars = _collect_parameters(cls.__orig_bases__)
+        # Look for Generic[T1, ..., Tn].
+        # If found, tvars must be a subset of it.
+        # If not found, tvars is it.
+        # Also check for and reject plain Generic,
+        # and reject multiple Generic[...].
+        gvars = None
+        for base in cls.__orig_bases__:
+            if (isinstance(base, _GenericAlias) and
+                    base.__origin__ is Generic):
+                if gvars is not None:
+                    raise TypeError(
+                        "Cannot inherit from Generic[...] multiple times.")
+                gvars = base.__parameters__
+        if gvars is not None:
+            tvarset = set(tvars)
+            gvarset = set(gvars)
+            if not tvarset <= gvarset:
+                s_vars = ', '.join(str(t) for t in tvars if t not in gvarset)
+                s_args = ', '.join(str(g) for g in gvars)
+                raise TypeError(f"Some type variables ({s_vars}) are"
+                                f" not listed in Generic[{s_args}]")
+            tvars = gvars
+    cls.__parameters__ = tuple(tvars)
 
-    def __typing_prepare_subst__(self, alias, args):
-        params = alias.__parameters__
-        i = params.index(self)
-        if i >= len(args):
-            raise TypeError(f"Too few arguments for {alias}")
-        # Special case where Z[[int, str, bool]] == Z[int, str, bool] in PEP 612.
-        if len(params) == 1 and not _is_param_expr(args[0]):
-            assert i == 0
-            args = (args,)
-        # Convert lists to tuples to help other libraries cache the results.
-        elif isinstance(args[i], list):
-            args = (*args[:i], tuple(args[i]), *args[i+1:])
-        return args
 
 def _is_dunder(attr):
     return attr.startswith('__') and attr.endswith('__')
@@ -1812,113 +1656,6 @@ class _UnpackGenericAlias(_GenericAlias, _root=True):
         return isinstance(self.__args__[0], TypeVarTuple)
 
 
-class Generic:
-    """Abstract base class for generic types.
-
-    A generic type is typically declared by inheriting from
-    this class parameterized with one or more type variables.
-    For example, a generic mapping type might be defined as::
-
-      class Mapping(Generic[KT, VT]):
-          def __getitem__(self, key: KT) -> VT:
-              ...
-          # Etc.
-
-    This class can then be used as follows::
-
-      def lookup_name(mapping: Mapping[KT, VT], key: KT, default: VT) -> VT:
-          try:
-              return mapping[key]
-          except KeyError:
-              return default
-    """
-    __slots__ = ()
-    _is_protocol = False
-
-    @_tp_cache
-    def __class_getitem__(cls, params):
-        """Parameterizes a generic class.
-
-        At least, parameterizing a generic class is the *main* thing this method
-        does. For example, for some generic class `Foo`, this is called when we
-        do `Foo[int]` - there, with `cls=Foo` and `params=int`.
-
-        However, note that this method is also called when defining generic
-        classes in the first place with `class Foo(Generic[T]): ...`.
-        """
-        if not isinstance(params, tuple):
-            params = (params,)
-
-        params = tuple(_type_convert(p) for p in params)
-        if cls in (Generic, Protocol):
-            # Generic and Protocol can only be subscripted with unique type variables.
-            if not params:
-                raise TypeError(
-                    f"Parameter list to {cls.__qualname__}[...] cannot be empty"
-                )
-            if not all(_is_typevar_like(p) for p in params):
-                raise TypeError(
-                    f"Parameters to {cls.__name__}[...] must all be type variables "
-                    f"or parameter specification variables.")
-            if len(set(params)) != len(params):
-                raise TypeError(
-                    f"Parameters to {cls.__name__}[...] must all be unique")
-        else:
-            # Subscripting a regular Generic subclass.
-            for param in cls.__parameters__:
-                prepare = getattr(param, '__typing_prepare_subst__', None)
-                if prepare is not None:
-                    params = prepare(cls, params)
-            _check_generic(cls, params, len(cls.__parameters__))
-
-            new_args = []
-            for param, new_arg in zip(cls.__parameters__, params):
-                if isinstance(param, TypeVarTuple):
-                    new_args.extend(new_arg)
-                else:
-                    new_args.append(new_arg)
-            params = tuple(new_args)
-
-        return _GenericAlias(cls, params)
-
-    def __init_subclass__(cls, *args, **kwargs):
-        super().__init_subclass__(*args, **kwargs)
-        tvars = []
-        if '__orig_bases__' in cls.__dict__:
-            error = Generic in cls.__orig_bases__
-        else:
-            error = (Generic in cls.__bases__ and
-                        cls.__name__ != 'Protocol' and
-                        type(cls) != _TypedDictMeta)
-        if error:
-            raise TypeError("Cannot inherit from plain Generic")
-        if '__orig_bases__' in cls.__dict__:
-            tvars = _collect_parameters(cls.__orig_bases__)
-            # Look for Generic[T1, ..., Tn].
-            # If found, tvars must be a subset of it.
-            # If not found, tvars is it.
-            # Also check for and reject plain Generic,
-            # and reject multiple Generic[...].
-            gvars = None
-            for base in cls.__orig_bases__:
-                if (isinstance(base, _GenericAlias) and
-                        base.__origin__ is Generic):
-                    if gvars is not None:
-                        raise TypeError(
-                            "Cannot inherit from Generic[...] multiple times.")
-                    gvars = base.__parameters__
-            if gvars is not None:
-                tvarset = set(tvars)
-                gvarset = set(gvars)
-                if not tvarset <= gvarset:
-                    s_vars = ', '.join(str(t) for t in tvars if t not in gvarset)
-                    s_args = ', '.join(str(g) for g in gvars)
-                    raise TypeError(f"Some type variables ({s_vars}) are"
-                                    f" not listed in Generic[{s_args}]")
-                tvars = gvars
-        cls.__parameters__ = tuple(tvars)
-
-
 class _TypingEllipsis:
     """Internal placeholder for ... (ellipsis)."""
 
@@ -2024,6 +1761,18 @@ def _lazy_load_getattr_static():
 
 _cleanups.append(_lazy_load_getattr_static.cache_clear)
 
+def _pickle_psargs(psargs):
+    return ParamSpecArgs, (psargs.__origin__,)
+
+copyreg.pickle(ParamSpecArgs, _pickle_psargs)
+
+def _pickle_pskwargs(pskwargs):
+    return ParamSpecKwargs, (pskwargs.__origin__,)
+
+copyreg.pickle(ParamSpecKwargs, _pickle_pskwargs)
+
+del _pickle_psargs, _pickle_pskwargs
+
 
 class _ProtocolMeta(ABCMeta):
     # This metaclass is really unfortunate and exists only because of
@@ -2943,7 +2692,7 @@ class NamedTupleMeta(type):
                                module=ns['__module__'])
         nm_tpl.__bases__ = bases
         if Generic in bases:
-            class_getitem = Generic.__class_getitem__.__func__
+            class_getitem = _generic_class_getitem
             nm_tpl.__class_getitem__ = classmethod(class_getitem)
         # update from user namespace without overriding special namedtuple attributes
         for key in ns: