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}
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.
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.
Assuming a is an instance of A and A has a class attribute bar:
a.bar = 4 creates an instance attribute bar which hides the class bar in the context of a
a.bar[4] = 2 only modifies the object that the class-level bar binds to (assuming it supports indexing)
a.bar += 1 - This one is nasty. If the class-level bar supports the += operation (e.g. by implementing __iadd__()), then it is modified in-place and no object-level attribute is created. Otherwise it is equivalent to a.bar = a.bar + 1 and a new instance attribute bar is created.
