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.
--
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