fix: mro_trait can be unbound when the class is not in mro (#824)

maartenbreddels · web-flow · commit 18814204c7e7 · 2023-01-08T17:04:51.000-06:00
Closes #823
diff --git a/traitlets/traitlets.py b/traitlets/traitlets.py
@@ -1020,6 +1020,7 @@ def setup_class(cls, classdict):  # noqa
                 cls._traits[name] = value
                 trait = value
                 default_method_name = "_%s_default" % name
+                mro_trait = mro
                 try:
                     mro_trait = mro[: mro.index(trait.this_class) + 1]  # type:ignore[arg-type]
                 except ValueError:
