actions: Have ParseEnum print a list of possible values on error

This will save the user a look at the program's manual and makes using
the program more convenient.

1 files changed, 3 insertions, 1 deletions

diff --git a/quarg/actions.py b/quarg/actions.py
index 9361ac2..70cde03 100644
--- a/quarg/actions.py
+++ b/quarg/actions.py
@@ -13,7 +13,9 @@ class ParseEnum(argparse.Action, metaclass=ABCMeta):
     def __call__(self, parser, namespace, value, option_string=None):
         key = value.upper()
         if key not in self.enumclass.__members__:
-            errx(f'Not a valid {self.enumclass.describe()}: {value}')
+            possible = ', '.join([e.name.lower() for e in self.enumclass])
+            desc = self.enumclass.describe()
+            errx(f'Not a valid {desc}: {value}\nPossible {desc}s are: {possible}')
 
         saved = getattr(namespace, self.dest) or []
         saved.append(self.enumclass[key])