How to use unittest's self.assertRaises with exceptions in a generator object?

Question

I got a generator object that I want to unittest. It goes through a loop and, when at the end of the loop a certain variable is still 0 I raise an exception. I want to unittest this, but I don't know how. Take this example generator:

class Example():
    def generatorExample(self):
        count = 0
        for int in range(1,100):
            count += 1
            yield count   
        if count > 0:
             raise RuntimeError, 'an example error that will always happen'

What I would like to do is

class testExample(unittest.TestCase):
    def test_generatorExample(self):
        self.assertRaises(RuntimeError, Example.generatorExample)

However, a generator object isn't calable and this gives

TypeError: 'generator' object is not callable

So how do you test if an exception is raised in a generator function?

Answer 1

assertRaises is a context manager since Python 2.7, so you can do it like this:

class testExample(unittest.TestCase):

    def test_generatorExample(self):
        with self.assertRaises(RuntimeError):
            list(Example().generatorExample())

If you have Python < 2.7 then you can use a lambda to exhaust the generator:

self.assertRaises(RuntimeError, lambda: list(Example().generatorExample()))