bpo-29298: Fix crash with required subparsers without dest

Author: asottile

Lib/argparse.py

@@ -704,6 +704,8 @@ def _get_action_name(argument):
         return argument.metavar
     elif argument.dest not in (None, SUPPRESS):
         return argument.dest
+    elif argument.choices:
+        return '{' + ','.join(argument.choices) + '}'
     else:
         return None
 

Lib/test/test_argparse.py

@@ -1934,6 +1934,12 @@ def test_required_subparsers_default(self):
         subparsers.add_parser('run')
         self._test_required_subparsers(parser)
 
+    def test_required_subparsers_no_destination_error(self):
+        parser = ErrorRaisingArgumentParser()
+        subparsers = parser.add_subparsers()
+        subparsers.add_parser('run')
+        self.assertArgumentParserError(parser.parse_args, ())
+
     def test_optional_subparsers(self):
         parser = ErrorRaisingArgumentParser()
         subparsers = parser.add_subparsers(dest='command', required=False)

Misc/NEWS.d/next/Library/2017-09-20-14-43-03.bpo-29298._78CSN.rst

@@ -0,0 +1,2 @@
+Fix ``TypeError`` when required subparsers without ``dest`` do not receive
+arguments. Patch by Anthony Sottile.