feat(config): ensure that the app loads correctly before the service starts

tests/test_cli.py

@@ -119,7 +119,10 @@ def test_cli_uds(uds_file: Path) -> None:  # pragma: py-win32
             result = runner.invoke(cli, ["tests.test_cli:App", "--workers=2", "--uds", str(uds_file)])
 
     assert result.exit_code == 0
-    assert result.output == ""
+    assert (
+        result.output
+        == "WARNING:  ASGI app factory detected. Using it, but please consider setting the --factory flag explicitly.\n"
+    )
     mock_bind_socket.assert_called_once()
     mock_run.assert_called_once()
     assert not uds_file.exists()

uvicorn/config.py

@@ -394,9 +394,31 @@ def configure_logging(self) -> None:
             logging.getLogger("uvicorn.access").handlers = []
             logging.getLogger("uvicorn.access").propagate = False
 
+    def check_load_app(self) -> Any:
+        try:
+            app = import_from_string(self.app)
+        except ImportFromStringError as exc:
+            logger.error("Error loading ASGI app. %s" % exc)
+            sys.exit(1)
+
+        try:
+            app = app()
+        except TypeError as exc:
+            if self.factory:
+                logger.error("Error loading ASGI app factory: %s", exc)
+                sys.exit(1)
+        else:
+            if not self.factory:
+                logger.warning(
+                    "ASGI app factory detected. Using it, but please consider setting the --factory flag explicitly."
+                )
+        return app
+
     def load(self) -> None:
         assert not self.loaded
 
+        self.loaded_app = self.check_load_app()
+
         if self.is_ssl:
             assert self.ssl_certfile
             self.ssl: ssl.SSLContext | None = create_ssl_context(
@@ -432,24 +454,6 @@ def load(self) -> None:
 
         self.lifespan_class = import_from_string(LIFESPAN[self.lifespan])
 
-        try:
-            self.loaded_app = import_from_string(self.app)
-        except ImportFromStringError as exc:
-            logger.error("Error loading ASGI app. %s" % exc)
-            sys.exit(1)
-
-        try:
-            self.loaded_app = self.loaded_app()
-        except TypeError as exc:
-            if self.factory:
-                logger.error("Error loading ASGI app factory: %s", exc)
-                sys.exit(1)
-        else:
-            if not self.factory:
-                logger.warning(
-                    "ASGI app factory detected. Using it, but please consider setting the --factory flag explicitly."
-                )
-
         if self.interface == "auto":
             if inspect.isclass(self.loaded_app):
                 use_asgi_3 = hasattr(self.loaded_app, "__await__")

uvicorn/main.py

@@ -564,6 +564,7 @@ def run(
         factory=factory,
         h11_max_incomplete_event_size=h11_max_incomplete_event_size,
     )
+    config.check_load_app()
     server = Server(config=config)
 
     if (config.reload or config.workers > 1) and not isinstance(app, str):
@@ -580,8 +581,8 @@ def run(
             Multiprocess(config, target=server.run, sockets=[sock]).run()
         else:
             server.run()
-    except KeyboardInterrupt:
-        pass  # pragma: full coverage
+    except KeyboardInterrupt:  # pragma: full coverage
+        pass
     finally:
         if config.uds and os.path.exists(config.uds):
             os.remove(config.uds)  # pragma: py-win32