Fix union from string

Author: krassowski

traitlets/tests/test_traitlets.py

@@ -3136,6 +3136,17 @@ def test_tcp_from_string(s, expected):
     _from_string_test(TCPAddress, s, expected)
 
 
+@pytest.mark.parametrize(
+    "s, expected",
+    [
+        ("[]", []),
+        ('{}', '{}')
+    ],
+)
+def test_union_of_list_and_unicode_from_string(s, expected):
+    _from_string_test(Union([List(), Unicode()]), s, expected)
+
+
 def test_all_attribute():
     """Verify all trait types are added to `traitlets.__all__`"""
     names = dir(traitlets)

traitlets/traitlets.py

@@ -2179,6 +2179,16 @@ def __or__(self, other):
         else:
             return Union(self.trait_types + [other])
 
+    def from_string(self, s):
+        for trait_type in self.trait_types:
+            try:
+                v = trait_type.from_string(s)
+                return trait_type.validate(None, v)
+            except TraitError:
+                continue
+        self.error(None, s)
+
+
 
 # -----------------------------------------------------------------------------
 # Basic TraitTypes implementations/subclasses