`UseEnum` does not work with str enum

まず、issue に示されている Color enum で報告された UseEnum の動作を再現し、次に UseEnum のバリデーションと既存のテストを調べます。str ベースの enum のカバレッジを追加し、基になる値 "green" を割り当てると Color.GREEN に解決され、既存の enum の割り当てが壊れないことを確認します。

Slightly adapting the UseEnum example to use a str based enum makes it fail to recognize the enum by the value:

import enum
from traitlets import HasTraits, UseEnum

class Color(str, enum.Enum):
    RED = "red"
    BLUE = "blue"
    GREEN = "green"

class MyEntity(HasTraits):
    color = UseEnum(Color, default_value=Color.BLUE)

entity = MyEntity(color=Color.RED)
entity.color = Color.GREEN
entity.color = "GREEN"  
entity.color = "Color.GREEN"  
entity.color = "green"
assert entity.color is Color.GREEN

Raises:

❯ python test_enum.py
Traceback (most recent call last):
  File "/home/mnoethe/Projects/test_enum.py", line 16, in <module>
    entity.color = "green"              # USE: number (as int)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 729, in __set__
    self.set(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 703, in set
    new_value = self._validate(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 735, in _validate
    value = self.validate(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 3582, in validate
    self.error(obj, value)
  File "/home/mnoethe/.local/conda/envs/cta-dev/lib/python3.9/site-packages/traitlets/traitlets.py", line 841, in error
    raise TraitError(e)
traitlets.traitlets.TraitError: The 'color' trait of a MyEntity instance expected any of ['RED', 'BLUE', 'GREEN'], not the str 'green'.