[MRG] add open_browser trait to ExtensionApp

[Zsailer]

Addresses #120.

Adds a trait to ExtensionApps to configure opening browser windows on launch.

[vidartf]

Should this also get a flag (inverted): --no-browser ?

[Zsailer]

The ExtensionApp actually already inherits the --no-browser flag from ServerApp before this PR (see here).

[vidartf]

That inherited flag reads as:

flags["no-browser"] = (
    {"ServerApp": {"open_browser": False}},
    _("Prevent the opening of the default url in the browser."),
)

Is it not an issue that it says ServerApp (the extension app does not inherit it)? If so, I imagine some other flags/aliases might have an issue too.

[echarles]

@vidartf As far as I have seen, the flags handling does not work as expected. I am looking at this and will open on issue (hopefully a PR). We can revisit this PR based on the findings/fixes we will get for the more global flag issue.

[echarles]

@Zsailer @vidartf

This branch fails on jupyter server --open-browser false [C 11:44:23.991 ServerApp] Unrecognized flag: '--open-browser'

I had that issue at other places (#117 and jupyterlab/jupyterlab#7416).

I have opened #140 to further discuss this before coming back here.

[vidartf]

@echarles That is not how it is intended to be used though. jupyter notebook --open-browser false also fails.

[vidartf]

PS: The right ways are either --NotebookApp.open_browser=False or via the flag --no-browser.

[vidartf]

PPS: Or in this case --ServerApp.open_browser=False or the flag.

[echarles]

After yesterday discussion at server weekly meeting, I now get the point that the extension flags should only be applied to an extension launched via its entrypoint.

BTW, this PR does not define flag for open_browser, so my previous attempt was out of context... Sorry for the confusion.

Going back to the parameters approach, I have tried to launch with --ServerApp.open_browser=False or --ExtensionApp.open_browser=False and it does not work (the browser if opened and it should not be).

Digging a bit more into the base ExtensionApp, I have the impression that the traits given via CLI are not taken into account by the ExtensionApp. e.g. jupyter simple-ext1 --ServerApp.custom_display_url=foo has no impact on the displayed URL.

Looking at the code that prepare the settings and printing those settings in the log, I feel that something must be done to feed the extension CLI config to the settings. Any input on this?

[Zsailer]

Is it not an issue that it says ServerApp (the extension app does not inherit it)? If so, I imagine some other flags/aliases might have an issue too.

@vidartf I don't think this is a problem. These traits should affect the ServerApp directly.

open_browser is a special case because ExtensionApps can affect the ServerApp.open_browser trait using its own open_browser trait.

[Zsailer]

Merging, since I've addressed the concerns above. We can open a future PR if needed.