bpo-44471: Change error type for bad objects in ExitStack.enter_context() (GH-26820)

A TypeError is now raised instead of an AttributeError in
ExitStack.enter_context() and AsyncExitStack.enter_async_context()
for objects which do not support the context manager or
asynchronous context manager protocols correspondingly.

Lib/contextlib.py

@@ -473,9 +473,14 @@ class _BaseExitStack:
         """
         # We look up the special methods on the type to match the with
         # statement.
-        _cm_type = type(cm)
-        _exit = _cm_type.__exit__
-        result = _cm_type.__enter__(cm)
+        cls = type(cm)
+        try:
+            _enter = cls.__enter__
+            _exit = cls.__exit__
+        except AttributeError:
+            raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does "
+                            f"not support the context manager protocol") from None
+        result = _enter(cm)
         self._push_cm_exit(cm, _exit)
         return result
 
@@ -600,9 +605,15 @@ class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager):
         If successful, also pushes its __aexit__ method as a callback and
         returns the result of the __aenter__ method.
         """
-        _cm_type = type(cm)
-        _exit = _cm_type.__aexit__
-        result = await _cm_type.__aenter__(cm)
+        cls = type(cm)
+        try:
+            _enter = cls.__aenter__
+            _exit = cls.__aexit__
+        except AttributeError:
+            raise TypeError(f"'{cls.__module__}.{cls.__qualname__}' object does "
+                            f"not support the asynchronous context manager protocol"
+                           ) from None
+        result = await _enter(cm)
         self._push_async_cm_exit(cm, _exit)
         return result