We could make the error message less wrong:

>>> p.parse_args(['d'])
usage: [-h] {a,b,c}
: error: argument a: invalid choice: 'd' (choose a value in 'abc')



% git diff
diff --git a/Lib/argparse.py b/Lib/argparse.py
index b44fa4f0f6..f03cc1f110 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2499,8 +2499,8 @@ def _check_value(self, action, value):
         # converted value must be one of the choices (if specified)
         if action.choices is not None and value not in action.choices:
             args = {'value': value,
-                    'choices': ', '.join(map(repr, action.choices))}
-            msg = _('invalid choice: %(value)r (choose from %(choices)s)')
+                    'choices': repr(action.choices)}
+            msg = _('invalid choice: %(value)r (choose a value in %(choices)s)')
             raise ArgumentError(action, msg % args)
 
     # =======================