Class Variable Access and Assignment

[Steven D'Aprano]

On Thu, 03 Nov 2005 15:13:52 +0100, Eric Nieuwland wrote:

> Stefan Arentz wrote:
>> It is really simple. When you say b.a then the instance variable 'a'
>> is looked up first. If it does not exist then a class variable lookup
>> is done.
> 
> This mixing of class and instance variable might be the cause of 
> confusion...
> 
> I think of it as follows:
> 1 When the class statement ends a class object is created which is 
> filled by all the statements inside the class statement
> 	This means all variables and functions (methods) are created according 
> to the description.
> 	NOTE This happens just once.

Yes.

> 2 When an instance of the class is created, what effectively happens is 
> that a shallow copy of the class object is made.
> 	Simple values and object references are copied.

No.

py> class Parrot:
...     var = 0
...
py> p = Parrot()
py> Parrot.var is p.var
True
py> Parrot.var = {"Hello world": [0, 1, 2]}
py> Parrot.var is p.var
True

It all boils down to inheritance. When Python does a look up of an
attribute, it looks for an instance attribute first (effectively trying
instance.__dict__['name']). If that fails, it looks up the class second
with instance.__class__.__dict__['name'], and if that fails it goes into a
more complex search path looking up any superclasses (if any).


> This explains:
> - why methods and complex objects (e.g. lists) are shared among 
> instances of a class and the class itself
> - simple values are not shared

No. it is all about the inheritance, and mutable/immutable objects.



-- 
Steven.