Python - Adding attribute to descriptor

Question

I've this descriptor:

# Generic descriptor
class Attribute(object):
    def __init__(self, value):
        self.value = value
    def __get__(self, instance, value):
        return self.value
    def __set__(self, instance, value):
        self.value = value

I would add an attribute 'modified' to check if an instance of descriptor is modified. Es.

# Generic descriptor
class Attribute(object):
    def __init__(self, value):
        self.value = value
        self.modified = False
    def __get__(self, instance, value):
        return self.value
    def __set__(self, instance, value):
        self.value = value
        self.modified = True

How I can do this ?

shx2 · Accepted Answer · 2013-09-26 13:17:38Z

2

Note that in your __get__ and __set__ methods, you actually want to access instance, not self (self being the actual Attribute object).

Here is one way of doing it:

class Attribute(object):
    def __init__(self, attr):
        self.attr = attr
    def __get__(self, instance, owner):
        return getattr(instance, self.attr)
    def __set__(self, instance, value):
        setattr(instance, self.attr, value)
        instance.modified = True

class A(object):
    def __init__(self):
        self._f = 0
        self.modified = False
    f = Attribute('_f')

a = A()
a.f
=> 0
a.modified
=> False
a.f = 33
a.modified
=> True

Of course, this snippet can be improved in many ways, depending on what you're trying to achieve.

answered Sep 26, 2013 at 13:17

shx2

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