[TIP] Should I always provide a return value when writing my test using mock library?

John Wong gokoproject at gmail.com
Thu Apr 12 12:18:47 PDT 2012

First, I hope I am not making duplicate (my first time using this kind of
mailing list...)

> The first thing is to be clear in your mind which code you are intending
> to test. In this case, it sounds like you want to test your
> "render_reverse" function, but without exercising Django's "reverse"
> (which makes it a unit test). Mocking "reverse" is a reasonable way to
> go about this.

Yes. Exactly. I want to perform a unittest on this function, which calls
django's reverse (for whatever reason why I am doing this...)

Here is the actual function from lib.py (there is no class involved, all
functions in lib.py)

from django.core.urlresolvers import reverse
def render_reverse(f, kwargs):
    kwargs is a dictionary, usually of the form {'args': [cbid]}
    return reverse(f, **kwargs)

Because the name lookup is `reverse`, so I should patch
`myproject.myapps.mylibrary.reverse` as opposed to

I did the first method. I want to assert that `render_reverse` generates
whatever `reverse` generates (by calling `reverse` directly). That's the
goal. So I have the following test code:

from lib import render_reverse, print_ls

class LibTest(unittest.TestCase):

def test_render_reverse_is_correct(self):
    with patch('myproject.myapps.mylibrary.reverse') as mock_reverse:
        from lib import render_reverse
        mock_f = MagicMock(name='f', return_value='dummy_views')
        mock_kwargs = MagicMock(name='kwargs',return_value={'args':['123']})
        mock_reverse.return_value = '/natrium/cb/details/123'
        response = render_reverse(mock_f(), mock_kwargs())

        print mock_reverse.mock_calls  # prints []
    print mock_reverse.mock_calls  # prints []
    self.assertTrue('/natrium/cb/details/' in response)

1. mock_reverse was never because `mock_calls` returns []. I've asserted
with `called` method, and it returns False.
2. using render_reverse like this, the name `dummy_views` must exists - it
must be a view function that actually exists in my Django project.
Otherwise, it will return an error.
3. using render_reverse like this, the speed takes 0.2 - 0.3 seconds, as
opposed to 0.06 s when I used `mock_reverse`.

Is there anything wrong the code? I understand that `mock_f` and
`mock_kwargs` should be just simple input, rather than mock objects.

Thanks, Carl.
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