Using assert raises unit testing in python

Question

Currently I'm writing a unittest for my class function.

def test_getitem(self):
    test1 = List(3)
    for i in range(3):
        test1.append(i)
    self.assertEqual(test1[2], 2)

    test1 = List(3)
    for i in range(3):
        test1.append(i)
    self.assertRaises(IndexError, test1[4])

the problem I'm having now is at the self.assertRaises part of my code. I'm not sure if that is how it's done but when I run the unittest, it produces an error Index out of range. By right, it should be "OK".

List is my class and List(3) creates an array based list. so when i test1.append(i), it is now [0,1,2].

test1[2] is a method of calling the getitem function in my class similar to self[index].

I'm wondering if I'm assert raising correctly? self.assertEqual is fine.

When you call assertRaises() like this you need to pass a callable as the second argument. So self.assertRaises(IndexError, lambda: test1[4]) would work, but @AK47's answer is the recommended way to do it. — Duncan
– Duncan, Commented Sep 13, 2017 at 13:03

Alan Kavanagh · Accepted Answer · 2017-09-13 12:59:56Z

7

You should use the with statement when you're asserting for exceptions being thrown

def test_getitem(self):
    test1 = List(3)
    for i in range(3):
        test1.append(i)
    self.assertEqual(test1[2], 2)

    test1 = List(3)
    for i in range(3):
        test1.append(i)
    with self.assertRaises(IndexError):
        test1[4]

answered Sep 13, 2017 at 12:59

Alan Kavanagh

9,9338 gold badges46 silver badges68 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

MSeifert · Accepted Answer · 2017-09-13 13:02:32Z

6

Arguments are evaluated before the function is called, so when you use test1[4] it's executed before the self.assertRaises is called. So it's not able to catch the exception.

That's also the reason why the 2-argument form has the signature assertRaises(exception, msg=None), the second argument here is the "msg" to match, not the function that is to be called. So it wouldn't have done the correct thing in any case.

You could use the context manager:

with self.assertRaises(IndexError):
    test1[4]

Or use the multiple argument form:

self.assertRaises(IndexError, test1.__getitem__, 4)

answered Sep 13, 2017 at 13:02

MSeifert

154k41 gold badges356 silver badges377 bronze badges

Collectives™ on Stack Overflow

Using assert raises unit testing in python

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related