class object's attribute is also the instance's attribute?

Thu Aug 30 10:11:06 EDT 2012

On Thursday, August 30, 2012 3:25:52 PM UTC+2, Hans Mulder wrote:
> On 30/08/12 14:34:51, Marco Nawijn wrote:
>>>> > Note that if you change 'd' it will change for all instances!
>>>> That depends on how you change it.
>>>> >>>> bobj = A()
>> >>>> bobj.d
>> > 'my attribute'
>> > 
>> >>>> A.d = 'oops...attribute changed'
>>>> Here you change the attribute on the class.
>> That will affect all instances:
>>>> >>>> aobj.d
>> > 'oops...attribute changed'
>> > 
>> >>>> bobj.d
>> > 'oops...attribute changed'
>>>> You can also set the attribute on an instance:
>>>> >>> bobj.d = 'For bobj only'
>> >>> bobj.d
>> 'For bobj only'
>> >>>> aobj.d
>> > 'oops...attribute changed'
>>>> So, if you specifically change it on one instance, thenit won't
>> change on other instances of the same class.
>>>> > If you want attributes to be local to the instance, you have
>> > to define them in the __init__ section of the class like this:
>>>> That's a good idea, but it's not required. You can set them
>> later, as shown above.
>>>>>> > class A(object):
>> > 
>> > def __init__(self):
>> > d = 'my attribute'
>>>> That will just set the global variable d.
>> You want to set the instance attribute:
>>>> self.d = 'my attribute'
>>>> >>>> aobj = A()
>> >>>> bobj = A()
>> > 
>> >>>> aobj.d
>> > 'my attribute'
>>>> Note that aobj.d will not find the global variable d,
>> if neither the instance, nor the class nor any of the
>> base classes have that attribute.
>>>> I don't know where this 'my attribute' comes from, but
>> it's not the instance attribute you tried to set in the
>> __init__ method. Maybe your class A still has a class
>> attribute with that value from an earlier experiment.
>>>>>> Hope this helps,
>>>> -- HansM

Learned my lesson today. Don't assume you know something. Test it first ;). I have done quite some programming in Python, but did not know that class attributes are still local to the instances. It is also a little surprising I must say. I always considered them like static variables in C++ (not that I am an expert in C++). 
I knew of course that you don't have to define a local attribute in the __init__ method of a class, but I consider it good style and since the OP is a self claimed newbie I left out the other option. 
The missing "self" in the code below was a typo
class A(object): 

 def __init__(self): 
 d = 'my attribute' # should be self.d 
Regards,
Marco