Why some class variables seem to act as static while others don't?

Question

Dictionaries and lists defined directly under the class definition act as static (e.g. this question)
How come other variables such as integer do not?

>>> class Foo():
        bar=1

>>> a=Foo()
>>> b=Foo()
>>> a.bar=4
>>> b.bar
1
>>> class Foo():
        bar={}

>>> a=Foo()
>>> b=Foo()
>>> a.bar[7]=8
>>> b.bar
{7: 8}

Answer 1

They are all class variables. Except for when you assigned a.bar=4 creating an instance variable. Basically Python has a lookup order on attributes. It goes:

instance -> class -> parent classes in MRO order (left to right)

So if you have

class Foo(object):
    bar = 1

This is a variable on the class Foo. Once you do

a = Foo()
a.bar = 2

you have created a new variable on the object a with the name bar. If you look at a.__class__.bar you will still see 1, but it is effectively hidden due to the order mentioned earlier.

The dict you created is at the class-level so it is shared between all instances of that class.

Answer 2

When you make the assignment

>>> a.bar=4

you are rebinding the Foo.bar name to 4, which is a new instance of an integer. On the other hand,

>>> a.bar[7]=8

Does not rebind Foo.bar to anything different, and simply modifies the dictionary that the name refers to.

If you do

>>> a.bar = {7: 8}

then you will rebind Foo.bar to a new dictionary.