019: class, objects and instance: property

属性在本质上来讲是一组方法，但是调用的时候却如同字段，换句话说，其实就是对字段的一种封装，在设定和读取的时候，可以很轻易的添加逻辑，而其调用方式其不会改变

在Pyhon中可以用@property来定义：

class Book(object):
    def __init__(self, title, price):
        self._title = title
        self._price = price
    
    @property
    def price(self):
        return "${}".format(self._price)        

    @price.setter    
    def price(self, value):
        self._price = value    

    @price.deleter
    def price(self):
        del self._price    

book = Book("Python Basic", 100)

print(book.price)        

book.price = 200
print(book.price)    

del book.price
print(book.price)    
    

运行结果：

$100
$200
Traceback (most recent call last):
  File "C:UsersMilespythonclass_object20160125_1.py", line 26, in <module>
    print(book.price)
  File "C:UsersMilespythonclass_object20160125_1.py", line 8, in price
    return "${}".format(self._price)
AttributeError: 'Book' object has no attribute '_price'