Why Doesn't An Attribute Value In A Dictionary Within A Class Refer To The Same Attribute Of The Class?

class test(object): def __init__(self): self.a = 5 self.d = {'alfa': self.a} I instantiate an object of the class, and assign a new value to it: a_test = te

Solution 1:

Because self.a contains the value 5 when you assign it in the dictionary. Integers are immutable so no reference is made. Please read about Python's data model.

Solution 2:

You create the dictionary when you call the constructor. Therefore the values in the dictionary are set during construction - they aren't passed by reference, but rather by value.

The values in the dictionary point to value of the object that is passed in, but not the object itself. So simply having self.a as a value doesn't automatically update the dictionary, unless self.a is mutable (eg. a list).

Solution 3:

Let me propose my DictObjclass, which I've been using for issues like this:

classDictObj(dict):
    """
    a dictionary like class that allows attribute-like access to its keys

    d = DictObj(a=2, b=7)
    d['a']  --> 2
    d.a     --> 2

    """def__getattr__(self, name):
        return self.__getitem__(name)


    def__setattr__(self, name, value):
        if name in self:
            self.__setitem__(name, value)
        else:
            super(DictObj, self).__setattr__(name, value)


    def__add__(self, other):
        res = DictObj(self)
        res.update(other)
        return res

Items are stored in the dictionary, but can alternatively be accessed as attributes of the object.

You can extend this easily with more magic methods to suit your needs.

Solution 4:

You should add a setter for manage your class instance:

defset_a(self, new_val):
   self.a = new_val
   self.d = {'alfa': self.a}

Solution 5:

>>>a = 5>>>b = {"key":a}>>>b
{'key': 5}
>>>a = 7>>>b
{'key': 5}
>>>

you can see here that at the assigning instance value of "a" passed to dictionary "b" and it is 5.

so b['key'] has value 5 not reference of a. So no matter you are changing value at reference of a.