python - Python中的“with”语句

Question

我的代码中实现了很多类。现在我意识到，对于为所有这些类调用的每个方法，我需要添加一行：

with service as object:

所以我正在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，我有输出：

hello A1!
hello A2!

但是当我取消注释该with语句时，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

您的__enter__方法正在返回None而不是返回对象。它应该是：

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