Django / Python unittest side_effect usage

The second in what might become a series in "unittesting strange code". I have the following function I am testing:

def filter_queryset(self):
        Filter our base queryset
    # Get our base queryset
    queryset = self.get_queryset()
    if self.tags:
            # Try the django-taggit way of filtering by tags
            queryset = queryset.filter(tags__name__in=self.tags)
        except FieldError:
            # Our queryset object is using django-taggable instead
            # filter by tags the ugly way...
            for tag in self.tags:
                queryset = queryset.filter(tags__icontains=tag)
    return queryset

First off let me assure you I realise that this is... ugly. It's a sad requirement for now that we are using two different django tag libraries, django-taggit and django-taggable. (Never use django-taggable). And this is about as generic a way as I can think of to have a filter function that works with both with the minimum amount of fuss. The issue that occurs when testing is that in order to test the functionality, I need to get queryset.filter() to raise a FieldError(). Fine, easily done with setting it up as a mock object with a side effect:

  def side_effect(*args, **kwargs):
        if kwargs.get("tags__name__in", None):
            return FieldError()

    mock_queryset = MagicMock()
    mock_queryset.filter.side_effect = side_effect

The issue with this is that because of the FieldError being raised, filter() can no longer be used in the except part of the function, meaning I cannot test for the correct called_count, or assert_any_call() etc.

Other than an upheaval of the codebase to only use one version of tagging, what are the ways around this?


Just make a separate function that raises FieldError if it's called one way, and doesn't in the other:

expected_return = MagicMock()

def fake_filter(**kwargs):
    if 'tags__name__in' in kwargs:
        raise FieldError()
    if 'tags__icontains' in kwargs:
        return expected_return
    raise ValueError("Neither expected kwarg present")

mock_queryset = MagicMock()
mock_queryset.filter = fake_filter

et cetera. I see now that you already do that with your side effect function, you can just do the same but more.

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