Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
301 views
in Technique[技术] by (71.8m points)

python - Decorating a class method after @property

I want to wrap every method of various objects except __init__ using a decorator.

class MyObject(object):

    def method(self):
        print "method called on %s" % str(self)

    @property
    def result(self):
        return "Some derived property"

def my_decorator(func):
    def _wrapped(*args, **kwargs):
        print "Calling decorated function %s" % func
        return func(*args, **kwargs)
    return _wrapped


class WrappedObject(object):

    def __init__(self, cls):
        for attr, item in cls.__dict__.items():
            if attr != '__init__' and (callable(item) or isinstance(item, property)):
                setattr(cls, attr, my_decorator(item))
        self._cls = cls

    def __call__(self, *args, **kwargs):
        return self._cls(*args, **kwargs)

inst = WrappedObject(MyObject)()

However, the wrapping of a property instance results is equivalent to this:

@my_decorator
@property
def result(self):
    return "Some derived property"

When the desired result is something equivalent to this:

@property
@my_decorator
def result(self):
    return "Some derived property"

It seems the attributes of a property object are read-only preventing modifying the function after property has wrapped it. I'm not too comfortable with the level of hackery required already and I'd rather not delve into the property object anyway.

The only other solution I can see is to generate a metaclass on the fly which I was hoping to avoid. Am I missing something obvious?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

There are a few other issues in this sample, but to atain to question, all you have to do is, when you are wrapping a property

When you are wrapping a property, wrap its __get__ method instead:

class MyObject(object):

    def method(self):
        print "method called on %s" % str(self)

    @property
    def result(self):
        return "Some derived property"

    def common(self, a=None):
        print self

def my_decorator(func):
    def _wrapped(*args, **kwargs):
        print "Calling decorated function %s" % func
        return func(*args, **kwargs)
    return _wrapped


class WrappedObject(object):

    def __init__(self, cls):
        for attr, item in cls.__dict__.items():
            if attr != '__init__' and callable(item):
                setattr(cls, attr, my_decorator(item))
            elif  isinstance(item, property):
                new_property = property(my_decorator(item.__get__), item.__set__, item.__delattr__)
                setattr(cls, attr, new_property)
        self._cls = cls

    def __call__(self, *args, **kwargs):
        return self._cls(*args, **kwargs)

inst = WrappedObject(MyObject)()

That is the simpelst modification to your code that does the job. I'd however change it to dinamically a subclass of the classs it is wrapping, in order to avoid re-writing its attributes. You can create a subclass programtically by simply caling type with the name, a tuple withe the bases, and a dict as parameters.

edit - changing code to subclass wrapped class

Actually, subclassing the given class requires almost no modification on the given code, but for the type call I indicated. I just tested it here - change your WrappedObject class to:

class WrappedObject(object):

    def __init__(self, cls):
        dct = cls.__dict__.copy()
        for attr, item in dct.items():
            if attr != '__init__' and callable(item):
                dct[attr] =  my_decorator(item)
            elif  isinstance(item, property):
                new_property = property(my_decorator(item.__get__), item.__set__, item.__delattr__)
                dct[attr] = new_property
        self._cls = type("wrapped_" + cls.__name__, (cls,), dct)

    def __call__(self, *args, **kwargs):
        return self._cls(*args, **kwargs)

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...