[FIX] Save Data: Reset writer upon changing the extension

[ajdapretnar]

Issue

Save Data doesn't change the writer when the user changes the extension.

Description of changes

Upon changing the extension, writer is set accordingly.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3437 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3437      +/-   ##
==========================================
+ Coverage   83.25%   83.25%   +<.01%     
==========================================
  Files         365      365              
  Lines       64249    64247       -2     
==========================================
+ Hits        53489    53490       +1     
+ Misses      10760    10757       -3

[janezd]

What about eliminating attribute self.writer, and calling get_writer_selected() at the two places when the widget needs the writer? There is no need to store data that can be trivially computed from the data you already have.

It's preferable to have a single source of truth whenever possible and practical. The bug that you're fixing here occurred because there were two and they were not properly synchronized.

[ajdapretnar]

@janezd Fixed as suggested. Could you perhaps give it another look and merge, if everything is ok. #3306 depends on this.

[janezd]

I think that tests shouldn't call update_ext because the widget doesn't call it either.

I thus removed this and the related redundant change of method name. This simplifies the PR so that it fixes only the problem with the writer.

@lanzagar, can you quickly check that it's still OK? Changes are now minimal.