[Tutor] Change to Class-level Variable

[Alan Gauld]

"Robert" <webtourist at gmail.com> wrote

>>>> class Foo( object ):
> ...     myid = 'Foo'
> ...     def __init__( self ):
> ...        pass
> ...
>>>> f1 = Foo()
>>>> f2 = Foo()
>>>> f1.myid = 'Bar'

This creates a myid instance variable in f1 which hides the class 
variable.
You should always modify class variables via the class not an instance
(you can read them either way but I prefer to use the class for both 
read
and write)


>>>> Foo.myid = 'SPAM'
>>>> f1.myid         <----------------------- Why is "f1" not affected 
>>>> by the Class variable change ?
> 'Bar'

Because this is accessing the instance variable not the class one.

You can get at the class variable from the instance usind __class__
but usually you don'tneed to. You know the class in context or you
want the instance data not the class data.

>>> f1.__class__.myid
'SPAM'

HTH,


-- 
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/