Merge pull request #3798 from plotly/fix/init-flask-run-command

fix blueprint registering and double init

Author: T4rk1n

dash/backends/_quart.py

@@ -122,7 +122,6 @@ def create_app(
     def register_assets_blueprint(
         self, blueprint_name: str, assets_url_path: str, assets_folder: str  # type: ignore[name-defined]
     ):
-
         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.
@@ -731,6 +733,17 @@ 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
+
+        # 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(

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