Issue #9348: Raise an early error if argparse nargs and metavar don't match.

1 files changed, 7 insertions, 0 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index de3cd11479..e46f919e02 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1294,6 +1294,13 @@ class _ActionsContainer(object):
         if not _callable(type_func):
             raise ValueError('%r is not callable' % type_func)
 
+        # raise an error if the metavar does not match the type
+        if hasattr(self, "_get_formatter"):
+            try:
+                self._get_formatter()._format_args(action, None)
+            except TypeError:
+                raise ValueError("length of metavar tuple does not match nargs")
+
         return self._add_action(action)
 
     def add_argument_group(self, *args, **kwargs):