为什么同一语句打印两个不同的值?

时间:2014-09-28 15:18:02

标签: python

当我想要理解python自我概念时,我遇到了这个我认为有用的例子。但是有一部分让我感到困惑。为什么print a.i会输出两个不同的值?在第一种情况下,输出为5,这对我来说很有意义。但是后来几行print a.i语句输出123

def say_hi():
    return 'hi!'

i = 789

class MyClass(object):

    i = 5

    def prepare(self):
        i = 10
        self.i = 123
        print i

    def say_hi(self):
        return 'Hi there!'

    def say_something(self):
        print say_hi()

    def say_something_else(self):
        print self.say_hi()

输出

>>> print say_hi()
hi!
>>> print i
789
>>> a = MyClass()
>>> a.say_something()
hi!
>>> a.say_something_else()
Hi there!
>>> print a.i
5
>>> a.prepare()
10
>>> print i
789
>>> print a.i
123

3 个答案:

答案 0 :(得分:5)

您正在使用具有相同名称的全局,本地和实例属性:

def say_hi():        # This is the global function 'say_hi'
    return 'hi!'    

i = 789              # This is the global 'i'

class MyClass(object):

    i = 5  # This is a class attribute 'i'

    def prepare(self):
        i = 10           # Here, you are creating a new 'i' (local to this function)
        self.i = 123     # Here, you are changing the instance attribute 'i'
        print i          # Here, you are printing the new'ed 'i' (now with value 10)

    def say_hi(self):         # This is the object method 'say_hi' function
        return 'Hi there!'

    def say_something(self):
        print say_hi()         # Here, you are calling the global 'say_hi' function

    def say_something_else(self):
        print self.say_hi()    # Here, you are calling the object method 'say_hi' function

所以输出是正确的:

>>> print say_hi()          # global
hi!
>>> print i                 # global
789
>>> a = MyClass()
>>> a.say_something()       # say_something calls the global version
hi!
>>> a.say_something_else()  # say_something_else calls the object version
Hi there!
>>> print a.i               # class attribute 'i'
5
>>> a.prepare()             # prints the local 'i' and change the class attribute 'i'
10
>>> print i                 # global 'i' is not changed at all
789
>>> print a.i               # class attribute 'i' changed to 123 by a.prepare()
123

答案 1 :(得分:4)

prepare方法中的以下陈述之前:

self.i = 123

self.i引用类属性MyClass.i(因为未设置实例属性)

执行self.i = ..语句后,self.i会引用新值123。 (这不会影响类属性MyClass.i,从而产生新的实例属性)

答案 2 :(得分:2)

您正在prepare()函数中将类变量i更改为123:

    self.i = 123

之后,您通过print a.i调用类变量,结果将打印123.