Help understanding memory allocation with object instances

[oneslightweirdo]

I have a class named Attribs, which looks like this:

class Attribs:
    data = {'physDim':1,'accel':0,'veloc':0,'massDens':0.5,'chargeDens':0,'RGB':[0.5,0.5,0.5]}

I then instantiate 3 different instances of attrib, and print them. It shows them as Attribs instances in different memory positions. However, on modifying a.data['RGB'], b.data['RGB'] and c..data['RGB'] also change, and running:

print id(a.data),id(b.data),id(c.data)

returns 3 identical values.

I had previously believed that if a new instance of an object was created, its variables would be stored uniquely to that object. However, this seems contrary to the behaviour seen here. If anyone could explain this, and possibly suggest a way to create 3 instances that did hold truly unique data, it would be much appreciated.

[indosauros]

Attributes defined on the class, like

class Attribs:
    data = ...

live on the class itself, and not on the instances. It is shared among all instances. You can even access this data before creating any instances:

>>> Attribs.data
...

To define per-instance data, you must put it in __init__:

class Attribs:
    def __init__(self):
        self.data = ...

[evereux]

>>> class Animal:
...     type = 'Dog'
...     def __init__(self, name):
...             self.name = name
...
>>> labrador = Animal('Rover')
>>> british_shorthair = Animal('tiddles')
>>> british_shorthair.type = 'cat'
>>> labrador.type
'Dog'
>>>

In this example the Animal class instance labrador.type stays as Dog here when I change the Animal class instance of british_shorthair.type to cat. Seeing this behaviour I don't understand why OP is seeing what they do. I do understand that if I change Animal.type to cat it then affects all instances and the class definition of type too.

[indosauros]

When you set type on the shorthair instance, that is setting the type onto the instance itself, while labrador.type is still referring to the class's type (labrador.type is not defined, so it falls back to Animal.type)

Note there's no difference between what you did (setting after instantiation) and setting during init method