Enum cannot set default value using a dictionary

[kitchoi]

Not a bug but suggestion for enhancement.

The following usage is currently not supported:

from traits.api import Dict, Enum, HasTraits, Int, Str

class EnumWithDictDemo(HasTraits):

    name_to_index = Dict(Str, Int)

    name = Enum(values="name_to_index")

demo = EnumWithDictDemo()
demo.name_to_index = {"A": 1, "B": 2}
print(demo.name)

Traceback (most recent call last):
  File "EnumEditor_Dict_demo.py", line 11, in <module>
    print(demo.name)
  File "/Users/kchoi/.edm/envs/traitsui/lib/python2.7/site-packages/traits/trait_types.py", line 2048, in _get
    value = values[0]
KeyError: 0

It would make sense to simply use value = next(iter(values)) for the default value?

[mdickinson]

Needs to be re-examined after #685 goes in, and before the 6.0 release.

[mdickinson]

This now apparently works after #685 went in, though I regard the fact that it works as accidental, and it should not be assumed that it will carry on working in the future. The supported types for the values collection are list, tuple and EnumMeta.

[kitchoi]

If users should not assume this to continue working and yet it works, it might be better if it did not work at all...
What about testing reversed(values) and raise if the values cannot be reversed (hence reopening the issue for future reconsideration)?

[kitchoi]

testing reversed(values)

Actually, list, tuple and EnumMeta satisfy the condition of isinstance(values, abc.Reversible) and isinstance(values, Collection).

[mdickinson]

it might be better if it did not work at all...

Agreed: I'd rather have it working, tested and documented, or not working at all. But adding new validation where there was none before is going to be a can of worms; we'd probably need at least a deprecation warning. This is something we could think about for 6.1.0

[kitchoi]

For future reference, #968 is going to make Enum support dictionary (and friends).