Class variables vs Class attributes

Language Help Python
1

I have a few questions about class variables and class attributes.

What are the differences between them?
When should they be used?
Can they be used interchangeably?

Thank you in advance

2

Hi, there!

I need some clarification before providing an answer. Which language are you requesting help in?

3 
class Foo:
    foo = "Class variable"
    def __init__(self, bar):
        self.bar = bar

>>> foo = Foo('instance variable')
>>> foo.foo
'Class variable'
>>> foo.bar
'instance variable'
>>> Foo.foo
'Class variable'
>>> Foo.bar
Traceback (most recent call last):
  File "<pyshell#39>", line 1, in <module>
    Foo.bar
AttributeError: type object 'Foo' has no attribute 'bar'
>>>
3 Likes
4

Something else to add to the kit: We cannot mutate the class variable with an instance. In that case we assign the new value to a newly created attribute that shadows the class variable.

>>> foo.foo = "class to instance variable"
>>> Foo.foo
'Class variable'
>>> foo.foo
'class to instance variable'
>>>

Note that foo now has its own ‘foo’ attribute and can no longer access the class variable, except by direct (Parent class) access.

>>> dir(foo)
[..., 'bar', 'foo']
>>>
5

Hey there, sorry this is coming 6 hours late. The language is Python

1 Like
6

Addendum

The situations that present with a need for class variables are for convenience and maybe even preference, but we would like to think there is a good reason for it.

Let’s try to think up a good reason. Hmmm. Oh! What about ID numbers?

>>> class Member:
...     id = 0
...     def get_id():
...         Member.id += 1
...         return Member.id
...     def __init__(self, name):
...         self.name = name
...         self.id = Member.get_id()
... 
...         
>>> a = Member('john')
>>> a.id
1
>>> b = Member('jane')
>>> b.id
2
>>> a = Member('jon')
>>> b = Member('jan')
>>> a.id
3
>>> b.id
4
>>>

Notice when we replace a and b they have new ids.

Aside

Python was assumed because it is in a Python category and the question was relatable.

2 Likes
7

This is very helpful, thank you so much.

1 Like
8

class Foo:
foo = “Class variable”
def init(self, bar):
self.bar = bar

I understand these lines. The class Foo has a class variable called foo. We then add a constructor to define the attributes of class Foo as self, and bar.

The following line is a bit confusing, and I’d appreciate some light shed on it.

foo = Foo(‘instance variable’)

I understand that this is an instance. What I don’t understand though is why Foo(self, bar) is being replaced with Foo('instance variable).

There is more I don’t understand. But If you will indulge me, could we go step by step, please?

Thank you so very much

9

self is not an attribute. It is the instance object. That is what, in that setting has the attribute, bar. When we instantiate a new object,

foo = Foo('instance variable')

the argument we have given is the value that will be assigned to the bar attribute. We’re not replacing anything.