diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index d9c2ee96b0..d00197688a 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -1894,7 +1894,7 @@ def _format_call_signature(name, args, kwargs):
     formatted_args = ''
     args_string = ', '.join([repr(arg) for arg in args])
     kwargs_string = ', '.join([
-        '%s=%r' % (key, value) for key, value in kwargs.items()
+        '%s=%r' % (key, value) for key, value in sorted(kwargs.items())
     ])
     if args_string:
         formatted_args = args_string
diff --git a/Lib/unittest/test/testmock/testmock.py b/Lib/unittest/test/testmock/testmock.py
index b65dc321cf..7019ee9012 100644
--- a/Lib/unittest/test/testmock/testmock.py
+++ b/Lib/unittest/test/testmock/testmock.py
@@ -1206,6 +1206,12 @@ class MockTest(unittest.TestCase):
         with self.assertRaises(AssertionError):
             m.hello.assert_not_called()
 
+    #Issue21256 printout of keyword args should be in deterministic order
+    def test_sorted_call_signature(self):
+        m = Mock()
+        m.hello(name='hello', daddy='hero')
+        text = "call(daddy='hero', name='hello')"
+        self.assertEquals(repr(m.hello.call_args), text)
 
     def test_mock_add_spec(self):
         class _One(object):
diff --git a/Misc/NEWS b/Misc/NEWS
index 90f2208302..9950725114 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -92,6 +92,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21256: Printout of keyword args should be in deterministic order in
+  a mock function call. This will help to write better doctests.
+
 - Issue #21677: Fixed chaining nonnormalized exceptions in io close() methods.
 
 - Issue #11709: Fix the pydoc.help function to not fail when sys.stdin is not a