url_base_pathname broken

[iynehz]

The vanilla Dash class can take url_base_pathname and set both requests_pathname_prefix and routes_pathname_prefix [1][2]. The JupyterDash class, however, seems to have different behavior that it sets its own requests_pathname_prefix attribute, and defaults it to "/" regardless if user specifies url_base_pathname.. So url_base_pathname itself as a init parameter results a broken JupyterDash app.

[1] https://github.com/plotly/dash/blob/e8ac94919105a91c76a966c21aca2ec7b0297e22/dash/_configs.py#L97 [2] https://github.com/plotly/dash/blob/e8ac94919105a91c76a966c21aca2ec7b0297e22/dash/_configs.py#L111 [3] https://github.com/plotly/jupyter-dash/blob/351cb7c24409b2312a474568b70005ce66eb31be/jupyter_dash/jupyter_app.py#L230