python - Pass by value or by reference

Question

I have an object with an attribute that is a list. For example: obj.a = [3, 4, 5] I would like to get the following behavior (but I can't manage to find a solution using magics/etc.) : l = obj.a obj.a[0] = 2 print(l) --> [3, 4, 5] print(obj.a) ---> [2, 4, 5] Of course I could simply use copy.deepcopy : l = copy.deepcopy(obj.a) but for several reasons I would like, somehow, to make this step automatic/hide it for my users. [EDIT] Using getattribute and returning a copy won't work of course: import copy class Test: def __init__(self): self.a = [] def __getattribute__(self, attr): if attr == 'a': return copy.deepcopy(super(Test, self).__getattribute__(attr)) Any help appreciated ! Thnak you, Thomas JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)

Answer 1

It's not possible to make the assignment l = obj.a make a copy of obj.a. As deceze said in a comment, you could make a a property that returns a copy of the value every time you access it, but that would, well, make a copy every time you access it, not just when you assign it to l. That's going to be inefficient, and probably not the behavior you want anyway There's no way for obj or obj.a to tell the difference between something like this: x = obj.a and this: obj.a[:2] Whatever happens when you access obj.a, it's going to happen in both cases. You can't "look ahead" to see whether it's going to be assigned to a variable, just to copy it in that particular case.