Made catch() raise TypeError on async handler (#69)

Co-authored-by: Alex Grönholm <alex.gronholm@nextday.fi>

CHANGES.rst

@@ -3,6 +3,11 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+- `catch()` now raises a `TypeError` if passed an async exception handler instead of
+  just giving a `RuntimeWarning` about the coroutine never being awaited. (#66, PR by
+  John Litborn)
+
 **1.1.2**
 
 - Changed handling of exceptions in exception group handler callbacks to not wrap a

src/exceptiongroup/_catch.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import inspect
 import sys
 from collections.abc import Callable, Iterable, Mapping
 from contextlib import AbstractContextManager
@@ -49,9 +50,15 @@ def handle_exception(self, exc: BaseException) -> BaseException | None:
             matched, excgroup = excgroup.split(exc_types)
             if matched:
                 try:
-                    handler(matched)
+                    result = handler(matched)
                 except BaseException as new_exc:
                     new_exceptions.append(new_exc)
+                else:
+                    if inspect.iscoroutine(result):
+                        raise TypeError(
+                            f"Error trying to handle {matched!r} with {handler!r}. "
+                            "Exception handler must be a sync function."
+                        ) from exc
 
             if not excgroup:
                 break

tests/test_catch.py

@@ -162,3 +162,17 @@ def test_catch_subclass():
     assert isinstance(lookup_errors[0], ExceptionGroup)
     exceptions = lookup_errors[0].exceptions
     assert isinstance(exceptions[0], KeyError)
+
+
+def test_async_handler(request):
+    async def handler(eg):
+        pass
+
+    def delegate(eg):
+        coro = handler(eg)
+        request.addfinalizer(coro.close)
+        return coro
+
+    with pytest.raises(TypeError, match="Exception handler must be a sync function."):
+        with catch({TypeError: delegate}):
+            raise ExceptionGroup("message", TypeError("uh-oh"))