通过迭代器进行测试，该怎么做？

Question

I have made a simple generator in Python, it is under a file called program.py

class Loop():
    def __init__(self):
        self.i=1

    def __iter__(self):
        return self

    def next(self):
        if self.i>10:
            raise StopIteration
        self.i=self.i+1
        return self.i

I have made a unittest module so that it tests the iterator and when there is a number 5 on the list then it prints that the test has failed in that part. I have do something like this:

class TestA(unittest.TestCase):
def test(self):
    for x in Loop():
        if not self.assertRaises(AssertionError,self.assertEqual(x,5)):
            print x

but I see that this breaks up after the first iteration, what am I doing wrong?

any help? Thanks

Answer 1

You're approaching the test incorrectly.

Your iterator is basically the equivalent of xrange hardcoded to start from 2 and goes to infinity.

The reason it starts from 2 is because you're initialising i to 1 and incrementing i inside the next before returning the first value.

I would write the test as follows

class TestA(unittest.TestCase):
    def test_loop_is_sliceable(self):
        slc = itertools.islice(Loop(), 5)
        self.assertEqual(range(2, 7), list(slc))

Also note that asserts are not predicates so they don't give you back a bool to use in conditional statements.