Python中魔法方法的学习

python基础

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2019-8-26

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概念解析

魔法方法是python的一种高级语法,他们是可以给你的类增加“magic”的特殊方法,比如:增加一些额外的功能。函数格式为_ xx _(被双下划线包围的函数)。

魔法方法

  • 基本定制
object.__init__(self, *args, **kwargs)  # 构造器,创建完对象后调用,进行当前实例的初始化
object.__new__(cls, *args, **kwargs)    # 构造器,创建对象时调用,返回值是一个对象
object.__del__(self)                    # 解构器,删除对象时调用
object.__str__(self)                    # 若类C中实现此方法,当str(C())时此方法被调用
object.__len__(self)                    # 若类C中实现此方法,当len(C())时此方法被调用

测试代码如下:

# coding=utf-8
class MagicDemo(object):
    def __init__(self):
        """对当前对象的实例的一些初始化"""
        print "call me second"
        self.value = [1, 2, 3]

    def __new__(cls, *args, **kwargs):
        """
        真正的构造函数,先于__init__调用,
        返回创建的对象
        """
        print "call me first"
        return super(MagicDemo, cls).__new__(cls, *args, **kwargs)

    def __del__(self):
        print "call __del__"
        del self

    def __len__(self):
        print "call __len__"
        return len(self.value)

    def __str__(self):
        return "call __str__"
if __name__ == '__main__':
    mag = MagicDemo()
    print mag.value
    print len(mag)
    print str(mag)
    del mag

运行结果如下:

call me first
call me second
[1, 2, 3]
call __len__
3
call __str__
call __del__
  • 比较函数
object.__lt__(self, obj)                # 小于比较 <
object.__le__(self, obj)                # 小于或等于 <=
object.__gt__(self, obj)                # 大于 >
object.__ge__(self, obj)                # 大于多等于 >=
object.__eq__(self, obj)                # 等于 ==
object.__ne__(self, obj)                # 不等于 !=

测试代码如下:

# coding=utf-8
class MagicDemo(object):
    def __init__(self, value):
        self.value = value

    def __lt__(self, obj):
        print "call the __lt__"
        return self.value < obj.value

    def __le__(self, obj):
        print "call the __le__"
        return self.value <= obj.value

    def __gt__(self, obj):
        print "call the __gt__"
        return self.value > obj.value

    def __ge__(self, obj):
        print "call the __ge__"
        return self.value >= obj.value

    def __eq__(self, obj):
        print "call the __eq__"
        return self.value == obj.value

    def __ne__(self, obj):
        print "call the __ne__"
        return self.value != obj.value
if __name__ == '__main__':
    mag1 = MagicDemo(5)
    mag2 = MagicDemo(6)
    print mag1 < mag2
    print mag1 <= mag2
    print mag1 > mag2
    print mag1 >= mag2
    print mag1 == mag2
    print mag1 != mag2

运行结果如下:

call the __lt__
True
call the __le__
True
call the __gt__
False
call the __ge__
False
call the __eq__
False
call the __ne__
True
  • 属性操作
object.__getattr__(self, attr)                   # 获取属性,仅当属性找不到时被调用
object.__setattr__(self, attr, val)              # 设置属性,当设置属性的值时被调用
object.__delattr__(self, attr)                   # 删除属性时被调用
object.__getattribute__(self, attr)              # 获取属性,总是被调用,发生异常时调用__getattr__
object.__get__(self, attr)                       #(描述符)获取属性
object.__set__(self, attr, val)                  #(描述符)设置属性
object.__delete__(self, attr)                    #(描述符)删除属性

测试代码(描述符的三个属性暂不说明)

# coding=utf-8
class MagicDemo(object):
    def __init__(self, value):
        self.value = value

    def __getattr__(self, attr):
        """找不到属性时被调用"""
        print "call __getattr__"
        return "%s no found" % attr

    def __setattr__(self, attr, value):
        print "call __setattr__"
        self.__dict__[attr] = value

    def __delattr__(self, attr):
        print "call __delattr__"
        del self.__dict__[attr]

    def __getattribute__(self, attr):
        """此方法只适用于新式类"""
        print "call __getattribute__"
        # return self.__dict__[name]
        # 为了避免陷入死循环
        return object.__getattribute__(self, attr)
if __name__ == '__main__':
    mag = MagicDemo(5)
    print "_______________"
    mag.value = 6
    print "_______________"
    print mag.value
    print "_______________"
    print mag.xxx

测试结果如下:

call __setattr__
call __getattribute__
_______________
call __setattr__
call __getattribute__
_______________
call __getattribute__
6
_______________
call __getattribute__
call __getattr__
xxx no found

可以看出,当访问对象的属性时,__getattribute__总是被调用(新式类中有效),当找不到属性时,将调用__getattr__。其中__getattribute__调用基类的方法是为了陷入死循环,因为__getattribute__总是无条件的被调用。

  • 自定义序列
object.__getitem__(self, key)                    # 得到单个序列元素
object.__setitem__(self, key, val)               # 设置单个序列元素
object.__delitem__(self, key)                    # 删除单个序列元素
object.__contains__(self, key)                   # 测试序列元素,使用in时调用
object.__iter__(self)                            # 返回一个迭代器

测试代码:

# coding=utf-8
class MagicDemo(object):
    def __init__(self):
        self.value = dict()

    def __getitem__(self, key):
        print "call the __getitem__"
        return self.value[key]

    def __setitem__(self, key, val):
        print "call the __setitem__"
        self.value[key] = val

    def __delitem__(self, key):
        print "call the __delitem__"
        del self.value[key]

    def __contains__(self, key):
        print "call the __contains__"
        return key in self.value

if __name__ == '__main__':
    mag = MagicDemo()
    mag["name"] = "hello"
    print "_______________"
    print mag["name"]
    print "_______________"
    print "xxx" in mag

测试结果如下:

call the __setitem__
_______________
call the __getitem__
hello
_______________
call the __contains__
False

需要注意的是当属性为dict时通过键值对进行赋值,当为list时,通过下标进行赋值。

作者:凌绝少爷