如何在新样式类中拦截对python"魔法"方法的调用？

Question

我试图在新的样式类中拦截对python的双下划线魔术方法的调用.这是一个简单的例子,但它显示了意图:

class ShowMeList(object):
    def __init__(self, it):
        self._data = list(it)

    def __getattr__(self, name):
        attr = object.__getattribute__(self._data, name)
        if callable(attr):
            def wrapper(*a, **kw):
                print "before the call"
                result = attr(*a, **kw)
                print "after the call"
                return result
            return wrapper
        return attr

如果我在列表周围使用该代理对象,我会获得非魔术方法的预期行为,但我的包装函数永远不会被魔术方法调用.

>>> l = ShowMeList(range(8))

>>> l #call to __repr__
<__main__.ShowMeList object at 0x9640eac>

>>> l.append(9)
before the call
after the call

>> len(l._data)
9

如果我不从对象继承(第一行class ShowMeList:),一切都按预期工作:

>>> l = ShowMeList(range(8))

>>> l #call to __repr__
before the call
after the call
[0, 1, 2, 3, 4, 5, 6, 7]

>>> l.append(9)
before the call
after the call

>> len(l._data)
9

如何使用新样式类完成此拦截？

Answer 1

出于性能原因,Python总是在类(和父类)中__dict__查找魔术方法,并且不使用普通的属性查找机制.解决方法是使用元类在创建类时自动为魔术方法添加代理; 例如,我已经使用这种技术来避免为包装类编写样板调用方法.

class Wrapper(object):
    """Wrapper class that provides proxy access to an instance of some
       internal instance."""

    __wraps__  = None
    __ignore__ = "class mro new init setattr getattr getattribute"

    def __init__(self, obj):
        if self.__wraps__ is None:
            raise TypeError("base class Wrapper may not be instantiated")
        elif isinstance(obj, self.__wraps__):
            self._obj = obj
        else:
            raise ValueError("wrapped object must be of %s" % self.__wraps__)

    # provide proxy access to regular attributes of wrapped object
    def __getattr__(self, name):
        return getattr(self._obj, name)

    # create proxies for wrapped object's double-underscore attributes
    class __metaclass__(type):
        def __init__(cls, name, bases, dct):

            def make_proxy(name):
                def proxy(self, *args):
                    return getattr(self._obj, name)
                return proxy

            type.__init__(cls, name, bases, dct)
            if cls.__wraps__:
                ignore = set("__%s__" % n for n in cls.__ignore__.split())
                for name in dir(cls.__wraps__):
                    if name.startswith("__"):
                        if name not in ignore and name not in dct:
                            setattr(cls, name, property(make_proxy(name)))

用法:

class DictWrapper(Wrapper):
    __wraps__ = dict

wrapped_dict = DictWrapper(dict(a=1, b=2, c=3))

# make sure it worked....
assert "b" in wrapped_dict                        # __contains__
assert wrapped_dict == dict(a=1, b=2, c=3)        # __eq__
assert "'a': 1" in str(wrapped_dict)              # __str__
assert wrapped_dict.__doc__.startswith("dict()")  # __doc__

Answer 2

使用__getattr__和__getattribute__是类的最后一个资源来响应获取属性.

考虑以下:

>>> class C:
    x = 1
    def __init__(self):
        self.y = 2
    def __getattr__(self, attr):
        print(attr)

>>> c = C()
>>> c.x
1
>>> c.y
2
>>> c.z
z

__getattr__只有在没有其他工作时才会调用该方法(它不适用于运算符,您可以在此处阅读).

在您的示例中,__repr__已经在object类中定义了许多其他魔术方法 .

有一件事可以做,思考,然后定义那些魔术方法然后调用__getattr__方法.我和它的答案(链接)检查这个问题以查看一些代码.