Make ExtensionHandler a Mixin

[echarles]

This is a first dry attempt to support extension handlers needed by JupyterLab jupyterlab/jupyterlab_server#79

The corresponding issue is #173

This current changes allow make the jlab extensions operational, but this implementation is suboptimal as it duplicates code (with minor changes) from the base package.

[echarles]

@Zsailer @kevin-bates As discussed during the jupyter server meeting, another option is making the ExtensionHandler a Mixin. I will give it a try in this PR.

[Zsailer]

I think it's more appropriate to make ExtensionHandler a mixin class. None of the base Jupyter Server Handler classes should have to depend on ExtensionHandler. Rather, the ExtensionHandlerMixin should extend (pardon the pun) the base server handlers by mixing in extension-specific logic.

Unfortunately, this is a backwards-incompatible change from 0.2.x. I think we need to backport this to 0.2.x, because this could cause issues for us later if other projects depend on 0.2.x first and then switch to 0.3.x.

[echarles]

None of the base Jupyter Server Handler classes should have to depend on ExtensionHandler. Rather, the ExtensionHandlerMixin should extend (pardon the pun) the base server handlers by mixing in extension-specific logic.

@Zsailer Are you saying that ExtensionHandlerMixin should extend all base server handlers (TerminalHandlers...)? I am not sure to understand. Maybe a small asciitext class hierarchy will help to be sure what you have in mind

[echarles]

@Zsailer @kevin-bates I have renamed that PR from [WIP] Add extension handlers to [WIP] Make ExtensionHandler a Mixin and force-pushed the needed changes for this. I am using this branch for the jupyterlab-server migration to extension handlers and test infrastructure (mock...) usage.

This works well so far for the main LabHandler and I will further work on the tests for the other handlers like WorkspacesHandler and SettingsHandler.

Please note the construct I use now and which is subject to discussion/update: class SettingsHandler(ExtensionHandlerMixin, ExtensionHandlerJinjaMixin, APIHandler):

[Zsailer]

This is looking great, @echarles!

Please note the construct I use now and which is subject to discussion/update: class SettingsHandler(ExtensionHandlerMixin, ExtensionHandlerJinjaMixin, APIHandler):

This is exactly what I had in mind. 😎

It does make the inheritance list a little long, but I think it's the most readable, transparent, and Pythonic form.

[echarles]

@Zsailer I have progressed on the jupyterlab-server SettingsHandler using the jupyter-server test plugin and the ExtensionHandlerMixin. All good so far. We can confirm this at today server meeting and take a decision to move forward.

[Zsailer]

LGTM, @echarles! Any reason not to merge this?