This notebook was prepared by Donne Martin. Source and license info is on GitHub.

- Can we assume the inputs are valid?
- No, check for None

- Can we assume this fits memory?
- Yes

* None input -> TypeError * 7, 5 -> 2 * -5, -7 -> 2 * -5, 7 -> -12 * 5, -7 -> 12

Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

In [ ]:

```
class Solution(object):
def sub_two(self, val):
# TODO: Implement me
pass
```

**The following unit test is expected to fail until you solve the challenge.**

In [ ]:

```
# %load test_sub_two.py
import unittest
class TestSubTwo(unittest.TestCase):
def test_sub_two(self):
solution = Solution()
self.assertRaises(TypeError, solution.sub_two, None)
self.assertEqual(solution.sub_two(7, 5), 2)
self.assertEqual(solution.sub_two(-5, -7), 2)
self.assertEqual(solution.sub_two(-5, 7), -12)
self.assertEqual(solution.sub_two(5, -7), 12)
print('Success: test_sub_two')
def main():
test = TestSubTwo()
test.test_sub_two()
if __name__ == '__main__':
main()
```

Review the Solution Notebook for a discussion on algorithms and code solutions.