Tweak detection of app factories (#914)

tests/test_config.py

@@ -1,4 +1,5 @@
 import json
+import logging
 import socket
 from copy import deepcopy
 
@@ -100,18 +101,23 @@ def test_app_unimportable_other(caplog):
     )
 
 
-def test_app_factory():
+def test_app_factory(caplog):
     def create_app():
         return asgi_app
 
     config = Config(app=create_app, factory=True, proxy_headers=False)
     config.load()
     assert config.loaded_app is asgi_app
 
-    # Flag missing.
-    config = Config(app=create_app)
-    with pytest.raises(SystemExit):
+    # Flag not passed. In this case, successfully load the app, but issue a warning
+    # to indicate that an explicit flag is preferred.
+    caplog.clear()
+    config = Config(app=create_app, proxy_headers=False)
+    with caplog.at_level(logging.WARNING):
         config.load()
+    assert config.loaded_app is asgi_app
+    assert len(caplog.records) == 1
+    assert "--factory" in caplog.records[0].message
 
     # App not a no-arguments callable.
     config = Config(app=asgi_app, factory=True)

uvicorn/config.py

@@ -310,18 +310,18 @@ def load(self):
             logger.error("Error loading ASGI app. %s" % exc)
             sys.exit(1)
 
-        if self.factory:
-            try:
-                self.loaded_app = self.loaded_app()
-            except TypeError as exc:
+        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)
-        elif not inspect.signature(self.loaded_app).parameters:
-            logger.error(
-                "APP seems to be an application factory. "
-                "Run uvicorn with the --factory flag."
-            )
-            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):