Correct type annotations in uvicorn/importer.py

#1046
#1067

The relationship between module imports and calls in uvicorn/config.py
was previously unclear. `import_from_string` was annotated as returning
a `ModuleType`, but was being used as if it were returning callables.
Mypy was raising "module not callable" errors, as reported in #1046.

Current use cases of `import_from_string` include:

- `uvicorn.Config.HTTP_PROTOCOLS`, `uvicorn.Config.WS_PROTOCOLS`:
  `Type[asyncio.Protocol]` (these are subclasses of asyncio protocols,
  so they are annotated with `Type[Class]` as explained in the mypy docs
- `uvicorn.Config.LOOP_SETUPS`: `Callable` (each loop is a function)
- `uvicorn.Config().loaded_app`: `ASGIApplication` (should be an ASGI
  application instance, like `Starlette()` or `FastAPI()`)

Ideally, the return type of `import_from_string` would reflect these use
cases, but the complex typing will be difficult to maintain. For easier
maintenance, the return type will be `Any`, and objects returned by
`import_from_string` will be annotated directly.

setup.cfg

@@ -14,6 +14,7 @@ files =
     uvicorn/middleware/message_logger.py,
     uvicorn/supervisors/basereload.py,
     uvicorn/importer.py,
+    tests/importer/test_importer.py,
     uvicorn/protocols/utils.py,
     uvicorn/loops,
     uvicorn/main.py,

tests/importer/test_importer.py

@@ -3,40 +3,40 @@
 from uvicorn.importer import ImportFromStringError, import_from_string
 
 
-def test_invalid_format():
+def test_invalid_format() -> None:
     with pytest.raises(ImportFromStringError) as exc_info:
         import_from_string("example:")
     expected = 'Import string "example:" must be in format "<module>:<attribute>".'
     assert expected in str(exc_info.value)
 
 
-def test_invalid_module():
+def test_invalid_module() -> None:
     with pytest.raises(ImportFromStringError) as exc_info:
         import_from_string("module_does_not_exist:myattr")
     expected = 'Could not import module "module_does_not_exist".'
     assert expected in str(exc_info.value)
 
 
-def test_invalid_attr():
+def test_invalid_attr() -> None:
     with pytest.raises(ImportFromStringError) as exc_info:
         import_from_string("tempfile:attr_does_not_exist")
     expected = 'Attribute "attr_does_not_exist" not found in module "tempfile".'
     assert expected in str(exc_info.value)
 
 
-def test_internal_import_error():
+def test_internal_import_error() -> None:
     with pytest.raises(ImportError):
         import_from_string("tests.importer.raise_import_error:myattr")
 
 
-def test_valid_import():
+def test_valid_import() -> None:
     instance = import_from_string("tempfile:TemporaryFile")
     from tempfile import TemporaryFile
 
     assert instance == TemporaryFile
 
 
-def test_no_import_needed():
+def test_no_import_needed() -> None:
     from tempfile import TemporaryFile
 
     instance = import_from_string(TemporaryFile)

uvicorn/_handlers/http.py

@@ -36,7 +36,7 @@ async def handle_http(
     connection_lost = loop.create_future()
 
     # Switch the protocol from the stream reader to our own HTTP protocol class.
-    protocol = config.http_protocol_class(
+    protocol = config.http_protocol_class(  # type: ignore[call-arg, operator]
         config=config,
         server_state=server_state,
         on_connection_lost=lambda: connection_lost.set_result(True),

uvicorn/config.py

@@ -316,12 +316,14 @@ def load(self) -> None:
         )
 
         if isinstance(self.http, str):
-            self.http_protocol_class = import_from_string(HTTP_PROTOCOLS[self.http])
+            http_protocol_class = import_from_string(HTTP_PROTOCOLS[self.http])
+            self.http_protocol_class: Type[asyncio.Protocol] = http_protocol_class
         else:
             self.http_protocol_class = self.http
 
         if isinstance(self.ws, str):
-            self.ws_protocol_class = import_from_string(WS_PROTOCOLS[self.ws])
+            ws_protocol_class = import_from_string(WS_PROTOCOLS[self.ws])
+            self.ws_protocol_class: Optional[Type[asyncio.Protocol]] = ws_protocol_class
         else:
             self.ws_protocol_class = self.ws
 
@@ -374,9 +376,13 @@ def load(self) -> None:
         self.loaded = True
 
     def setup_event_loop(self) -> None:
-        loop_setup = import_from_string(LOOP_SETUPS[self.loop])
-        if loop_setup is not None:
-            loop_setup()
+        loop_setup_str: Optional[str] = LOOP_SETUPS[self.loop]
+        if loop_setup_str:
+            loop_setup: Callable = import_from_string(loop_setup_str)
+            if not inspect.isfunction(loop_setup):
+                raise ImportFromStringError("Asyncio event loop must be a callable.")
+            else:
+                loop_setup()
 
     def bind_socket(self) -> socket.socket:
         family = socket.AF_INET

uvicorn/importer.py

@@ -1,13 +1,12 @@
 import importlib
-from types import ModuleType
-from typing import Union
+from typing import Any
 
 
 class ImportFromStringError(Exception):
     pass
 
 
-def import_from_string(import_str: Union[ModuleType, str]) -> ModuleType:
+def import_from_string(import_str: Any) -> Any:
     if not isinstance(import_str, str):
         return import_str