Let UseEnum select a member by value, not only by name or number

tests/test_traitlets_enum.py

@@ -34,6 +34,18 @@ class CSColor(enum.Enum):
     YeLLoW = 4
 
 
+class StrColor(str, enum.Enum):
+    RED = "red"
+    GREEN = "green"
+    BLUE = "blue"
+
+
+class ShadowedColor(str, enum.Enum):
+    # A member name that collides with a different member's value
+    red = "green"
+    green = "blue"
+
+
 color_choices = ["red", "Green", "BLUE", "YeLLoW"]
 
 
@@ -129,6 +141,33 @@ def test_assign_bad_enum_value_number__raises_error(self):
             with self.assertRaises(TraitError):
                 example.color = value
 
+    def test_assign_str_enum_value(self):
+        # -- CONVERT: value (as string) => Enum value (item)
+        class Example(HasTraits):
+            color = UseEnum(StrColor)
+
+        for enum_item in StrColor.__members__.values():
+            example = Example()
+            example.color = enum_item.value
+            self.assertIs(example.color, enum_item)
+
+    def test_assign_str_enum_name_wins_over_value(self):
+        # -- A name lookup must keep taking precedence over a value lookup
+        class Example(HasTraits):
+            color = UseEnum(ShadowedColor)
+
+        example = Example()
+        example.color = "green"
+        self.assertIs(example.color, ShadowedColor.green)
+
+    def test_assign_bad_str_enum_value__raises_error(self):
+        class Example(HasTraits):
+            color = UseEnum(StrColor)
+
+        example = Example()
+        with self.assertRaises(TraitError):
+            example.color = "purple"
+
     def test_ctor_without_default_value(self):
         # -- IMPLICIT: default_value = Color.red (first enum-value)
         class Example2(HasTraits):

traitlets/traitlets.py

@@ -4275,6 +4275,15 @@ def select_by_name(self, value: str, default: t.Any = Undefined) -> t.Any:
             value = value.replace(self.name_prefix, "", 1)
         return self.enum_class.__members__.get(value, default)
 
+    def select_by_value(self, value: t.Any, default: t.Any = Undefined) -> t.Any:
+        """Selects enum-value by using its value-constant."""
+        enum_members = self.enum_class.__members__
+        for enum_item in enum_members.values():
+            if enum_item.value == value:
+                return enum_item
+        # -- NOT FOUND:
+        return default
+
     def validate(self, obj: t.Any, value: t.Any) -> t.Any:
         if isinstance(value, self.enum_class):
             return value
@@ -4288,6 +4297,11 @@ def validate(self, obj: t.Any, value: t.Any) -> t.Any:
             value2 = self.select_by_name(value)
             if value2 is not Undefined:
                 return value2
+            # -- CONVERT: value (as string) => enum_value (item), for str-based enums
+            # whose member names differ from their values
+            value2 = self.select_by_value(value)
+            if value2 is not Undefined:
+                return value2
         elif value is None:
             if self.allow_none:
                 return None