Add DeprecationWarning when using Mapping for options

[jtpio]

Fixes #1958.

For Selection Widgets, add a DeprecationWarning whenever a Mapping (dictionaries) is used as options.

I'll do a second pass on the documentation to remove all mentions to dictionaries as possible values for options.

TODO

• Fix Python 2 test
• Second pass on the documentation

[jasongrout]

Thank you!

Can you remove the model_id changes in the PR for the notebook? That's churn that breaks the saved widget state in the notebook. We'll run all the notebooks before release and commit one big update with the updated widget state.

(to be clear, please keep the source text changes in the notebook, and the one place where we have printed output that would change).

[jtpio]

Sure thing! And it also makes the diff cleaner.

[jasongrout]

Adding to 7.3 - let's see if we can get this in...

[jasongrout]

Looks like there is a test failure. We'll try to get this in first thing for 7.4.

[jtpio]

The failure is related to the new TestDropdown test and seems to be only happening for Python 2.7.

Not sure what it is yet as the tests pass locally with Python 2.7.14

[jtpio]

Tests are now green after the last commit.

There seems to be a different behavior between Python 2.7 and Python 3 when it comes to testing warnings (even though the documentation looks identical between the two versions: https://docs.python.org/3.7/library/warnings.html#testing-warnings).

Python 2 requires to manually clear the warning registry if we want to be able to catch them:

import inspect
module = inspect.getmodule(Dropdown)
getattr(module, '__warningregistry__', {}).clear()

This has also been mentioned in other project testing for warning being triggered, for example with Matplotlib: matplotlib/matplotlib@0017d3e

[SylvainCorlay]

@jtpio these seems to be a conflict. Could you please rebase your PR on master?

[jasongrout]

@SylvainCorlay - We should try to get this in for 7.4 as well.

[jtpio]

test_widget_selection.py was added in both master and this branch.

Should be good now 👍

[jasongrout]

That's an awesome problem, that we're tripping over ourselves to add more testing :).