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__