Python: attribute sharing between class and nested class

Question

There's something about indirectly linking to an attribute that apparently I don't understand. If I have a class, and I create a nested class which has a variable that points to the parent/controller class variable, I thought it should change when the controllers variable changes.

Example:

class A():
    def __init__(self):
        self.A_value = 10
        print("A_value in A after init:     " + str(self.A_value))
        self.B = self.B(self)
        print("")
        print("A_value in A after B init:   " + str(self.A_value))
        print("B_value in B after B init:   " + str(self.B.B_value))

    class B():
        def __init__(self, controller):
            self.B_value = controller.A_value
            print("B_value in B after init:     " + str(self.B_value))
            controller.A_value = 1
            print("B_value in B after change:   " + str(self.B_value))

A()

which results in

A_value in A after init:     10
B_value in B after init:     10
B_value in B after change:   10

A_value in A after B init:   1
B_value in B after B init:   10

What I expect to happen is B_value to be 1 at the end too, since it points to A_value. Why doesn't B_value change? Honestly, I don't understand why half my code works if this doesn't. And I'm having a hard time coming up with a way to search for the correct, I know that this is definitely answered in some way somewhere. (tested in Python 2.7 and 3.7)

There is no subclasses in the code you posted. subclass != nested class — DeepSpace
– DeepSpace, Commented Jan 14, 2020 at 12:34

chepner · Accepted Answer · 2020-01-14 12:43:06Z

2

You aren't changing the existing value that the A_value attribute references; you are changing what the name itself refers to. That has no effect on the original object still referenced by the B_value attribute. You'll want to read https://nedbatchelder.com/text/names.html for a good overview of how exactly Python names work.

self.A_value = 10 makes self.A_value reference a particular object in memory; call it M1, which is an int object representing 10.

self.B_value = controller.A_value results in self.B_value referencing the same object M1.

controller.A_value = 1 does not affect M1 at all; it makes controller.A_value refer to a new object (call it M2) that represents 1. The B_value object still refers to M1.

edited Jan 14, 2020 at 12:43

answered Jan 14, 2020 at 12:34

chepner

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3 Comments

wiseboar Over a year ago

thanks for the Link, this is very helpful indeed. I'm amazed how I came this far without running into this issue.

wiseboar Over a year ago

maybe as a bit of addendum: so if I assign a mutable, for example 'self.A_value = [1]', and then referenced it with 'self.B_value = controller.A_value' and changed it via 'self.B_value[0] = 10', it would actually change the value (which explains why most of my code works anyways, because I'm using lists and dictionaries all over the place)

chepner Over a year ago

That change isn't really an assignment. It looks like an assignment, but it's really a mutating method call, equivalent to setattr(self.B_value, 0, 10). You aren't changing the references, only the object both refer to.

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