Fix cyclic import context help

[feerrenrut]

Link to issue number:

#11950

Summary of the issue:

When installing add-ons which imported gui in their InstallTasks module NVDA failed to restart properly.

This was caused by the fact that when gui was imported it also imported contextHelp.ContextHelpMixin and contextHelp in turn also imports gui causing circular import.

Description of how this pull request fixes the issue:

Alternative fix to #11951
Breaks the circular import in contextHelp by making imports of gui, ui, and queueHandler dynamic imports.

This exposes another problem, the order of imports mattered. Several modules imported by gui, depended on ContextHelpMixin (imported name) in the gui module. This is fixed by referring directly to gui.contextHelp.ContextHelpMixin rather than gui.ContextHelpMixin in all those files. To prevent this from happening again (or being used by add-ons) it has been imported as _ContextHelpMixin.

Testing performed:

Tested as per #11950

Known issues with pull request:

Dependencies and layout of code in gui package needs attention. This would be an API breaking change, as such it can happen in 2021.1

Change log entry:

None

[lukaszgo1]

Aside from failing Linter checks and some cosmetic changes which I've requested below this looks fine to me and resolves this issue on my side. Nice work!

[feerrenrut]

I don't think these imports should be considered redundant. The gui package now no longer explicitly exposes that name, if no other module imported ContextHelp it wouldn't be found in the gui package. Depending on this is depending on an implementation detail. Instead I think it is better to be explicit.

For more (and better) descriptions of this see:

• the submodules are added to the package namespace trap
• this stack overflow answer