init app check instead of backend blueprint check

Author: T4rk1n

dash/backends/_fastapi.py

@@ -272,11 +272,6 @@ def create_app(name: str = "__main__", config: Dict[str, Any] | None = None):
     def register_assets_blueprint(
         self, blueprint_name: str, assets_url_path: str, assets_folder: str
     ):
-        # Check if route is already mounted to avoid duplicate registration
-        # This can happen when init_app() is called multiple times
-        for route in self.server.routes:
-            if getattr(route, "name", None) == blueprint_name:
-                return
         try:
             self.server.mount(
                 assets_url_path,

dash/backends/_flask.py

@@ -86,10 +86,6 @@ def create_app(name: str = "__main__", config: Dict[str, Any] | None = None):
     def register_assets_blueprint(
         self, blueprint_name: str, assets_url_path: str, assets_folder: str
     ):
-        # Check if blueprint is already registered to avoid duplicate registration
-        # This can happen when init_app() is called multiple times (e.g., with flask run)
-        if blueprint_name in self.server.blueprints:
-            return
         bp = Blueprint(
             blueprint_name,
             __name__,

dash/backends/_quart.py

@@ -122,10 +122,6 @@ def create_app(
     def register_assets_blueprint(
         self, blueprint_name: str, assets_url_path: str, assets_folder: str  # type: ignore[name-defined]
     ):
-        # Check if blueprint is already registered to avoid duplicate registration
-        # This can happen when init_app() is called multiple times (e.g., with quart run)
-        if blueprint_name in self.server.blueprints:
-            return
         bp = Blueprint(
             blueprint_name,
             __name__,

dash/dash.py

@@ -135,6 +135,8 @@
 _ID_STORE = "_pages_store"
 _ID_DUMMY = "_pages_dummy"
 
+_UNINITIALIZED = object()  # Sentinel for tracking init_app state
+
 DASH_VERSION_URL = "https://dash-version.plotly.com:8080/current_version"
 
 # Handles the case in a newly cloned environment where the components are not yet generated.
@@ -734,6 +736,14 @@ def init_app(self, app: Optional[Any] = None, **kwargs) -> None:
             # Also update the backend's server reference so routes are registered
             # on the correct server (important when using server=False pattern)
             self.backend.server = app
+
+        # Skip registration if already initialized on this server
+        # This prevents double registration when init_app() is called multiple times
+        # (e.g., with flask run pattern where __init__ calls init_app, then user does too)
+        if getattr(self, "_initialized_server", _UNINITIALIZED) is self.server:
+            return
+        self._initialized_server = self.server
+
         bp_prefix = config.routes_pathname_prefix.replace("/", "_").replace(".", "_")
         assets_blueprint_name = f"{bp_prefix}dash_assets"
         self.backend.register_assets_blueprint(