def test_peek_does_not_change_the_stack_size():
stack = Stack()
stack.push(42)
previous_size = stack.size
stack.peek()
assert stack.size == previous_size
def test_push_sets_the_element_on_top():
stack = Stack()
stack.push(42)
assert stack.peek() == 42
def test_peek_on_empty_stack_returns_none():
stack = Stack()
assert stack.peek() == None
def test_peek_returns_the_element_on_top():
stack = Stack()
stack.push(42)
assert stack.peek() == 42