bpo-41559: Change PEP 612 implementation to pure Python (#25449)

Lib/_collections_abc.py

@@ -443,6 +443,18 @@ class _CallableGenericAlias(GenericAlias):
             ga_args = args
         return super().__new__(cls, origin, ga_args)
 
+    @property
+    def __parameters__(self):
+        params = []
+        for arg in self.__args__:
+            # Looks like a genericalias
+            if hasattr(arg, "__parameters__") and isinstance(arg.__parameters__, tuple):
+                params.extend(arg.__parameters__)
+            else:
+                if _is_typevarlike(arg):
+                    params.append(arg)
+        return tuple(dict.fromkeys(params))
+
     def __repr__(self):
         if _has_special_args(self.__args__):
             return super().__repr__()
@@ -458,16 +470,50 @@ class _CallableGenericAlias(GenericAlias):
 
     def __getitem__(self, item):
         # Called during TypeVar substitution, returns the custom subclass
-        # rather than the default types.GenericAlias object.
-        ga = super().__getitem__(item)
-        args = ga.__args__
-        # args[0] occurs due to things like Z[[int, str, bool]] from PEP 612
-        if not isinstance(ga.__args__[0], tuple):
-            t_result = ga.__args__[-1]
-            t_args = ga.__args__[:-1]
-            args = (t_args, t_result)
-        return _CallableGenericAlias(Callable, args)
+        # rather than the default types.GenericAlias object.  Most of the
+        # code is copied from typing's _GenericAlias and the builtin
+        # types.GenericAlias.
 
+        # A special case in PEP 612 where if X = Callable[P, int],
+        # then X[int, str] == X[[int, str]].
+        param_len = len(self.__parameters__)
+        if param_len == 0:
+            raise TypeError(f'There are no type or parameter specification'
+                            f'variables left in {self}')
+        if (param_len == 1
+                and isinstance(item, (tuple, list))
+                and len(item) > 1) or not isinstance(item, tuple):
+            item = (item,)
+        item_len = len(item)
+        if item_len != param_len:
+            raise TypeError(f'Too {"many" if item_len > param_len else "few"}'
+                            f' arguments for {self};'
+                            f' actual {item_len}, expected {param_len}')
+        subst = dict(zip(self.__parameters__, item))
+        new_args = []
+        for arg in self.__args__:
+            if _is_typevarlike(arg):
+                arg = subst[arg]
+            # Looks like a GenericAlias
+            elif hasattr(arg, '__parameters__') and isinstance(arg.__parameters__, tuple):
+                subparams = arg.__parameters__
+                if subparams:
+                    subargs = tuple(subst[x] for x in subparams)
+                    arg = arg[subargs]
+            new_args.append(arg)
+
+        # args[0] occurs due to things like Z[[int, str, bool]] from PEP 612
+        if not isinstance(new_args[0], (tuple, list)):
+            t_result = new_args[-1]
+            t_args = new_args[:-1]
+            new_args = (t_args, t_result)
+        return _CallableGenericAlias(Callable, tuple(new_args))
+
+def _is_typevarlike(arg):
+    obj = type(arg)
+    # looks like a TypeVar/ParamSpec
+    return (obj.__module__ == 'typing'
+            and obj.__name__ in {'ParamSpec', 'TypeVar'})
 
 def _has_special_args(args):
     """Checks if args[0] matches either ``...``, ``ParamSpec`` or