Index: Doc/library/os.rst
===================================================================
--- Doc/library/os.rst	(revision 82985)
+++ Doc/library/os.rst	(working copy)
@@ -1113,12 +1113,13 @@
    Compose a raw device number from the major and minor device numbers.
 
 
-.. function:: mkdir(path[, mode])
+.. function:: mkdir(path[, mode[, exist_ok]])
 
    Create a directory named *path* with numeric mode *mode*. The default *mode*
    is ``0o777`` (octal).  On some systems, *mode* is ignored.  Where it is used,
    the current umask value is first masked out.  If the directory already
-   exists, :exc:`OSError` is raised.
+   exists, :exc:`OSError` is raised if *exist_ok* is False, otherwise no
+   exception is raised.
 
    It is also possible to create temporary directories; see the
    :mod:`tempfile` module's :func:`tempfile.mkdtemp` function.
@@ -1126,7 +1127,7 @@
    Availability: Unix, Windows.
 
 
-.. function:: makedirs(path[, mode])
+.. function:: makedirs(path[, mode[, exist_ok]])
 
    .. index::
       single: directory; creating
Index: Lib/os.py
===================================================================
--- Lib/os.py	(revision 82985)
+++ Lib/os.py	(working copy)
@@ -119,8 +119,8 @@
 # Super directory utilities.
 # (Inspired by Eric Raymond; the doc strings are mostly his)
 
-def makedirs(name, mode=0o777):
-    """makedirs(path [, mode=0o777])
+def makedirs(name, mode=0o777, exist_ok=False):
+    """makedirs(path [, mode=0o777[, exist_ok=False]])
 
     Super-mkdir; create a leaf directory and all intermediate ones.
     Works like mkdir, except that any intermediate path segment (not
@@ -133,14 +133,14 @@
         head, tail = path.split(head)
     if head and tail and not path.exists(head):
         try:
-            makedirs(head, mode)
+            makedirs(head, mode, exist_ok)
         except OSError as e:
             # be happy if someone already created the path
             if e.errno != errno.EEXIST:
                 raise
         if tail == curdir:           # xxx/newdir/. exists if xxx/newdir exists
             return
-    mkdir(name, mode)
+    mkdir(name, mode, exist_ok)
 
 def removedirs(name):
     """removedirs(path)
Index: Lib/test/test_os.py
===================================================================
--- Lib/test/test_os.py	(revision 82985)
+++ Lib/test/test_os.py	(working copy)
@@ -609,6 +609,14 @@
                             'dir5', 'dir6')
         os.makedirs(path)
 
+    def test_exist_ok(self):
+        base = support.TESTFN
+        path = os.path.join(support.TESTFN, 'dir1')
+        os.makedirs(path)
+        self.assertRaises(OSError, os.makedirs, path)
+        self.assertRaises(OSError, os.makedirs, path, exist_ok=False)
+        os.makedirs(path, exist_ok=True)
+
     def tearDown(self):
         path = os.path.join(support.TESTFN, 'dir1', 'dir2', 'dir3',
                             'dir4', 'dir5', 'dir6')
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 82985)
+++ Modules/posixmodule.c	(working copy)
@@ -2727,26 +2727,29 @@
 #endif /* MS_WINDOWS */
 
 PyDoc_STRVAR(posix_mkdir__doc__,
-"mkdir(path [, mode=0777])\n\n\
+"mkdir(path [, mode=0o777[, exist_ok=False]])\n\n\
 Create a directory.");
 
 static PyObject *
-posix_mkdir(PyObject *self, PyObject *args)
+posix_mkdir(PyObject *self, PyObject *args, PyObject *kwdict)
 {
     int res;
     PyObject *opath;
     char *path;
     int mode = 0777;
+    int exist_ok = 0;
+    static char *kwlist[] = {"path", "mode", "exist_ok", NULL};
 
 #ifdef MS_WINDOWS
     PyUnicodeObject *po;
-    if (PyArg_ParseTuple(args, "U|i:mkdir", &po, &mode)) {
+    if (PyArg_ParseTupleAndKeywords(args, kwdict, "U|ii:mkdir",
+                        kwlist, &po, &mode, &exist_ok)) {
         Py_BEGIN_ALLOW_THREADS
         /* PyUnicode_AS_UNICODE OK without thread lock as
            it is a simple dereference. */
         res = CreateDirectoryW(PyUnicode_AS_UNICODE(po), NULL);
         Py_END_ALLOW_THREADS
-        if (!res)
+        if (!res && !(res == ERROR_ALREADY_EXISTS && exist_ok))
             return win32_error_unicode("mkdir", PyUnicode_AS_UNICODE(po));
         Py_INCREF(Py_None);
         return Py_None;
@@ -2754,8 +2757,8 @@
     /* Drop the argument parsing error as narrow strings
        are also valid. */
     PyErr_Clear();
-    if (!PyArg_ParseTuple(args, "O&|i:mkdir",
-                          PyUnicode_FSConverter, &opath, &mode))
+    if (!PyArg_ParseTupleAndKeywords(args, kwdict, "O&|ii:mkdir", kwlist,
+                    PyUnicode_FSConverter, &opath, &mode, &exist_ok))
         return NULL;
     path = PyBytes_AsString(opath);
     Py_BEGIN_ALLOW_THREADS
@@ -2763,7 +2766,7 @@
        it is a simple dereference. */
     res = CreateDirectoryA(path, NULL);
     Py_END_ALLOW_THREADS
-    if (!res) {
+    if (!res && !(res == ERROR_ALREADY_EXISTS && exist_ok)) {
         win32_error("mkdir", path);
         Py_DECREF(opath);
         return NULL;
@@ -2773,8 +2776,8 @@
     return Py_None;
 #else
 
-    if (!PyArg_ParseTuple(args, "O&|i:mkdir",
-                          PyUnicode_FSConverter, &opath, &mode))
+    if (!PyArg_ParseTupleAndKeywords(args, kwdict, "O&|ii:mkdir", kwlist,
+                          PyUnicode_FSConverter, &opath, &mode, &exist_ok))
         return NULL;
     path = PyBytes_AsString(opath);
     Py_BEGIN_ALLOW_THREADS
@@ -2784,7 +2787,7 @@
     res = mkdir(path, mode);
 #endif
     Py_END_ALLOW_THREADS
-    if (res < 0)
+    if (res < 0 && !(errno == EEXIST && exist_ok))
         return posix_error_with_allocated_filename(opath);
     Py_DECREF(opath);
     Py_INCREF(Py_None);
@@ -7579,7 +7582,8 @@
 #endif /* HAVE_LINK */
     {"listdir",         posix_listdir, METH_VARARGS, posix_listdir__doc__},
     {"lstat",           posix_lstat, METH_VARARGS, posix_lstat__doc__},
-    {"mkdir",           posix_mkdir, METH_VARARGS, posix_mkdir__doc__},
+    {"mkdir",           (PyCFunction)posix_mkdir, METH_VARARGS | METH_KEYWORDS,
+                posix_mkdir__doc__},
 #ifdef HAVE_NICE
     {"nice",            posix_nice, METH_VARARGS, posix_nice__doc__},
 #endif /* HAVE_NICE */