Python descriptor not hooking class level attribute

Question

I wanted to make a similar feature to ECMAScript 5 "frozen" object where you can't change anything on it.

Did this:

class Fixed(object):
        frzn = 'I AM AWESOME'
        def holdset(_,val):
                _.frzn = _.frzn
                print "frozen is frozen not setting to ", val
        frozen = property(fset=holdset)

        @classmethod
        def __setattr__(self, name, val):
                if name=="frozen":
                        print "You can't unfreeze the frozen"
                else:   
                        self.name = val

dd = Fixed()
dd.frozen = 12312312
dd.frozen = 'AA'
Fixed.frozen = {}
print dd.frozen
print Fixed.frozen

Which gives

You can't unfreeze the frozen You can't unfreeze the frozen {} {}

The first two assignments fail, when they reference the attribute as an instance member. The third assignment succeeds, when the attribute is referenced as a class member.

I don't understand why.

How can I make

Fixed.frozen = {}

Fail in the same way the other assignments did? And why doesn't it?

Ashwini Chaudhary · Accepted Answer · 2014-02-26 19:37:56Z

3

You need to override __setattr__ of the Metaclass. Metaclass is a class that creates a class (Similar to instance created by a class), So, to override things related to class object you need to modify the special methods on the Metaclass.

class Meta(type):

    def __setattr__(self, attr, val):
        if attr == "frozen":
            print "You can't unfreeze the frozen"
        else:   
            cls.name = val

class Fixed(object):
        __metaclass__ = Meta
        ...

edited Feb 26, 2014 at 19:37

answered Feb 26, 2014 at 19:27

Ashwini Chaudhary

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