fix blueprint registering and double init

Author: T4rk1n

dash/backends/_fastapi.py

@@ -272,6 +272,11 @@ 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

@@ -86,6 +86,10 @@ 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

@@ -122,7 +122,10 @@ 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

@@ -731,6 +731,9 @@ def init_app(self, app: Optional[Any] = None, **kwargs) -> None:
         )
         if app is not None:
             self.server = app
+            # 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
         bp_prefix = config.routes_pathname_prefix.replace("/", "_").replace(".", "_")
         assets_blueprint_name = f"{bp_prefix}dash_assets"
         self.backend.register_assets_blueprint(

tests/unit/test_configs.py

@@ -524,3 +524,43 @@ def test_csrf_config_read_only():
         app.config.csrf_token_name = "something_else"
     with pytest.raises(AttributeError):
         app.config.csrf_header_name = "something_else"
+
+
+def test_init_app_with_flask_run_pattern():
+    """Test that init_app works correctly with the flask run pattern.
+
+    This tests the fix for https://github.com/plotly/dash/issues/3787
+    where using flask run would cause a ValueError about duplicate blueprint
+    registration because init_app was called twice (once automatically in
+    __init__ with server=True, and once by the user in their create_app factory).
+    """
+    # Simulate the flask run pattern where:
+    # 1. Dash app is created with server=True (default)
+    # 2. User's create_app factory calls init_app with their Flask server
+    external_server = Flask("external_test")
+    app = Dash(__name__)
+
+    # This should NOT raise "ValueError: The name '_dash_assets' is already registered"
+    app.init_app(external_server)
+
+    # Verify the backend now uses the external server
+    assert app.server is external_server
+    assert app.backend.server is external_server
+
+
+def test_init_app_server_false_pattern():
+    """Test that init_app works correctly when server=False is used.
+
+    This tests the fix for https://github.com/plotly/dash/issues/3787
+    where using server=False and then calling init_app would result in
+    404 errors because the backend's server reference was not updated.
+    """
+    external_server = Flask("external_test_false")
+    app = Dash(__name__, server=False)
+
+    # Call init_app with the external server
+    app.init_app(external_server)
+
+    # Verify both the app and backend use the external server
+    assert app.server is external_server
+    assert app.backend.server is external_server