属性的代理访问

简单来说，代理是一种编程模式，它将某个操作转移给另外一个对象来实现。 最简单的形式可能是像下面这样：

class A:
    def spam(self, x):
        pass

    def foo(self):
        pass


class B1:
    """简单的代理"""

    def __init__(self):
        self._a = A()

    def spam(self, x):
        # Delegate to the internal self._a instance
        return self._a.spam(x)

    def foo(self):
        # Delegate to the internal self._a instance
        return self._a.foo()

    def bar(self):
        pass
如果仅仅就两个方法需要代理，那么像这样写就足够了。但是，如果有大量的方法需要代理， 那么使用 __getattr__() 方法或许或更好些：

class B2:
    """使用__getattr__的代理，代理方法比较多时候"""

    def __init__(self):
        self._a = A()

    def bar(self):
        pass

    # Expose all of the methods defined on class A
    def __getattr__(self, name):
        """这个方法在访问的attribute不存在的时候被调用
        the __getattr__() method is actually a fallback method
        that only gets called when an attribute is not found"""
        return getattr(self._a, name)
__getattr__ 方法是在访问attribute不存在的时候被调用，使用演示：

b = B()
b.bar() # Calls B.bar() (exists on B)
b.spam(42) # Calls B.__getattr__('spam') and delegates to A.spam
另外一个代理例子是实现代理模式，例如：

# A proxy class that wraps around another object, but
# exposes its public attributes
class Proxy:
    def __init__(self, obj):
        self._obj = obj

    # Delegate attribute lookup to internal obj
    def __getattr__(self, name):
        print('getattr:', name)
        return getattr(self._obj, name)

    # Delegate attribute assignment
    def __setattr__(self, name, value):
        if name.startswith('_'):
            super().__setattr__(name, value)
        else:
            print('setattr:', name, value)
            setattr(self._obj, name, value)

    # Delegate attribute deletion
    def __delattr__(self, name):
        if name.startswith('_'):
            super().__delattr__(name)
        else:
            print('delattr:', name)
            delattr(self._obj, name)
使用这个代理类时，你只需要用它来包装下其他类即可：

class Spam:
    def __init__(self, x):
        self.x = x

    def bar(self, y):
        print('Spam.bar:', self.x, y)

# Create an instance
s = Spam(2)
# Create a proxy around it
p = Proxy(s)
# Access the proxy
print(p.x)  # Outputs 2
p.bar(3)  # Outputs "Spam.bar: 2 3"
p.x = 37  # Changes s.x to 37
通过自定义属性访问方法，你可以用不同方式自定义代理类行为(比如加入日志功能、只读访问等)。