在python中获取属性的属性

Question

I have a decorator which simply caches return values (called @cached in my example) and I wish to use it in conjunction with @property . This works just fine normally. The problem I am facing occurs when I try and use an expire attribute added by @cached .

def cached(f):
    cache = [None]

    def inner(*args, **kwargs):
        if cache[0]:
            cache[0] = f(*args, **kwargs)
        return cache[0]

    def expire():
        cache[0] = None

    inner.expire = expire
    return inner

class Example(object):
    @property
    @cached
    def something_expensive(self):
        print("expensive")
        return "hello"


e = Example()
e.something_expensive
e.something_expensive.expire()

How am I able to get access to the expire function? added to the function after its replaced by @property . I understand why this doesn't work I am interested in a way of working around that problem.

Some restrictions:

• I cannot change the @cached decorator its in a library I don't control
• I would really rather not remove @property because I want to expire in my unit tests and they make my code much nicer to use.

One solution that I think is rather bad is (because In reality I have a lot of properties that I want to do this for):

class Example(object):
    @cached
    def _something_expensive(self):
        return "hello"

    @property
    def something_expensive(self):
        return self._something_expensive()

Answer 1

You can access it using the class dictionary:

type(e).__dict__['something_expensive'].fget.expire()

In general e.something_expensive is equivalent to:

type(e).__dict__['something_expensive'].__get__(e, type(e))

For more details read up: Descriptor HowTo Guide

Note that inside the expiry function you're not setting cache from the outer function cached function as None , you're simply creating a new local variable. You may want to do something like this:

def expire():
    del cache[:]
    cache.append(None)

In Python 3 it's even easier to update cache using the nonlocal keyword.