Python Object Reference

Question

I am a bit confused on how object values persist in python.

Let say I have an class object a that has a specific set of attributes attached to it. Multiple classes B, C have their own references to class a when each of them are created. Only class C can modify stuff in class a. When it is modified, does class B pick them up? From creating my own example they seem to do so.

My question is are there any instances where I should be aware of when changes to wont be picked up? When does python create an copy of class a? More specifically, what will create a copy of it that wont affect the original a.

class A:

    def __init__(self):
        self.x = 1

    def changeX(self,num):
        self.x = num


class B:

    def __init__(self,classA):
        self.x = classA

class C:
    def __init__(self,classA):
        self.x = classA

    def ChangeA(self,num):
        self.x.changeX(num)

c_a = A()
c_b = B(c_a)
c_c = C(c_a)


c_c.ChangeA(2)

# All returns 2
c_a.x
c_b.x.x
c_c.x.x

AdamMcKay · Accepted Answer · 2015-02-22 01:35:09Z

1

Assignment statements in Python don't copy objects, they create bindings between the target and an object.

An easy to understand guide on how this different from other languages is available here.

When you need to change an assigned object so it does not affect other assigned objects you can use the copy module. (https://docs.python.org/3.4/library/copy.html)

import copy

class A:

    def __init__(self):
        self.x = 1

    def changeX(self,num):
        self.x = num


class B:

    def __init__(self,classA):
        self.x = classA

class C:
    def __init__(self,classA):
        self.x = classA

    def ChangeA(self,num):
        self.x.changeX(num)

c_a = A()
c_b = B(c_a)
c_c = copy.deepcopy(C(c_a))


c_c.ChangeA(2)

c_a.x #1
c_b.x.x #1
c_c.x.x #2

answered Feb 22, 2015 at 1:35

AdamMcKay

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Comments

Simeon Visser · Accepted Answer · 2015-02-22 01:30:34Z

1

Unless you create a copy Python won't create a copy for you. You can use the copy module to create copies of objects or you can create a new object with the same values.

In your example there is only once instance of class A and each of the c_a, c_b and c_c have access to it. That's why you're seeing 2 in each case as they're all the same attribute on the same object.

answered Feb 22, 2015 at 1:30

Simeon Visser

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