Python 描述符以及property

descripter.py

class Integer(object):
    def __init__(self, name):
        self.name = name
    def __get__(self, instance, cls):
        if instance is None:
            return self
        else:
            return instance.__dict__[self.name]
    
    def __set__(self, instance, value):
        if not isinstance(value, int):
            raise TypeError('Expected an int')
        instance.__dict__[self.name] = value
        
class Point(object):
    x = Integer('x')
    y = Integer('y')
    def __init__(self, x, y):
        self.x = x
        self.y = y
        
 
if __name__ == '__main__':
    p = Point(2,3)
    
    # __get__ 描述符参数
    p.x  # Point.x.__get__(p, Point)
    
    Point.x # Point.x.__get__(None, Point)

memoized_property.py

# sqlalchemy.util
class memoized_property(object):
    """
    like cached_property, memoized_property is fget property
    
    so we can get more infomation about property
    
    __get__ can insert into obj an attr 
    """
    def __init__(self, fget, doc=None):
        self.fget = fget
        self.__doc__ = fget.__doc__ or doc
        self.__name__ = fget.__name__
    
    def __get__(self, obj, cls):
        if obj is None:
            return self
        obj.__dict__[self.__name__] = result = self.fget(obj)  # 传 obj 是绑定实例
        return result
    
    def _reset(self, obj):
        memoized_property.reset(obj, self.__name__)
        
    @classmethod
    def reset(obj, name):
        obj.__dict__.pop(name, None)
        
class Object(object):
    # info = memoized_property(info)
    @memoized_property
    def info(self):
        return {}

property.py

class A(object):
    def _get_value(self):
        return 'get'
    
    def _set_value(self, value):
        return value
    
    value = property(_get_value, _set_value, doc='')
    del _get_value, _set_value
   

def cache_property(key, empty, type):
    return property(lambda x: x._get_cache_value(key, empty, type),
                    lambda x, v: x._set_cache_value(key, v, type),
                    lambda x: x._del_cache_value(key),
                    'accessor for %r' % key)

class _CacheControl(UpdateDictMixin, dict):
    no_cache = cache_property('no-cache', '*', None)

1. __get__ 参数为 owner 的instance 和 Owner 的Class

property 的使用方法

1. 针对单属性的get set del, 是很好的方法
2. 针对多个属性的操作, 如果功能相同, 第二种操作很好

trick 的点:
首先 property 的参数有fget, fset, fdel 三个参数
每个参数都是可以调用的对象
注意第一种方法中的 property的声明，　说明了该函数在 class A 的命名空间中, 没有带类名
同时三个函数对象调用时第一参数是一个object 实例
不同的是 fset 的参数有两个