diff --git a/Lib/argparse.py b/Lib/argparse.py
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1151,16 +1151,18 @@
         # the special argument "-" means sys.std{in,out}
         if string == '-':
             if 'r' in self._mode:
-                return _sys.stdin
+                file = _sys.stdin.fileno()
             elif 'w' in self._mode:
-                return _sys.stdout
+                file = _sys.stdout.fileno()
             else:
                 msg = _('argument "-" with mode %r') % self._mode
                 raise ValueError(msg)
+        else:
+            file = string
 
         # all other arguments are used as file names
         try:
-            return open(string, self._mode, self._bufsize)
+            return open(file, self._mode, self._bufsize)
         except IOError as e:
             message = _("can't open '%s': %s")
             raise ArgumentTypeError(message % (string, e))
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -1439,6 +1439,19 @@
         return self.name == other.name == text
 
 
+class FilenoMode(object):
+
+    def __init__(self, file, mode):
+        self.fileno = file.fileno()
+        self.mode = mode
+
+    def __eq__(self, other):
+        return self.fileno == other.fileno() and self.mode == other.mode
+
+    def __repr__(self):
+        return "FilenoMode({}, {!r})".format(self.fileno, self.mode)
+
+
 class TestFileTypeR(TempDirMixin, ParserTestCase):
     """Test the FileType option/argument type for reading files"""
 
@@ -1459,7 +1472,8 @@
         ('foo', NS(x=None, spam=RFile('foo'))),
         ('-x foo bar', NS(x=RFile('foo'), spam=RFile('bar'))),
         ('bar -x foo', NS(x=RFile('foo'), spam=RFile('bar'))),
-        ('-x - -', NS(x=sys.stdin, spam=sys.stdin)),
+        ('-x - -', NS(x=FilenoMode(sys.stdin, 'r'),
+                      spam=FilenoMode(sys.stdin, 'r'))),
         ('readonly', NS(x=None, spam=RFile('readonly'))),
     ]
 
@@ -1483,7 +1497,8 @@
         ('foo', NS(x=None, spam=RFile('foo'))),
         ('-x foo bar', NS(x=RFile('foo'), spam=RFile('bar'))),
         ('bar -x foo', NS(x=RFile('foo'), spam=RFile('bar'))),
-        ('-x - -', NS(x=sys.stdin, spam=sys.stdin)),
+        ('-x - -', NS(x=FilenoMode(sys.stdin, 'rb'),
+                      spam=FilenoMode(sys.stdin, 'rb'))),
     ]
 
 
@@ -1522,7 +1537,8 @@
         ('foo', NS(x=None, spam=WFile('foo'))),
         ('-x foo bar', NS(x=WFile('foo'), spam=WFile('bar'))),
         ('bar -x foo', NS(x=WFile('foo'), spam=WFile('bar'))),
-        ('-x - -', NS(x=sys.stdout, spam=sys.stdout)),
+        ('-x - -', NS(x=FilenoMode(sys.stdout, 'w'),
+                      spam=FilenoMode(sys.stdout, 'w'))),
     ]
 
 
@@ -1537,7 +1553,8 @@
         ('foo', NS(x=None, spam=WFile('foo'))),
         ('-x foo bar', NS(x=WFile('foo'), spam=WFile('bar'))),
         ('bar -x foo', NS(x=WFile('foo'), spam=WFile('bar'))),
-        ('-x - -', NS(x=sys.stdout, spam=sys.stdout)),
+        ('-x - -', NS(x=FilenoMode(sys.stdout, 'wb'),
+                      spam=FilenoMode(sys.stdout, 'wb'))),
     ]