How to mock a readonly property with mock?

Solution 1:

I think the better way is to mock the property as PropertyMock, rather than to mock the __get__ method directly.

It is stated in the documentation, search for unittest.mock.PropertyMock: A mock intended to be used as a property, or other descriptor, on a class. PropertyMock provides __get__ and __set__ methods so you can specify a return value when it is fetched.

Here is how:

class MyClass:
    @property
    def last_transaction(self):
        # an expensive and complicated DB query here
        pass

def test(unittest.TestCase):
    with mock.patch('MyClass.last_transaction', new_callable=PropertyMock) as mock_last_transaction:
        mock_last_transaction.return_value = Transaction()
        myclass = MyClass()
        print myclass.last_transaction
        mock_last_transaction.assert_called_once_with()

Solution 2:

Actually, the answer was (as usual) in the documentation, it's just that I was applying the patch to the instance instead of the class when I followed their example.

Here is how to do it:

class MyClass:
    @property
    def last_transaction(self):
        # an expensive and complicated DB query here
        pass

In the test suite:

def test():
    # Make sure you patch on MyClass, not on a MyClass instance, otherwise
    # you'll get an AttributeError, because mock is using settattr and
    # last_transaction is a readonly property so there's no setter.
    with mock.patch(MyClass, 'last_transaction') as mock_last_transaction:
        mock_last_transaction.__get__ = mock.Mock(return_value=Transaction())
        myclass = MyClass()
        print myclass.last_transaction

Solution 3:

If the object whose property you want to override is a mock object, you don't have to use patch.

Instead, can create a PropertyMock and then override the property on the type of the mock. For example, to override mock_rows.pages property to return (mock_page, mock_page,):

mock_page = mock.create_autospec(reader.ReadRowsPage)
# TODO: set up mock_page.
mock_pages = mock.PropertyMock(return_value=(mock_page, mock_page,))
type(mock_rows).pages = mock_pages