- Run tests, from main folder, via `python -m unittest`; test files have to start with `test_` prefix, and be located in `test/` folder

cube.py

class Cube:
    def __init__(self, length):
        self.length = length

    def surface_area(self):
        return 6 * self.length ** 2

    def volume(self):
        return self.length ** 3

    def shrink(self, delta):
        if self.length - delta < 0:
            raise ValueError('Length must be >= 0')
        self.length -= delta

    def grow(self, delta):
        self.length += delta

test_cube.py

import unittest

from cube import Cube

class TestCube(unittest.TestCase):
   def setUp(self):
       self.cube = Cube(3)

   def test_surface_area_calculation(self):
       self.assertEqual(self.cube.surface_area(), 54)

   def test_volume_calculation(self):
       self.assertEqual(self.cube.volume(), 27)

   def test_shrink_decreases_length(self):
       self.cube.shrink(2)
       self.assertEqual(self.cube.length, 1)

   def test_grow_increases_length(self):
       self.cube.grow(4)
       self.assertEqual(self.cube.length, 7)

   def test_shrink_throws_exception_if_negative_length(self):
       with self.assertRaises(ValueError):
           self.cube.shrink(4)