Reverse map discrete options using identity

[philippjfr]

Using a dictionary to map options from their value to their label did not work well because the values may be non-hashable or hash inconsistently. Instead this PR switches to using the value's identity to do the reverse mapping which should work reliably.

[jsignell]

This seems like a more straightforward approach. Are there downsides? Is it slower?

[jbednar]

Sounds good. Should Param do the same for selectors?

[philippjfr]

I doubt it's much slower, if anything I'd expect it to be cheaper not to do the hashing. That said the approach isn't quite right since __contains__ will fall back to testing for equality which will break for certain types (e.g. numpy arrays). I'll have to add a small is_in utility.

[philippjfr]

Sounds good. Should Param do the same for selectors?

Yes, the named_objs implementation in param seems to suffer from the same issues.

[codecov-io]

Codecov Report

Merging #305 into master will decrease coverage by 0.06%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
- Coverage    89.3%   89.24%   -0.07%     
==========================================
  Files          63       63              
  Lines        6528     6565      +37     
==========================================
+ Hits         5830     5859      +29     
- Misses        698      706       +8

Impacted Files	Coverage Δ
panel/widgets/player.py	86.27% <100%> (ø)	⬆️
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/util.py	70.25% <85.71%> (-2.6%)	⬇️
panel/widgets/select.py	89.51% <92.5%> (-0.41%)	⬇️

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[jsignell]

LGTM