def test_extend_positive(self):
"""Sign extension of a positive number doesn't change it. Note high
bit in field must be zero. 7 is a positive number in a 3-bit field,
but a (different) negative number in a 3-bit field.
"""
self.assertEqual(sign_extend(7, 4), 7)
self.assertNotEqual(sign_extend(7, 3), 7)
self.assertTrue(sign_extend(7, 3) < 0)
def test_extend_negative(self):
"""For negative numbers, sign extension restores the high bits"""
chunk = (-3) & 0b111
self.assertEqual(sign_extend(chunk, 3), -3)