When the--autoreload flag is passed, the webapp will watch for any changes to its Python source. On change, all changed packages will be reimported and the webapp will restart. Also works on Python source files in Jupyter server extensions. Implemented using the built inautoreload parameter in the constructor oftornado.web.Application, whichNotebookApp.web_app is a subclass of.

While I was working on aserver extension for Jupyterlab, I got pretty sick of having to kill and restart Jupyterlab every time I made changes to the Python source. This PR is an attempt to enable hot reloading for the sources ofnotebook and any extensions.

Though the autoreload behavior is currently a bit cumbersome (the Notebook server will reload completely from scratch, including (on Jupyterlab, at least) launching a new browser window with a new security token), I believe that it is useful as is. I also think that in the future it can definitely be improved, most obviously by recycling certain resources on reload, such as browser windows and security tokens.