bpo-14074: argparse doesn't allow metavar to be a tuple

Currently argparse allows nargs>1 for positional arguments but doesn't
allow metavar to be a tuple:

    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument('foo', nargs=2, metavar=('bar', 'baz'))
    args = parser.parse_args()

The above code will raise exception both with and without '--help'.

This patch fixed some functions about metavar processing so the above
code will pass.

Author: cykerway

Lib/argparse.py

@@ -553,6 +553,8 @@ def _format_action(self, action):
     def _format_action_invocation(self, action):
         if not action.option_strings:
             default = self._get_default_metavar_for_positional(action)
+            if isinstance(action.nargs, int):
+                return ' '.join(self._metavar_formatter(action, default)(1))
             metavar, = self._metavar_formatter(action, default)(1)
             return metavar
 
@@ -730,6 +732,9 @@ def _get_action_name(argument):
     elif argument.option_strings:
         return  '/'.join(argument.option_strings)
     elif argument.metavar not in (None, SUPPRESS):
+        if isinstance(
+            argument.nargs, int) and isinstance(argument.metavar, tuple):
+            return ' '.join(argument.metavar)
         return argument.metavar
     elif argument.dest not in (None, SUPPRESS):
         return argument.dest

Lib/test/test_argparse.py

@@ -4226,6 +4226,28 @@ class TestHelpTupleMetavar(HelpTestCase):
     version = ''
 
 
+class TestHelpTupleMetavarPositional(HelpTestCase):
+    """Test specifying metavar on a Positional as a tuple"""
+
+    parser_signature = Sig(prog='PROG')
+    argument_signatures = [
+        Sig('foo', help='foo help', nargs=2, metavar=('bar', 'baz'))
+    ]
+    argument_group_signatures = []
+    usage = '''\
+        usage: PROG [-h] bar baz
+        '''
+    help = usage + '''\
+
+        positional arguments:
+          bar baz     foo help
+
+        options:
+          -h, --help  show this help message and exit
+        '''
+    version = ''
+
+
 class TestHelpRawText(HelpTestCase):
     """Test the RawTextHelpFormatter"""
 

Misc/NEWS.d/next/Library/2018-12-04-07-36-27.bpo-14074.fMLKCu.rst

@@ -0,0 +1,2 @@
+Fix :mod:`argparse` metavar processing to allow positional arguments to have a
+tuple metavar.