From 4143e7eb0c3e474fe3cb031666b4620cf1e0d832 Mon Sep 17 00:00:00 2001
From: azhu <azhu@tower-research.com>
Date: Fri, 19 Apr 2024 11:03:18 +1200
Subject: [PATCH] Fix assigning string value to Set

Assigning a string value to a Set trait used to implicitly
cast to a set containing the string, but it accidentally
became a set of the string's characters after #883. Fix this
and add a unit test.
---
 tests/test_traitlets.py | 18 ++++++++++++++++++
 traitlets/traitlets.py  |  7 +------
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/tests/test_traitlets.py b/tests/test_traitlets.py
index dfcf3f0f..f9f623b4 100644
--- a/tests/test_traitlets.py
+++ b/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
 
diff --git a/traitlets/traitlets.py b/traitlets/traitlets.py
index 1d1675ab..ecd0d7cc 100644
--- a/traitlets/traitlets.py
+++ b/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)