Testing Django querysets: self.assertListEqual vs self.assertQuerysetEqual

Question

Testing in Django I've found two different ways to test functions that involve querysets. The first one is:

test_instance = FooFactory()
self.assertListEqual(list(Foo.objects.all()), [test_instance])

And the second one is:

test_instance = FooFactory()
self.assertQuerysetEqual(Foo.objects.all(), map(repr, [test_instance]))

Which of these two are better? I read from the Django docs that this is what assertQuerysetEqual does:

Asserts that a queryset qs returns a particular list of values values.

The comparison of the contents of qs and values is performed using the function transform; by default, this means that the repr() of each value is compared. Any other callable can be used if repr() doesn’t provide a unique or helpful comparison.

By default, the comparison is also ordering dependent. If qs doesn’t provide an implicit ordering, you can set the ordered parameter to False, which turns the comparison into a collections.Counter comparison. If the order is undefined (if the given qs isn’t ordered and the comparison is against more than one ordered values), a ValueError is raised.

I see two important things here: one is that the repr() of each value is compared, and the other is that assertQuerysetEqual allows you to make ordering optional.

For the first point, I think it's better to compare the models themselves instead of their repr values since then you know that they are exactly the same (although one could also make the argument that repr values should be distinct).

For the second, I can see how unordered comparisons could be useful. Then again, if you're specifically testing something where ordering isn't important/is easily simulated, I don't see the advantage there.

The only argument I can think of for assertQuerysetEqual (with those assumptions above) is that it's more readable/direct about its purpose.

Is there any practical difference between self.assertListEqual(list(<queryset>), [<expected_models_in_correct_order>]) and self.assertQuerysetEqual(<queryset>, map(repr, [<expected_models_in_correct_order>]) that I failed to account for?

Answer 1

The only practical difference is in how python does object and string comparison.

self.assertListEqual: compares list of model instances delegating to an __eq__ method which provides a rule for how comparison is done. See how Django does comparison for model instances.

self.assertQuerysetEqual: compares list of strings using the ord value of each character in each string.

Depending on how __eq__ and __repr__ is implemented for the object, one may provide slight performance gains over the other.