如何在描述符外部访问存储在python描述符对象中的状态？

Question

Is it possible to get at state stored in a python descriptor?

For example:

class Price(object):
    def __init__(self, dollars):
        self.dollars = dollars

    class Convert(object):
        def __init__(self, rate):
            self.rate = rate
        def __get__(self, instance, owner):
            return instance.dollars * self.rate
    euros = Convert(0.75)

p = Price(20.0)

print p.dollars
print p.euros

works as expected; however, I'd like to get at the rate that is stored within the Convert descriptor that is managing p.euros is that possible? Clearly p.euros.rate won't work but I'm not sure how to get at the euros instance of Converter.

(I realize that in this simplified case making euros a property and putting rate on price makes sense it was just a simple example)

Answer 1

Actually @yak's comment on his own answer above gives the right way to do it: using the class __dict__ attribute.

Therefore, you can get the descriptor by doing, (on your code, which lists p as an instance of Price ) by doing: p.__class__.__dict__["euros"]

Note however that the descriptor itself is a "class variable" - so, still on your example, if you need different "Price" objects to have different "rate" attributes on the "euro" attribute, the Convert descriptor has to keep the "rate" not as self.rate, but in some (possibly name mangled) attribute of the "instance" parameter passed to the "__get__" and "__set__" methods.

If that is the case, you could access that attribute directly on the instance itself - or implement some "get" and "setters" (or a nested property) for dealing with the descirptor attribute you want to manipulate in the descriptor's class itself.

Answer 2

If you access the descriptor through the class, the instance argument of __get__ will be None. You can check for that and return the descriptor itself:

class Price(object):
    def __init__(self, dollars):
        self.dollars = dollars

    class Convert(object):
        def __init__(self, rate):
            self.rate = rate

        def __get__(self, instance, owner):
            if instance is None:
                # Accessed through class, return the descriptor instead.
                return self
            return instance.dollars * self.rate

    euros = Convert(0.75)

Then you get the rate like this:

>>> Price.euros.rate
0.75