diff -up traitlets-4.3.2/traitlets/config/loader.py.orig traitlets-4.3.2/traitlets/config/loader.py
--- traitlets-4.3.2/traitlets/config/loader.py.orig	2019-09-11 21:32:08.604367480 -0600
+++ traitlets-4.3.2/traitlets/config/loader.py	2019-09-11 21:36:20.012251166 -0600
@@ -792,7 +792,7 @@ class KVArgParseConfigLoader(ArgParseCon
                 nargs = '?'
             else:
                 nargs = None
-            if len(key) is 1:
+            if len(key) == 1:
                 paa('-'+key, '--'+key, type=text_type, dest=value, nargs=nargs)
             else:
                 paa('--'+key, type=text_type, dest=value, nargs=nargs)
@@ -801,7 +801,7 @@ class KVArgParseConfigLoader(ArgParseCon
                 #
                 self.alias_flags[self.aliases[key]] = value
                 continue
-            if len(key) is 1:
+            if len(key) == 1:
                 paa('-'+key, '--'+key, action='append_const', dest='_flags', const=value)
             else:
                 paa('--'+key, action='append_const', dest='_flags', const=value)