为什么它打印“创建的实例数：0”而不是“创建的实例数：3”？

Question

这是垃圾邮件.py：  class Spam:        numInstances = 0        def __init__(self):            Spam.numIntances = Spam.numInstances + 1        def printNumInstances():            print("Number of instances created: %s" % Spam.numInstances)这是另一个文件.py：from spam import Spama = Spam()b = Spam()c = Spam()Spam.printNumInstances()a.printNumInstances()为什么它打印“创建的实例数：0”而不是“创建的实例数：3”？

Answer 1

你有一些错别字，printNumInstances应该是一个类方法。

class Spam:

    numInstances = 0

    def __init__(self):

        Spam.numInstances = Spam.numInstances + 1

    @classmethod

    def printNumInstances(cls):

        print("Number of instances created: %s" % cls.numInstances)

a = Spam()

b = Spam()

c = Spam()

Spam.printNumInstances()

a.printNumInstances()

输出：

Number of instances created: 3

Number of instances created: 3