”with“在Python中的语句

Question

我在我的代码中实现了很多类。现在我认识到了调用所有这些类的每一个方法，我需要添加一行：

with service as object: 

所以我尝试使用代理模式来自动完成这项工作，这是我的示例代码

class A(object): 
    def __init__(self, name): 
     self.name = name 
    def hello(self): 
     print 'hello %s!' % (self.name) 
    def __enter__(self): 
     print 'Enter the function' 
    def __exit__(self, exc_type, exc_value, traceback): 
     print 'Exit the function' 
#   
class Proxy(object): 
    def __init__(self, object_a): 
#  object.__setattr__(self, '_object_a', object_a) 
     self._object_a = object_a 

    def __getattribute__(self, name): 
     service = object.__getattribute__(self, '_object_a') 
#  with service as service: 
     result = getattr(service, name) 
     return result  

if __name__=='__main__': 
    a1 = A('A1') 
    b = Proxy(a1) 
    b.hello() 
    a2 = A('A2') 
    b = Proxy(a2) 
    b.hello() 

一切正常发现，我有输出：

hello A1! 
hello A2! 

但是，当我取消了with声明，我有错误

Enter the function 
Exit the function 
Traceback (most recent call last): 
    File "/home/hnng/workspace/web/src/test.py", line 30, in <module> 
    b.hello() 
    File "/home/hnng/workspace/web/src/test.py", line 24, in __getattribute__ 
    result = getattr(service, name) 
AttributeError: 'NoneType' object has no attribute 'hello' 

Answer 1

你__enter__方法返回None而不是返回的对象。它应该是：

def __enter__(self): 
    print 'Enter the function' 
    return self