Python错误：缺少1个必需的位置参数：'self'

我是Python的新手，我正在尝试学习如何使用类。有人知道这怎么不起作用吗？关于关键字“自我”的任何其他提示将不胜感激。

编码：

class Enemy:
    life = 3

    def attack(self):
        print('ouch!')
        self.life -= 1

    def checkLife(self):
        if self.life <= 0:
            print('I am dead')
        else:
            print(str(self.life) + "life left")


enemy1 = Enemy
enemy1.attack()
enemy1.checkLife()

错误：

C:\Users\Liam\AppData\Local\Programs\Python\Python36-32\python.exe C:/Users/Liam/PycharmProjects/YouTube/first.py
Traceback (most recent call last):
  File "C:/Users/Liam/PycharmProjects/YouTube/first.py", line 16, in <module>
    enemy1.attack()
TypeError: attack() missing 1 required positional argument: 'self'

Process finished with exit code 1