How to use unittest to assert an assert?

Question

Let's say I have this function:

def to_upper(var):
    assert type(var) is str, 'The input for to_upper should be a string'
    return var.upper()

And a file for unit testing using unittest :

class Test(unittest.TestCase):

    def test_1(self):
        -- Code here --


if __name__ == '__main__':
    unittest.main()

How do I test that if I call to_upper(9) it throws an assertion error?

Answer 1

You can assert an assertion with assertRaises(AssertionError) :

def test_1(self):
    with self.assertRaises(AssertionError):
        to_upper(9)

Answer 2

Assertions are for debugging. If there is some precondition that is important enough to verify with a unit test, check for it explicitly and raise a ValueError or TypeError , as appropriate, if it fails.

In this case, you don't actually care the var is a str , though; you just need it to have an upper method.

def to_upper(var):
    try:
        return var.upper()
    except AttributeError:
        raise TypeError("Argument has no method 'upper'")