How to mock a readonly property with mock?

Question

How do you mock a readonly property with mock?

I tried:

setattr(obj.__class__, 'property_to_be_mocked', mock.Mock()) 

but the issue is that it then applies to all instances of the class... which breaks my tests.

Do you have any other idea? I don't want to mock the full object, only this specific property.

Answer 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() 

Answer 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