[FIX] MDS: Handle subset data

[VesnaT]

Issue

Fixes #3617

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3620 into master will decrease coverage by <.01%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #3620      +/-   ##
==========================================
- Coverage   84.07%   84.07%   -0.01%     
==========================================
  Files         370      370              
  Lines       67256    67267      +11     
==========================================
+ Hits        56546    56553       +7     
- Misses      10710    10714       +4

[janezd]

I agree with extracting _handle_subset_data to reduce the complexity of handleNewSignals.

But I'm not too happy with the fix. The root cause of the bug is that handleNewSignals does not call super().handleNewSignals(). We'll get similar bugs every time somebody adds something to OWDataProjectionWidget.handleNewSignals (or OWDataProjectionWidget.setup_plot(): OWDataProjectionWidget.handleNewSignals calls it and OWMDS.handleNewSignals doesn't!).

Derived methods that don't call inherited methods are a violation of OOP principles and a potential source of problems (especially when unexpected) that we should avoid. (Please don't say look, who's preaching! :))

You can:

• change OWMDS.handleNewSignals to call the inherited method, possibly moving some code from OWMDS.handleNewSignals to OWMDS.setup_plot, or
• remove OWMDS.handleNewSignals entirely and move all of its (additional) code to OWMDS.setup_plot (I would prefer this solution because this code indeed seems to belong there),
• convince me that above solutions are too impractical.