Add SelectorObjects wrapper for forward and backward compatibility

[philippjfr]

We have long struggled with an approach to improving Selector, ListSelector and ObjectSelector that would allow users to easily update the objects. Due to unfortunate past decisions this is quite difficult because users can supply both a list or a dict in the constructor and we decided to store objects as a simple list while storing the optional names as a dictionary on the names attribute. This meant that if you instantiated objects as a dictionary you had to manage updates to both of these attributes at the same time.

To handle this issue we create a wrapper object around the objects attribute which has hybrid behavior that allows both list-like and dict-like updates and warns appropriately if for instance you are trying to update objects with the list-like API but have supplied names for other objects previously. An additional benefit of this wrapper object is that if you use any of the list or dict API to modify the objects we can trigger an event, while in the past a user would manually have to trigger such an event if they wanted Panel or some other downstream library to update in response to the change in objects.

Fixes #309
Fixes #331
Fixes #645
Fixes #398

• Add tests
• Update docs

[philippjfr]

Will need some internal remapping to allow users to subscribe to parameter:objects despite the fact the slot is now called _objects.

[codecov-commenter]

Codecov Report

Merging #598 (b6ba65a) into master (7d702ad) will decrease coverage by 1.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
- Coverage   81.95%   80.57%   -1.38%     
==========================================
  Files           4        4              
  Lines        3020     3110      +90     
==========================================
+ Hits         2475     2506      +31     
- Misses        545      604      +59     

Impacted Files	Coverage Δ
param/parameterized.py	82.20% <100.00%> (+0.02%)	⬆️
numbergen/__init__.py	76.05% <0.00%> (-3.87%)	⬇️

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[philippjfr]

@jbednar This is ready for review. I would suggest instead of reviewing the code, review the test cases and check whether they match your understanding and intuitions.

One thing to note is that I've decided to allow dictionary style updates to list-like objects by upgrading the list to a dictionary, i.e. you can do something like this:

>>> p.selector.objects = ['A', 'B', 'C']

>>> p.selector.objects['D'] = 4

>>> dict(p.selector.objects)

{'A': 'A', 'B': 'B', 'C': 'C', 'D': 4}

[jbednar]

Looks good to me. I have to admit that my eyes glazed over both on the list/dict methods and on the tests; they all look reasonable but I have low confidence I'd detect any problems. I think we'll have to test this with lots of existing code to see if we find any problems. Have you tried it against the Panel test suite?

[philippjfr]

This is ready to merge once we drop py2.7 on main.

[maximlt]

Does it fix #398 too?

[philippjfr]

Yes, it absolutely should.

[philippjfr]

Just confirmed that it does.

[maximlt]

The default value of names is now an empty dict when objects is a list, instead of None. The test suite fails now, since I added a bunch of tests that check the default values.

Seems like this change was intentional, @philippjfr can you confirm? If so, I can update the tests accordingly.

[maximlt]

I assumed this was an intentional change and updated the tests accordingly. Can always amend before 2.0 is release if need be.