Example(s) of how to force iteration of lazy iterators.

force_iter.py

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import unittest


def force(it, num=None):
    """Force an iterator to completely exhaust itself.

    This is useful for iterators that work via side effect."""
    if num is None:
        for _ in it:
            pass
    else:
        if num > 0:
            for i, _ in enumerate(it, start=1):
                if i >= num:
                    break


class TestForceIter(unittest.TestCase):

    def test_without_num(self):
        expected = ['result 1', 'result 2', 'result 3']
        result = []

        # A function that works by side effect
        def func(i):
            result.append('result {0}'.format(i))

        self.assertEqual(result, [])
        force(map(func, range(1, 4)))
        self.assertEqual(result, expected)

    def test_with_num(self):
        expected = ['result 1', 'result 2', 'result 3']
        result = []

        # A function that works by side effect
        def func(i):
            result.append('result {0}'.format(i))

        self.assertEqual(result, [])
        force(map(func, range(1, 5)), num=3)
        self.assertEqual(result, expected)

    def test_with_num0(self):
        result = []

        # A function that works by side effect
        def func(i):
            result.append('result {0}'.format(i))

        self.assertEqual(result, [])
        force(map(func, range(1, 5)), num=0)
        self.assertEqual(result, [])



def _test():
    unittest.main()


if __name__ == '__main__':
    _test()