def test_stack_opper_swap_subtract(self):
stack = Stack()
stack.append(5)
stack.append(10)
stack.swap()
stack.math.subtract()
assert len(stack) is 1 and stack[-1] is 5
def test_swap(self):
stack = Stack('abc')
stack.swap()
assert stack[-1] is 'b' and stack[-2] is 'c'