Changing instance attributes in for loop in method

Question

I have a problem where I want to change the class of some child elements in a class recursively.

I have a working solution, which is the following:

class PerformanceAggregator:
    def __init__(self, collection: pf.GenericPortfolioCollection):
        self.collection = collection
        self.change_children()

    def __getattr__(self, item):
        return getattr(self.collection, item, None)

    def change_children(self):
        for i in range(len(self.children)):
            if isinstance(self.children[i], pf.GenericPortfolioCollection):
                self.children[i] = PerformanceAggregator(self.children[i])

However, the change_children method is not very pythonic. It would be better with

class PerformanceAggregator2:
    def __init__(self, collection: pf.GenericPortfolioCollection):
        self.collection = collection
        self.change_children()

    def __getattr__(self, item):
        return getattr(self.collection, item, None)

    def change_children(self):
        for child in self.children:
            if isinstance(child, pf.GenericPortfolioCollection):
                child = PerformanceAggregator(child)

But this method does not work as intended. It does not replace the "child" element as the first method does. Do anybody have an idea about what is wrong?

In your second example, `child` is a variable that assigned to one of `self.children`. When you reassign it using `child = PerformanceAggregator(child)`, you are not changing the reference in `self.children` at all. You are just assigning the variable `child` to the new instance, but nothing else happens with it since you never update the parent object. — James, Dec 12 '19 at 11:03
Does this answer your question? [Assigning values to variables in a list using a loop](https://stackoverflow.com/questions/4687364/assigning-values-to-variables-in-a-list-using-a-loop) — Raj, Dec 12 '19 at 11:04
Answer is in the duplicate. Mandatory reading for you: [this article](https://nedbatchelder.com/text/names.html) explains in great length and depth all you need to know about python "variables". — bruno desthuilliers, Dec 12 '19 at 11:08
And for the pythonic way to deal with this case: [use `enumerate()`](https://docs.python.org/3/library/functions.html#enumerate) so you get both the child and it's index. — bruno desthuilliers, Dec 12 '19 at 11:09

score 1 · Accepted Answer · edited Dec 12 '19 at 11:17

When you're looping through self.children, you're assigning child to PerformanceAggregator(child) but not necessarily updating the child element in self.children.

This should work:

     def change_children(self):
         for (val,child) in enumerate(self.children):
             if isinstance(child, pf.GenericPortfolioCollection):
                 self.children[val] = PerformanceAggregator(child)

Changing instance attributes in for loop in method

1 Answers1