added validation to AppRegistry to make sure there are no duplicate app names

Author: dantownsend

piccolo/conf/apps.py

@@ -2,6 +2,7 @@
 from dataclasses import dataclass, field
 from importlib import import_module
 import inspect
+import itertools
 from types import ModuleType
 import typing as t
 
@@ -108,10 +109,32 @@ class AppRegistry:
 
     def __post_init__(self):
         self.app_configs: t.Dict[str, AppConfig] = {}
+        app_names = []
+
         for app in self.apps:
             app_conf_module = import_module(app)
             app_config: AppConfig = getattr(app_conf_module, "APP_CONFIG")
             self.app_configs[app_config.app_name] = app_config
+            app_names.append(app_config.app_name)
+
+        self._validate_app_names(app_names)
+
+    @staticmethod
+    def _validate_app_names(app_names: t.List[str]):
+        """
+        Raise a ValueError if an app_name is repeated.
+        """
+        app_names.sort()
+        grouped = itertools.groupby(app_names)
+        for key, value in grouped:
+            count = len([i for i in value])
+            if count > 1:
+                raise ValueError(
+                    f"There are {count} apps with the name `{key}`. This can "
+                    "cause unexpected behavior. Make sure each app has a "
+                    "unique name, and you haven't registered the same app "
+                    "multiple times."
+                )
 
     def get_app_config(self, app_name: str) -> t.Optional[AppConfig]:
         return self.app_configs.get(app_name)

piccolo/engine/postgres.py

@@ -223,7 +223,7 @@ def get_version(self) -> float:
             )
 
         try:
-            response: t.Sequence[t.Dict] = future.result()
+            response: t.Sequence[t.Dict] = future.result()  # type: ignore
         except ConnectionRefusedError as exception:
             # Suppressing the exception, otherwise importing piccolo_conf.py
             # containing an engine will raise an ImportError.

tests/conf/test_apps.py

@@ -9,6 +9,18 @@ def test_init(self):
         app_config = app_registry.get_app_config(app_name="user")
         self.assertTrue(isinstance(app_config, AppConfig))
 
+    def test_duplicate_app_names(self):
+        """
+        An exception should be if apps with duplicate names are registered.
+        """
+        with self.assertRaises(ValueError):
+            AppRegistry(
+                apps=[
+                    "piccolo.apps.user.piccolo_app",
+                    "piccolo.apps.user.piccolo_app",
+                ]
+            )
+
 
 class TestTableFinder(TestCase):
     def test_table_finder(self):