How can I override the value of an object parameter in Python?

Question

I'm trying to change the given value when I instantiate an object in Python. The arguments are 2 lists (list_A and list_B) that appear in a GUI. I can manipulate the value of list_A, all the values are boolean-type. I want the list_B to show some values, but to be dependent on the values of the list_A. For example, if the first value of list_A is true, then show one half of list_B, if the second is true, then show the second half of list_B, and if both are true, show the whole list. But I need to completely instantiate the object first, in order to read the values of list_A, and then modify list_B.

I have no access to the class, that's why I cannot modify the workflow, I need to do it this way.

instance_var = foo( list_A, list_B ) # this is how I instantiate the object

So I don't know if, after the instantiation, I can access and modify list_B.

Thank you

Sorry, it would be Foo( list_A, list_B ). It's a class, not a function. — Gerardo Lankenau
– Gerardo Lankenau, Commented Jan 17, 2020 at 19:09
If you pass list_A into the constructor, you already have access to it. So why do you feel like you have to modify list_B inside the class instead of before passing it? — cadolphs
– cadolphs, Commented Jan 17, 2020 at 19:55
I don't understand the problem. You can modify the lists before you pass them in; it's not like they're iterators. Please provide a minimal reproducible example. You can edit the question. Also check out How to Ask if you want more advice. — wjandrea
– wjandrea, Commented Jan 18, 2020 at 1:50

SpecZynk · Accepted Answer · 2020-01-18 14:43:06Z

In Python, at the moment you creat an object and you set any attribute you can access it by __dict__, so when you instaciated your class you setted two attributes, so they were stored in __dict__, which means you can access them, modify them and so on easely through __dict__.

Let me show you an example, take a look:

class MyClass:

    def __init__(self, list_a, list_b):
        self.list_a = list_a
        self.list_b = list_b

    def print_list_b(self):
        print(f'list_b = {self.list_b}')


if __name__ == "__main__":

    list_a = [1, 2, 3] 
    list_b = ["a", "b", "c"]

    p1 = MyClass(list_a,list_b)
    p1.print_list_b()     #Output: list_b = ['a', 'b', 'c']

    print(p1.__dict__)    #Output: {'list_a': [1, 2, 3], 'list_b': ['a', 'b', 'c']}
    attr_list_b = p1.__dict__['list_b'] #Getting list_b
    print(attr_list_b)                  #Output: ['a', 'b', 'c']                 

    attr_list_b[2] = "X"    #Modifying list_b
    print(attr_list_b)      #Output: ['a', 'b', 'X']

    p1.print_list_b()       #Output: list_a = ['a', 'b', 'X']

As you can see we can easily change the behavior of our classes through __dict__.

For a better understanding of __dict__, I suggest you read about it in this answer here.

NOTE: YOU CAN USE ALL DICTIONARY' METHODS THROUGH __dict__, SINCE IT'S A DICTIONARY.

May help you, bye!

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