gh-128398: improve error messages when incorrectly using `with` and `async with` (#132218)

Improve the error message with a suggestion when an object supporting the synchronous
(resp. asynchronous) context manager protocol is entered using `async with` (resp. `with`)
instead of `with` (resp. `async with`).

2 files changed, 22 insertions, 5 deletions

diff --git a/Lib/unittest/async_case.py b/Lib/unittest/async_case.py
index 6000af1cef0..a1c0d6c368c 100644
--- a/Lib/unittest/async_case.py
+++ b/Lib/unittest/async_case.py
@@ -75,9 +75,17 @@ class IsolatedAsyncioTestCase(TestCase):
             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
+            msg = (f"'{cls.__module__}.{cls.__qualname__}' object does "
+                   "not support the asynchronous context manager protocol")
+            try:
+                cls.__enter__
+                cls.__exit__
+            except AttributeError:
+                pass
+            else:
+                msg += (" but it supports the context manager protocol. "
+                        "Did you mean to use enterContext()?")
+            raise TypeError(msg) from None
         result = await enter(cm)
         self.addAsyncCleanup(exit, cm, None, None, None)
         return result
diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 10c3b7e1223..884fc1b21f6 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -111,8 +111,17 @@ def _enter_context(cm, addcleanup):
         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
+        msg = (f"'{cls.__module__}.{cls.__qualname__}' object does "
+               "not support the context manager protocol")
+        try:
+            cls.__aenter__
+            cls.__aexit__
+        except AttributeError:
+            pass
+        else:
+            msg += (" but it supports the asynchronous context manager "
+                    "protocol. Did you mean to use enterAsyncContext()?")
+        raise TypeError(msg) from None
     result = enter(cm)
     addcleanup(exit, cm, None, None, None)
     return result