需求:有一个计数器（属性），统计总共实例化了多少个对象
代码如下：
1 class Student: 2 school = 'Luffycity' 3 count = 0 4 5 def __init__(self, name, age, sex): 6 self.name = name 7 self.age = age 8 self.sex = sex 9 self.count += 1 10 11 def learn(self): 12 print('%s is learning' % self.name) 13 14 stu1 = Student('alex', 'male', 38) 15 stu2 = Student('jinxin', 'female', 78) 16 stu3 = Student('Egon', 'male', 18) 17 18 print(Student.count) 19 print(stu1.count) 20 print(stu2.count) 21 print(stu3.count)
结果为：
0
1
1
1
从以上结果可以看出，如果写成self.count ,他就会变成对象的私有属性，所以说虽然实例化了3次，但是类的count值为0，每个对象的count值为1
从以下验证代码可以看出：
1 print(stu1.__dict__) 2 print(stu2.__dict__) 3 print(stu3.__dict__) 4 5 结果为 6 7 {'name': 'alex', 'age': 'male', 'sex': 38, 'count': 1} 8 {'name': 'jinxin', 'age': 'female', 'sex': 78, 'count': 1} 9 {'name': 'Egon', 'age': 'male', 'sex': 18, 'count': 1}
所以说正确的代码实例如下：
1 class Student: 2 school = 'Luffycity' 3 count = 0 4 5 def __init__(self, name, age, sex): 6 self.name = name 7 self.age = age 8 self.sex = sex 9 # self.count += 1 10 Student.count += 1 11 12 def learn(self): 13 print('%s is learning' % self.name) 14 15 stu1 = Student('alex', 'male', 38) 16 stu2 = Student('jinxin', 'female', 78) 17 stu3 = Student('Egon', 'male', 18) 18 19 print(Student.count) 20 print(stu1.count) 21 print(stu2.count) 22 print(stu3.count) 23 print(stu1.__dict__) 24 print(stu2.__dict__) 25 print(stu3.__dict__) 26 27 结果为： 28 29 3 30 3 31 3 32 3 33 {'name': 'alex', 'age': 'male', 'sex': 38} 34 {'name': 'jinxin', 'age': 'female', 'sex': 78} 35 {'name': 'Egon', 'age': 'male', 'sex': 18}