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.

2 files changed, 55 insertions, 2 deletions

diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index dbc3f5f..9c27866 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -661,6 +661,25 @@ class TestBaseExitStack:
             result.append(2)
         self.assertEqual(result, [1, 2, 3, 4])
 
+    def test_enter_context_errors(self):
+        class LacksEnterAndExit:
+            pass
+        class LacksEnter:
+            def __exit__(self, *exc_info):
+                pass
+        class LacksExit:
+            def __enter__(self):
+                pass
+
+        with self.exit_stack() as stack:
+            with self.assertRaisesRegex(TypeError, 'the context manager'):
+                stack.enter_context(LacksEnterAndExit())
+            with self.assertRaisesRegex(TypeError, 'the context manager'):
+                stack.enter_context(LacksEnter())
+            with self.assertRaisesRegex(TypeError, 'the context manager'):
+                stack.enter_context(LacksExit())
+            self.assertFalse(stack._exit_callbacks)
+
     def test_close(self):
         result = []
         with self.exit_stack() as stack:
@@ -886,9 +905,11 @@ class TestBaseExitStack:
     def test_instance_bypass(self):
         class Example(object): pass
         cm = Example()
+        cm.__enter__ = object()
         cm.__exit__ = object()
         stack = self.exit_stack()
-        self.assertRaises(AttributeError, stack.enter_context, cm)
+        with self.assertRaisesRegex(TypeError, 'the context manager'):
+            stack.enter_context(cm)
         stack.push(cm)
         self.assertIs(stack._exit_callbacks[-1][1], cm)
 
diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
index cbc82df..7904abf 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -483,7 +483,7 @@ class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
             1/0
 
     @_async_test
-    async def test_async_enter_context(self):
+    async def test_enter_async_context(self):
         class TestCM(object):
             async def __aenter__(self):
                 result.append(1)
@@ -505,6 +505,26 @@ class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
         self.assertEqual(result, [1, 2, 3, 4])
 
     @_async_test
+    async def test_enter_async_context_errors(self):
+        class LacksEnterAndExit:
+            pass
+        class LacksEnter:
+            async def __aexit__(self, *exc_info):
+                pass
+        class LacksExit:
+            async def __aenter__(self):
+                pass
+
+        async with self.exit_stack() as stack:
+            with self.assertRaisesRegex(TypeError, 'asynchronous context manager'):
+                await stack.enter_async_context(LacksEnterAndExit())
+            with self.assertRaisesRegex(TypeError, 'asynchronous context manager'):
+                await stack.enter_async_context(LacksEnter())
+            with self.assertRaisesRegex(TypeError, 'asynchronous context manager'):
+                await stack.enter_async_context(LacksExit())
+            self.assertFalse(stack._exit_callbacks)
+
+    @_async_test
     async def test_async_exit_exception_chaining(self):
         # Ensure exception chaining matches the reference behaviour
         async def raise_exc(exc):
@@ -536,6 +556,18 @@ class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
         self.assertIsInstance(inner_exc, ValueError)
         self.assertIsInstance(inner_exc.__context__, ZeroDivisionError)
 
+    @_async_test
+    async def test_instance_bypass_async(self):
+        class Example(object): pass
+        cm = Example()
+        cm.__aenter__ = object()
+        cm.__aexit__ = object()
+        stack = self.exit_stack()
+        with self.assertRaisesRegex(TypeError, 'asynchronous context manager'):
+            await stack.enter_async_context(cm)
+        stack.push_async_exit(cm)
+        self.assertIs(stack._exit_callbacks[-1][1], cm)
+
 
 class TestAsyncNullcontext(unittest.TestCase):
     @_async_test