Use string.ascii_letters instead of string.letters (SF bug #226706).
diff --git a/Lib/cmd.py b/Lib/cmd.py
index 10c11a2..eacd498 100644
--- a/Lib/cmd.py
+++ b/Lib/cmd.py
@@ -40,7 +40,7 @@
 __all__ = ["Cmd"]
 
 PROMPT = '(Cmd) '
-IDENTCHARS = string.letters + string.digits + '_'
+IDENTCHARS = string.ascii_letters + string.digits + '_'
 
 class Cmd:
     prompt = PROMPT