python - Python 中的代理模式

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'
        return self
    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:

Enter the function A1
Exit the function A1
hello A1!
Enter the function A2
Exit the function A2
hello A2!

但这并不是我真正需要的，因为我需要的是:

with a1 as a1:
    a1.hello()

我必须有输出:

Enter the function A1
hello A1!
Exit the function A1
Enter the function A2
hello A2!
Exit the function A2

我需要什么才能得到那个结果？谢谢

Answer 1

我会使用装饰器:

class Proxy(object):
    def __init__(self, object_a):
        self._object_a = object_a

    def decorateEnterExit(self, obj, f):
        def inner(*args, **kwargs):
            with obj as _:
                return f(*args, **kwargs)
        return inner

    def __getattribute__(self, name):
        obj = object.__getattribute__(self, '_object_a')
        dee = object.__getattribute__(self, 'decorateEnterExit')
        return dee(obj, getattr(obj, name))

这样，只有在执行函数时才会计算 with 。按照您执行此操作的方式，它将首先评估 with(包括退出它)，然后 function 将被返回以供调用。现在我们返回一个函数，它本身将进入 with 并调用里面的函数。

>>> Proxy(A('Ax')).hello()
Enter the function
hello Ax!
Exit the function

请注意，您需要在A 的__enter__ 方法中返回self:

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