We have a proper running Dash 4.1 application. Upon testing 4.2.0rc3 and starting the application with flask run we were greeted with: ValueError: The name '_dash_assets' is already registered for a different blueprint. Use 'name=' to provide a unique name.
Full stack:
(venv) PS C:\Users\user\Python\dash-site> flask run
2026-05-14 13:39:02 - INFO - Settings class: dash.config.LocalDevelopmentConfig
2026-05-14 13:39:03 - INFO - Database URI: sqlite:///C:\Users\user\Python\dash-site\instance\dwsc.sqlite3
2026-05-14 13:39:06 - CRITICAL - Uncaught exception
Traceback (most recent call last):
File "<frozen runpy>", line 198, in _run_module_as_main
File "<frozen runpy>", line 88, in _run_code
File "C:\Users\user\Python\dash-site\venv\Scripts\flask.exe\__main__.py", line 5, in <module>
sys.exit(main())
~~~~^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 1131, in main
cli.main()
~~~~~~~~^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 1435, in main
rv = self.invoke(ctx)
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 1902, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 1298, in invoke
return ctx.invoke(self.callback, **ctx.params)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 853, in invoke
return callback(*args, **kwargs)
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\decorators.py", line 93, in new_func
return ctx.invoke(f, obj, *args, **kwargs)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\click\core.py", line 853, in invoke
return callback(*args, **kwargs)
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 979, in run_command
raise e from None
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 963, in run_command
app: WSGIApplication = info.load_app() # pyright: ignore
~~~~~~~~~~~~~^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 349, in load_app
app = locate_app(import_name, name)
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 262, in locate_app
return find_best_app(module)
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\cli.py", line 72, in find_best_app
app = app_factory()
File "C:\Users\user\Python\dash-site\dash\__init__.py", line 180, in create_app
dash_app.init_app(app)
~~~~~~~~~~~~~~~~~^^^^^
File "C:\Users\user\Python\dash-site\dash\dash_app.py", line 505, in init_app
app.init_app(server)
~~~~~~~~~~~~^^^^^^^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\dash\dash.py", line 734, in init_app
self.backend.register_assets_blueprint(
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
assets_blueprint_name,
^^^^^^^^^^^^^^^^^^^^^^
config.routes_pathname_prefix + self.config.assets_url_path.lstrip("/"),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
self.config.assets_folder,
^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\dash\backends\_flask.py", line 95, in register_assets_blueprint
self.server.register_blueprint(bp)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\sansio\scaffold.py", line 47, in wrapper_func
return f(self, *args, **kwargs)
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\sansio\app.py", line 595, in register_blueprint
blueprint.register(self, options)
~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
File "C:\Users\user\Python\dash-site\venv\Lib\site-packages\flask\sansio\blueprints.py", line 310, in register
raise ValueError(
...<3 lines>...
)
ValueError: The name '_dash_assets' is already registered for a different blueprint. Use 'name=' to provide a unique name.
(venv) PS C:\Users\user\Python\dash-site>I realize this is not a MRE. If this is needed to troubleshoot, I can invest the time to try to create one.
We have a proper running Dash 4.1 application. Upon testing 4.2.0rc3 and starting the application with
flask runwe were greeted with:ValueError: The name '_dash_assets' is already registered for a different blueprint. Use 'name=' to provide a unique name.Full stack:
I realize this is not a MRE. If this is needed to troubleshoot, I can invest the time to try to create one.