bpo-37828: Fix default mock_name in unittest.mock.assert_called error (GH-16166)

In the format string for assert_called the evaluation order is incorrect and hence for mock's without name, 'None' is printed whereas it should be 'mock' like for other messages. The error message is ("Expected '%s' to have been called." % self._mock_name or 'mock').
(cherry picked from commit 5f5f11faf9)

Co-authored-by: Abraham Toriz Cruz <awonderfulcode@gmail.com>

Lib/unittest/mock.py

@@ -873,7 +873,7 @@ class NonCallableMock(Base):
         """
         if self.call_count == 0:
             msg = ("Expected '%s' to have been called." %
-                   self._mock_name or 'mock')
+                   (self._mock_name or 'mock'))
             raise AssertionError(msg)
 
     def assert_called_once(self):

Lib/unittest/test/testmock/testmock.py

@@ -388,6 +388,14 @@ class MockTest(unittest.TestCase):
         _check(mock)
 
 
+    def test_assert_called_exception_message(self):
+        msg = "Expected '{0}' to have been called"
+        with self.assertRaisesRegex(AssertionError, msg.format('mock')):
+            Mock().assert_called()
+        with self.assertRaisesRegex(AssertionError, msg.format('test_name')):
+            Mock(name="test_name").assert_called()
+
+
     def test_assert_called_once_with(self):
         mock = Mock()
         mock()

Misc/NEWS.d/next/Library/2019-09-15-21-31-18.bpo-37828.gLLDX7.rst

@@ -0,0 +1,2 @@
+Fix default mock name in :meth:`unittest.mock.Mock.assert_called` exceptions.
+Patch by Abraham Toriz Cruz.