Reject generator functions in sync_to_async

asgiref/sync.py

@@ -426,10 +426,15 @@ def __init__(
         executor: Optional["ThreadPoolExecutor"] = None,
         context: Optional[contextvars.Context] = None,
     ) -> None:
+        func_call = getattr(func, "__call__", func)
         if (
             not callable(func)
             or iscoroutinefunction(func)
-            or iscoroutinefunction(getattr(func, "__call__", func))
+            or iscoroutinefunction(func_call)
+            or inspect.isgeneratorfunction(func)
+            or inspect.isgeneratorfunction(func_call)
+            or inspect.isasyncgenfunction(func)
+            or inspect.isasyncgenfunction(func_call)
         ):
             raise TypeError("sync_to_async can only be applied to sync functions.")
 

tests/test_sync.py

@@ -74,6 +74,22 @@ def test_sync_to_async_fail_non_function():
     )
 
 
+def test_sync_to_async_fail_generator_function():
+    """
+    sync_to_async raises a TypeError when applied to a generator function.
+    """
+
+    def test_function():
+        yield 1
+
+    with pytest.raises(TypeError) as excinfo:
+        sync_to_async(test_function)
+
+    assert excinfo.value.args == (
+        "sync_to_async can only be applied to sync functions.",
+    )
+
+
 @pytest.mark.asyncio
 async def test_sync_to_async_fail_async():
     """
@@ -120,6 +136,17 @@ async def __call__(self):
         sync_to_async(CallableClass())
 
 
+def test_sync_to_async_raises_typeerror_for_generator_callable_instance():
+    class CallableClass:
+        def __call__(self):
+            yield 1
+
+    with pytest.raises(
+        TypeError, match="sync_to_async can only be applied to sync functions."
+    ):
+        sync_to_async(CallableClass())
+
+
 @pytest.mark.asyncio
 async def test_sync_to_async_decorator():
     """