Fix assigning string value to Set (#903)

tests/test_traitlets.py

@@ -1658,6 +1658,24 @@ def coerce(self, value):
         return value
 
 
+class SetTrait(HasTraits):
+    value = Set(Unicode())
+
+
+class TestSet(TraitTestBase):
+    obj = SetTrait()
+
+    _default_value: t.Set[str] = set()
+    _good_values = [{"a", "b"}, "ab"]
+    _bad_values = [1]
+
+    def coerce(self, value):
+        if isinstance(value, str):
+            # compatibility handling: convert string to set containing string
+            value = {value}
+        return value
+
+
 class Foo:
     pass
 

traitlets/traitlets.py

@@ -3698,12 +3698,7 @@ def validate_elements(self, obj: t.Any, value: t.Any) -> t.Any:
 
     def set(self, obj: t.Any, value: t.Any) -> None:
         if isinstance(value, str):
-            return super().set(
-                obj,
-                set(
-                    value,
-                ),
-            )
+            return super().set(obj, {value})
         else:
             return super().set(obj, value)