Re: Re: Single assert / Single mock rule in practice

On 14 January 2014 16:31, Avi Kessner <[email protected]> wrote:

>
>
> "I could have replaced
> self.assertTrue(item_exists)
> with
> foo.xdep.get_item.assert_called_once_with(1)"
>
> These two things are not equivalent tests.
>

Yes, I've come up to this conclusion after some time.


> your first assert, is correct and is testing the interface.  You expect
> the item to exist if get_item returns the value of 1.
> Your second assertion is testing the implementation.
>


Indeed, and that is a matter of my concern.
I'm testing the implementation and somewhat implicitly I'm assuming this
particular test
verifies the foo.exists() - as the foo.exists() is implemented in such way
that if get_item returns
id equal to the id that is being tested.

Yes, I agree it is an unfortunate test, or it is a different test, hence my
questions.


What if you later change the get_item function to return an object instead
> of the item id?  Do you really want to re-write this test?
>


Yes, I will have to re-write the test and as far as I understand it is
nothing uncommon.
I admit, this also confuses me a bit.

Isn't a unit test somewhat tightly coupled to implementation?

Isn't the fact that I create a mock for a dependency used deep in
implementation of method/behaviour under test, isn't it exposing
implementation details, hence coupling the test and increasing risk
of reimplementing test if the method/behaviour under test changes?

AFAIU, this risk is included in price of unit tests, low level tests, isn't
it?

 Best regards,
-- 
Mateusz  Łoskot, http://mateusz.loskot.net