實例屬性和類屬性

練習
為了統計學生人數，可以給Student類增加一個類屬性，每創建一個實例，該屬性自動增加：

# -*- coding: utf-8 -*-

class Student(object):
    count = 0

    def __init__(self, name):
        self.name = name
        Student.count+=1

# 測試:
if Student.count != 0:
    print('測試失敗!')
else:
    bart = Student('Bart')
    if Student.count != 1:
        print('測試失敗!')
    else:
        lisa = Student('Bart')
        if Student.count != 2:
            print('測試失敗!')
        else:
            print('Students:', Student.count)
            print('測試通過!')

　　在類的函數中也不能直接引用count類屬性，需要Student.count