Raise exception if you put app.function on a class method

modal/_utils/function_utils.py

@@ -47,10 +47,18 @@ def inner(filename):
     return inner
 
 
-def is_global_object(object_qual_name):
+def is_global_object(object_qual_name: str):
     return "<locals>" not in object_qual_name.split(".")
 
 
+def is_top_level_function(f: Callable) -> bool:
+    """Returns True if this function is defined in global scope.
+
+    Returns False if this function is locally scoped (including on a class).
+    """
+    return f.__name__ == f.__qualname__
+
+
 def is_async(function):
     # TODO: this is somewhat hacky. We need to know whether the function is async or not in order to
     # coerce the input arguments to the right type. The proper way to do is to call the function and
@@ -104,15 +112,6 @@ def __init__(
         elif f is None and user_cls:
             # "service function" for running all methods of a class
             self.function_name = f"{user_cls.__name__}.*"
-        elif f.__qualname__ != f.__name__ and not serialized:
-            # single method of a class - should be only @build-methods at this point
-            if len(f.__qualname__.split(".")) > 2:
-                raise InvalidError(
-                    f"Cannot wrap `{f.__qualname__}`:"
-                    " functions and classes used in Modal must be defined in global scope."
-                    " If trying to apply additional decorators, they may need to use `functools.wraps`."
-                )
-            self.function_name = f"{user_cls.__name__}.{f.__name__}"
         else:
             self.function_name = f.__qualname__
 

modal/app.py

@@ -4,6 +4,7 @@
 import warnings
 from io import TextIOWrapper
 from pathlib import PurePosixPath
+from textwrap import dedent
 from typing import Any, AsyncGenerator, Callable, ClassVar, Dict, List, Optional, Sequence, Tuple, Union
 
 from google.protobuf.message import Message
@@ -14,7 +15,7 @@
 from ._ipython import is_notebook
 from ._output import OutputManager
 from ._utils.async_utils import synchronize_api
-from ._utils.function_utils import FunctionInfo
+from ._utils.function_utils import FunctionInfo, is_global_object, is_top_level_function
 from ._utils.mount_utils import validate_volumes
 from .app_utils import (  # noqa: F401
     _list_apps,
@@ -533,7 +534,9 @@ def wrapped(
 
             # 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.")
+                raise TypeError(
+                    "The `@app.function` decorator cannot be used on a class. Please use `@app.cls` instead."
+                )
 
             if isinstance(f, _PartialFunction):
                 # typically for @function-wrapped @web_endpoint and @asgi_app
@@ -547,6 +550,35 @@ def wrapped(
                 if webhook_config and interactive:
                     raise InvalidError("interactive=True is not supported with web endpoint functions")
             else:
+                if not is_global_object(f.__qualname__) and not serialized:
+                    raise InvalidError(
+                        dedent(
+                            """
+                            The `@app.function` decorator must apply to functions in global scope,
+                            unless `serialize=True` is set.
+                            If trying to apply additional decorators, they may need to use `functools.wraps`.
+                            """
+                        )
+                    )
+
+                if not is_top_level_function(f) and is_global_object(f.__qualname__):
+                    raise InvalidError(
+                        dedent(
+                            """
+                            The `@app.function` decorator cannot be used on class methods.
+                            Please use `@app.cls` with `@modal.method` instead. Example:
+
+                            ```python
+                            @app.cls()
+                            class MyClass:
+                                @modal.method()
+                                def f(self, x):
+                                    ...
+                            ```
+                            """
+                        )
+                    )
+
                 info = FunctionInfo(f, serialized=serialized, name_override=name)
                 webhook_config = None
                 raw_f = f

test/function_test.py

@@ -790,3 +790,15 @@ def test_warn_on_local_volume_mount(client, servicer):
     assert modal.is_local()
     with pytest.warns(match="local"):
         dummy_function.local()
+
+
+class X:
+    def f(self):
+        ...
+
+
+def test_function_decorator_on_method():
+    app = modal.App()
+
+    with pytest.raises(InvalidError, match="@app.cls"):
+        app.function()(X.f)

test/function_utils_test.py

@@ -1,9 +1,7 @@
 # Copyright Modal Labs 2023
-import pytest
 
 from modal import method, web_endpoint
 from modal._utils.function_utils import FunctionInfo, method_has_params
-from modal.exception import InvalidError
 
 
 def hasarg(a):
@@ -49,14 +47,6 @@ def test_method_has_params():
     assert method_has_params(Cls().buz)
 
 
-def test_nonglobal_function():
-    def f():
-        ...
-
-    with pytest.raises(InvalidError, match=r"Cannot wrap `test_nonglobal_function.<locals>.f"):
-        FunctionInfo(f)
-
-
 class Foo:
     def __init__(self):
         pass