Python中变量前面加不加self的区别

class animal():
    def __init__(self):
        self.a = 5


class human():
    def __init__(self):
        a = 123

if __name__ == '__main__':
  animal = animal()
  print(animal.a)
  human = human()
  print(human.a)  # AttributeError: 'human' object has no attribute 'a'
class animal():
    def __init__(self)