diff --git a/Lib/argparse.py b/Lib/argparse.py
index 0ee8c0865e8..f77c0c2d75f 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1714,10 +1714,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
             if action.dest is not SUPPRESS:
                 if not hasattr(namespace, action.dest):
                     if action.default is not SUPPRESS:
-                        default = action.default
-                        if isinstance(action.default, str):
-                            default = self._get_value(action, default)
-                        setattr(namespace, action.dest, default)
+                        setattr(namespace, action.dest, action.default)
 
         # add any parser defaults that aren't present
         for dest in self._defaults:
@@ -1945,12 +1942,22 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
         if positionals:
             self.error(_('too few arguments'))
 
-        # make sure all required actions were present
+        # make sure all required actions were present, and convert defaults.
         for action in self._actions:
-            if action.required:
-                if action not in seen_actions:
+            if action not in seen_actions:
+                if action.required:
                     name = _get_action_name(action)
                     self.error(_('argument %s is required') % name)
+                else:
+                    # Convert action default now instead of doing it before
+                    # parsing arguments to avoid calling convert functions
+                    # twice (which may fail) if the argument was given, but
+                    # only if it was defined already in the namespace
+                    if (action.default is not None and
+                            hasattr(namespace, action.dest) and
+                            action.default is getattr(namespace, action.dest)):
+                        setattr(namespace, action.dest,
+                                self._get_value(action, action.default))
 
         # make sure all required groups had one option present
         for group in self._mutually_exclusive_groups:
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 3a2976126f2..cc051607e97 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -1462,6 +1462,22 @@ class TestFileTypeR(TempDirMixin, ParserTestCase):
         ('readonly', NS(x=None, spam=RFile('readonly'))),
     ]
 
+class TestFileTypeDefaults(TempDirMixin, ParserTestCase):
+    """Test that a file is not created unless the default is needed"""
+    def setUp(self):
+        super(TestFileTypeDefaults, self).setUp()
+        file = open(os.path.join(self.temp_dir, 'good'), 'w')
+        file.write('good')
+        file.close()
+
+    argument_signatures = [
+        Sig('-c', type=argparse.FileType('r'), default='no-file.txt'),
+    ]
+    # should provoke no such file error
+    failures = ['']
+    # should not provoke error because default file is created
+    successes = [('-c good', NS(c=RFile('good')))]
+
 
 class TestFileTypeRB(TempDirMixin, ParserTestCase):
     """Test the FileType option/argument type for reading files"""
@@ -4468,6 +4484,38 @@ class TestArgumentTypeError(TestCase):
         else:
             self.fail()
 
+# ================================================
+# Check that the type function is called only once
+# ================================================
+
+class TestTypeFunctionCallOnlyOnce(TestCase):
+
+    def test_type_function_call_only_once(self):
+        def spam(string_to_convert):
+            self.assertEqual(string_to_convert, 'spam!')
+            return 'foo_converted'
+
+        parser = argparse.ArgumentParser()
+        parser.add_argument('--foo', type=spam, default='bar')
+        args = parser.parse_args('--foo spam!'.split())
+        self.assertEqual(NS(foo='foo_converted'), args)
+
+# ================================================================
+# Check that the type function is called with a non-string default
+# ================================================================
+
+class TestTypeFunctionCallWithNonStringDefault(TestCase):
+
+    def test_type_function_call_with_non_string_default(self):
+        def spam(int_to_convert):
+            self.assertEqual(int_to_convert, 0)
+            return 'foo_converted'
+
+        parser = argparse.ArgumentParser()
+        parser.add_argument('--foo', type=spam, default=0)
+        args = parser.parse_args([])
+        self.assertEqual(NS(foo='foo_converted'), args)
+
 # ======================
 # parse_known_args tests
 # ======================
diff --git a/Misc/ACKS b/Misc/ACKS
index a46ee843e5d..f534143cb60 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -305,6 +305,7 @@ Nils Fischbeck
 Frederik Fix
 Matt Fleming
 Hernán Martínez Foffani
+Arnaud Fontaine
 Michael Foord
 Amaury Forgeot d'Arc
 Doug Fort
diff --git a/Misc/NEWS b/Misc/NEWS
index 8b0b145b787..c78dc96ee75 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -112,6 +112,12 @@ Core and Builtins
 Library
 -------
 
+- Issue #12776,#11839: call argparse type function (specified by add_argument)
+  only once. Before, the type function was called twice in the case where the
+  default was specified and the argument was given as well.  This was
+  especially problematic for the FileType type, as a default file would always
+  be opened, even if a file argument was specified on the command line.
+
 - Issue #13370: Ensure that ctypes works on Mac OS X when Python is
   compiled using the clang compiler