In python, how to update attribute based on attribute of another object passed as an argument?

Question

Alright, let's start with the code, because it's pretty explicit :

class c_1:
    def __init__(self):
        self.a = 5

class c_2:
    def __init__(self,other_obj):
        self.other = other_obj
        self.b = self.other.a

And the output:

obj_1 = c_1()
obj_2 = c_2(obj_1)
print(obj_2.b)
# Output is 5
obj_1.a = 8
print(obj_2.b)
# Output is still 5

This is the problem. I know the second call of obj_2.b should return 5, but I want it to return 8.

I think that what I actually want, is for the value of obj_1.a to be passed by reference for obj_2.b (this example is pretty simple, but in my actual code, there are more attributes from obj_1 that obj_2 uses.)

Is there a way, without calling another method, to automatically update obj_2.b when the value of obj_1.a gets changed ? Thank you.

Answer 1

It is possible, but your mileage may vary, depending on your use cases:

One approach, shown hereunder, is to implement a Var class that handles this for you; encapsulating the values inside an object circumvents the 'passing by value', and opens the 'values' to mutability. here is an incomplete (and brittle) example; there are quite a few corner cases to iron out to make it work smoothly, but to answer your question: yes, it is definitely possible:

Other approaches might use the inspect module, or metaclasses, or the implementation of callbacks.

tkinter in the python standard library uses specialized classes, and a callback approach.

class Var:
    def __init__(self, value):
        self._value = value

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, value):
        self._value = value

    def __repr__(self):
        return str(self._value)


class IntVar(Var):

    def __iadd__(self, other: int):
        self._value = self._value + other
        return self.value


class c_1:
    def __init__(self):
        self._a = IntVar(5)

    @property
    def a(self):
        return self._a

    @a.setter
    def a(self, value):
        self._a.value = value


class c_2:
    def __init__(self, other_obj):
        self.other = other_obj
        self.b = self.other.a


obj_1 = c_1()
obj_2 = c_2(obj_1)
print(obj_2.b)
obj_1.a = 8
print(obj_2.b)
obj_1.a += 2
print(obj_2.b)

Output:

5
8
10