Question

I have this (Py2.7.2):

class MyClass(object):
    def __init__(self, dict_values):
        self.values = dict_values
        self.changed_values = {} #this should track changes done to the values{}
        ....

I can use it like this:

var = MyClass()
var.values['age'] = 21
var.changed_values['age'] = 21

But I want to use it like this:

var.age = 21
print var.changed_values #prints {'age':21}

I suspect I can use properties to do that, but how?

UPDATE:

I don’t know the dict contents at the design time. It will be known at run-time only. And it will likely to be not empty

Answer 1

You can create a class that inherits from a dict and override the needed functions

class D(dict):
    def __init__(self):
        self.changed_values = {}
        self.__initialized = True

    def __setitem__(self, key, value):
        self.changed_values[key] = value
        super(D, self).__setitem__(key, value)

    def __getattr__(self, item):
        """Maps values to attributes.
        Only called if there *isn't* an attribute with this name
        """
        try:
            return self.__getitem__(item)
        except KeyError:
            raise AttributeError(item)

    def __setattr__(self, item, value):
        """Maps attributes to values.
        Only if we are initialised
        """
        if not self.__dict__.has_key('_D__initialized'):  # this test allows attributes to be set in the __init__ method
            return dict.__setattr__(self, item, value)
        elif self.__dict__.has_key(item):       # any normal attributes are handled normally
            dict.__setattr__(self, item, value)
        else:
            self.__setitem__(item, value)

a = D()
a['hi']  = 'hello'
print a.hi
print a.changed_values

a.hi = 'wow'
print a.hi
print a.changed_values

a.test = 'test1'
print a.test
print a.changed_values

output

>>hello
>>{'hi': 'hello'}
>>wow
>>{'hi': 'wow'}
>>test1
>>{'hi': 'wow', 'test': 'test1'}