python中的self和cls

self用在对象方法中，是第一个参数，表示一个具体的实例本身。

Cls是类方法的第一个参数，表示类本身。

在对象方法中，也可以访问类，但用的是类名。

下面例子中，对象方法__init__和die都访问了类，使用类名Robot。

Print方法中的{:d}，是format函数所要求，:后面可带填充字符，无则填充空格。d表示十进制。

{:#>8d}是一个完整的例子，:后面是填充字符#，>表示右对齐，8表示宽度。

^、<、>分别是居中、左对齐、右对齐，后面带宽度。

例：

class Robot:

    """表示有一个带名字的机器人。"""

    population=0

    def __init__(self,name):

        self.name=name

        print("(Initializing {})".format(self.name))

        Robot.population+=1

    def die(self):

        """我挂了。"""

        print("{} is being destroyed!".format(self.name))

        Robot.population-=1

        if Robot.population==0:

            print("{} was the last one.".format(self.name))

        else:

            print("There are still {:d} robots working.".format(Robot.population))

           

    def say_hi(self):

        """来自机器人的诚挚问候

        没问题，你做得到。"""

        print("Greetings,my masters call me {}.".format(self.name))

    @classmethod

    def how_many(cls):

        """打印出当前的人口数量"""

        print("We have {:d} robots.".format(cls.population))

    droid1=Robot("R2_D2")

    droid1.say_hi()

    Robot.how_many()

    droid2=Robot("C_3PO")

    droid2.say_hi()

    Robot.how_many()

    print("\nRobots can do some word here.\n")

    print("Robots have finished their work.So let's destroy them.")

    droid1.die()

    droid2.die()

    Robot.how_many()

结果：

(Initializing R2_D2)

Greetings,my masters call me R2_D2.

We have 1 robots.

(Initializing C_3PO)

Greetings,my masters call me C_3PO.

We have 2 robots.

Robots can do some word here.

Robots have finished their work.So let's destroy them.

R2_D2 is being destroyed!

There are still 1 robots working.

There are still #######1 robots working.

C_3PO is being destroyed!

C_3PO was the last one.

We have 0 robots.