diff -r 3884a7e3df1c Doc/library/re.rst
--- a/Doc/library/re.rst	Sun Oct 16 19:14:23 2016 +0200
+++ b/Doc/library/re.rst	Sun Oct 16 23:26:27 2016 +0300
@@ -1064,11 +1064,17 @@ Match objects support the following meth
 
    Return a dictionary containing all the *named* subgroups of the match, keyed by
    the subgroup name.  The *default* argument is used for groups that did not
-   participate in the match; it defaults to ``None``.  For example:
+   participate in the match.  For example:
 
-      >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds")
-      >>> m.groupdict()
-      {'first_name': 'Malcolm', 'last_name': 'Reynolds'}
+      >>> m = re.match(r"(?P<title>Mrs |Mr |Dr )?(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds")
+      >>> m.groupdict(None)
+      {'title': None, 'first_name': 'Malcolm', 'last_name': 'Reynolds'}
+
+   .. deprecated:: 3.7
+
+      If the *default* argument is not specified, the result is the same as
+      if call :meth:`~match.groupdict` with ``None``.  This will be changed
+      in future releases of Python.
 
 
 .. method:: match.start([group])
diff -r 3884a7e3df1c Lib/test/test_re.py
--- a/Lib/test/test_re.py	Sun Oct 16 19:14:23 2016 +0200
+++ b/Lib/test/test_re.py	Sun Oct 16 23:26:27 2016 +0300
@@ -578,9 +578,13 @@ class ReTests(unittest.TestCase):
         self.checkPatternError(r'(abc\1)', 'cannot refer to an open group', 4)
 
     def test_groupdict(self):
-        self.assertEqual(re.match('(?P<first>first) (?P<second>second)',
-                                  'first second').groupdict(),
-                         {'first':'first', 'second':'second'})
+        m = re.match('(?P<first>first) ?(?P<second>second)? ?(?P<third>third)',
+                     'first third')
+        self.assertEqual(m.groupdict('N/A'),
+                         {'first': 'first', 'second': 'N/A', 'third': 'third'})
+        with self.assertWarns(FutureWarning):
+            self.assertEqual(m.groupdict(),
+                             {'first': 'first', 'second': None, 'third': 'third'})
 
     def test_expand(self):
         self.assertEqual(re.match("(?P<first>first) (?P<second>second)",
diff -r 3884a7e3df1c Modules/_sre.c
--- a/Modules/_sre.c	Sun Oct 16 19:14:23 2016 +0200
+++ b/Modules/_sre.c	Sun Oct 16 23:26:27 2016 +0300
@@ -2019,7 +2019,7 @@ match_getslice_by_index(MatchObject* sel
 
     if (self->string == Py_None || self->mark[index] < 0) {
         /* return default value if the string or group is undefined */
-        Py_INCREF(def);
+        Py_XINCREF(def);
         return def;
     }
 
@@ -2163,7 +2163,7 @@ static PyObject *
 /*[clinic input]
 _sre.SRE_Match.groupdict
 
-    default: object = None
+    default: object = NULL
         Is used for groups that did not participate in the match.
 
 Return a dictionary containing all the named subgroups of the match, keyed by the subgroup name.
@@ -2171,7 +2171,7 @@ Return a dictionary containing all the n
 
 static PyObject *
 _sre_SRE_Match_groupdict_impl(MatchObject *self, PyObject *default_value)
-/*[clinic end generated code: output=29917c9073e41757 input=0ded7960b23780aa]*/
+/*[clinic end generated code: output=29917c9073e41757 input=cba63577c6f64449]*/
 {
     PyObject* result;
     PyObject* keys;
@@ -2193,8 +2193,20 @@ static PyObject *
         if (!key)
             goto failed;
         value = match_getslice(self, key, default_value);
-        if (!value)
-            goto failed;
+        if (!value) {
+            if (PyErr_Occurred())
+                goto failed;
+            if (PyErr_WarnEx(PyExc_FutureWarning,
+                             "The result of groupdict() without argument "
+                             "includes unmatched groups.  This will be "
+                             "changed in future.  Use groupdict(None) to "
+                             "silence the warning.",
+                             1) < 0) {
+                goto failed;
+            }
+            value = Py_None;
+            Py_INCREF(value);
+        }
         status = PyDict_SetItem(result, key, value);
         Py_DECREF(value);
         if (status < 0)
diff -r 3884a7e3df1c Modules/clinic/_sre.c.h
--- a/Modules/clinic/_sre.c.h	Sun Oct 16 19:14:23 2016 +0200
+++ b/Modules/clinic/_sre.c.h	Sun Oct 16 23:26:27 2016 +0300
@@ -540,7 +540,7 @@ static PyObject *
     PyObject *return_value = NULL;
     static const char * const _keywords[] = {"default", NULL};
     static _PyArg_Parser _parser = {"|O:groupdict", _keywords, 0};
-    PyObject *default_value = Py_None;
+    PyObject *default_value = NULL;
 
     if (!_PyArg_ParseStack(args, nargs, kwnames, &_parser,
         &default_value)) {
@@ -728,4 +728,4 @@ static PyObject *
 {
     return _sre_SRE_Scanner_search_impl(self);
 }
-/*[clinic end generated code: output=a4a246bca1963bc9 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=2dd1305659b6db7f input=a9049054013a1b77]*/