Better error messages when 'dest' occurs twice for positional arguments

3 files changed, 14 insertions, 0 deletions

diff --git a/NEWS.txt b/NEWS.txt
index 4e527c4..60ed0c9 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -3,6 +3,7 @@ What's New in argparse 1.1
 
 - Issue #36: Get "from argparse import *" working
 - Issue #37: Stop treating two character alphanumeric flags as numbers
+- Issue #38: Better error messages when 'dest' occurs twice for positional arguments
 - Issue #39: Better error messages for invalid actions
 
 
diff --git a/argparse.py b/argparse.py
index 503cbaf..3cc3b6a 100644
--- a/argparse.py
+++ b/argparse.py
@@ -1249,6 +1249,8 @@ class _ActionsContainer(object):
         # argument
         chars = self.prefix_chars
         if not args or len(args) == 1 and args[0][0] not in chars:
+            if args and 'dest' in kwargs:
+                raise ValueError('dest supplied twice for positional argument')
             kwargs = self._get_positional_kwargs(*args, **kwargs)
 
         # otherwise, we're adding an optional argument
diff --git a/test/test_argparse.py b/test/test_argparse.py
index c1717dc..a1c674c 100644
--- a/test/test_argparse.py
+++ b/test/test_argparse.py
@@ -3624,6 +3624,17 @@ class TestInvalidArgumentConstructors(TestCase):
             msg = 'expected %r, found %r' % (expected, e)
             self.failUnless(expected in str(e), msg)
 
+    def test_multiple_dest(self):
+        parser = argparse.ArgumentParser()
+        parser.add_argument(dest='foo')
+        try:
+            parser.add_argument('bar', dest='baz')
+        except ValueError:
+            e = sys.exc_info()[1]
+            expected = 'dest supplied twice for positional argument'
+            msg = 'expected %r, found %r' % (expected, e)
+            self.failUnless(expected in str(e), msg)
+
     def test_no_argument_actions(self):
         for action in ['store_const', 'store_true', 'store_false',
                        'append_const', 'count']: