Catch when users decorate class with @app.function() (#1801)

modal/app.py

@@ -549,6 +549,10 @@ def wrapped(
         ) -> _Function:
             nonlocal keep_warm, is_generator
 
+            # Check if the decorated object is a class
+            if inspect.isclass(f):
+                raise TypeError("The @app.function decorator cannot be used on a class. Please use @app.cls instead.")
+
             if isinstance(f, _PartialFunction):
                 f.wrapped = True
                 info = FunctionInfo(f.raw_f, serialized=serialized, name_override=name, cls=_cls)

test/app_test.py

@@ -275,6 +275,15 @@ def f():
     assert "function(image=image)" in str(excinfo.value)
 
 
+def test_function_decorator_on_class():
+    app = App()
+    with pytest.raises(TypeError, match="cannot be used on a class"):
+
+        @app.function()
+        class Foo:
+            pass
+
+
 @pytest.mark.asyncio
 async def test_deploy_disconnect(servicer, client):
     app = App()
@@ -356,6 +365,7 @@ def test_function_named_app():
     app = App()
 
     with pytest.warns(match="app"):
+
         @app.function(serialized=True)
         def app():
             ...