Python之回调魔法

Python中魔法(前后又下划线)会在对象的生命周期被回调. 借助这种回调, 可以实现AOP或者拦截器的思想.

在Python语言中提供了类似于C++的运算符重在功能：
一下为Python运算符重在调用的方法如下：
Method        Overloads        Call for
__init__        构造函数        X=Class()
__del__        析构函数        对象销毁
__add__        +                X+Y,X+=Y
__or__        |                X|Y,X|=Y
__repr__        打印转换        print X，repr(X)
__str__        打印转换        print X，str(X)
__call__        调用函数        X()
__getattr_   限制            X.undefine
__setattr__    取值            X.any=value
__getitem__    索引            X[key]，
__len__        长度            len(X)
__cmp__        比较            X==Y,X<Y
__lt__        小于            X<Y
__eq__        等于            X=Y
__radd__        Right-Side +        +X
__iadd__        +=                X+=Y
__iter__        迭代            For In
7.1    减法重载

Python代码

1. class Number:   
2. def __init__(self, start):   
3. self.data = start   
4. def __sub__(self, other): #minus method
5. return Number(self.data - other)   
6. number = Number(20)   
7. y = number – 10 # invoke __sub__ method

[python] view plaincopy

1. <span style="font-family: 宋体;">class Number:  
2. def __init__(self, start):  
3. self.data = start  
4. def __sub__(self, other): #minus method
5. return Number(self.data - other)  
6. number = Number(20)  
7. y = number – 10 # invoke __sub__ method</span>

7.2    迭代重载

Python代码

1. class indexer:   
2. def __getitem__(self, index): #iter override
3. return index ** 2
4. X = indexer()   
5. X[2]   
6. for i in range(5):   
7. print X[i] 

[python] view plaincopy

1. <span style="font-family: 宋体;">class indexer:  
2. def __getitem__(self, index): #iter override
3. return index ** 2
4. X = indexer()  
5. X[2]  
6. for i in range(5):  
7. print X[i]</span> 

7.3    索引重载

Python代码

1. class stepper:   
2. def __getitem__(self, i):   
3. return self.data[i]   
4. X = stepper()   
5. X.data = 'Spam'
6. X[1] #call __getitem__
7. for item in X: #call __getitem__
8. print item 

[python] view plaincopy

1. <span style="font-family: 宋体;">class stepper:  
2. def __getitem__(self, i):  
3. return self.data[i]  
4. X = stepper()  
5. X.data = 'Spam'
6. X[1] #call __getitem__
7. for item in X: #call __getitem__
8. print item</span> 

7.4    getAttr/setAttr重载

Python代码

1. class empty:   
2. def __getattr__(self,attrname):   
3. if attrname == 'age':   
4. return 40
5. else:   
6. raise AttributeError,attrname   
7. X = empty()   
8. print X.age #call__getattr__
9. class accesscontrol:   
10. def __setattr__(self, attr, value):   
11. if attr == 'age':   
12. # Self.attrname = value loops!
13. self.__dict__[attr] = value   
14. else:   
15. print attr   
16. raise AttributeError, attr + 'not allowed'
17. X = accesscontrol()   
18. X.age = 40 #call __setattr__
19. X.name = 'wang' #raise exception

[python] view plaincopy

1. <span style="font-family: 宋体;">class empty:  
2. def __getattr__(self,attrname):  
3. if attrname == 'age':  
4. return 40
5. else:  
6. raise AttributeError,attrname  
7. X = empty()  
8. print X.age #call__getattr__
9. class accesscontrol:  
10. def __setattr__(self, attr, value):  
11. if attr == 'age':  
12. # Self.attrname = value loops!
13. self.__dict__[attr] = value  
14. else:  
15. print attr  
16. raise AttributeError, attr + 'not allowed'
17. X = accesscontrol()  
18. X.age = 40 #call __setattr__
19. X.name = 'wang' #raise exception
20. </span> 

7.5    打印重载

Python代码

1. class adder:   
2. def __init__(self, value=0):   
3. self.data = value   
4. def __add__(self, other):   
5. self.data += other   
6. class addrepr(adder):   
7. def __repr__(self):   
8. return 'addrepr(%s)' % self.data   
9. x = addrepr(2)  #run __init__
10. x + 1 #run __add__
11. print x     #run __repr__

[python] view plaincopy

1. <span style="font-family: 宋体;">class adder:  
2. def __init__(self, value=0):  
3. self.data = value  
4. def __add__(self, other):  
5. self.data += other  
6. class addrepr(adder):  
7. def __repr__(self):  
8. return 'addrepr(%s)' % self.data  
9. x = addrepr(2)  #run __init__
10. x + 1 #run __add__
11. print x     #run __repr__</span>

7.6    Call调用函数重载

Python代码

1. class Prod:   
2. def __init__(self, value):   
3. self.value = value   
4. def __call__(self, other):   
5. return self.value * other   
6. p = Prod(2) #call __init__
7. print p(1) #call __call__
8. print p(2) 

[python] view plaincopy

1. <span style="font-family: 宋体;">class Prod:  
2. def __init__(self, value):  
3. self.value = value  
4. def __call__(self, other):  
5. return self.value * other  
6. p = Prod(2) #call __init__
7. print p(1) #call __call__
8. print p(2)</span> 

7.7    析构函数重载

Python代码

1. class Life:   
2. def __init__(self, name='name'):   
3. print 'Hello', name   
4. self.name = name   
5. def __del__(self):   
6. print 'Goodby', self.name   
7. brain = Life('Brain') #call __init__
8. brain = 'loretta' # call __del__