On 30/08/12 16:48:24, Marco Nawijn wrote:
> On Thursday, August 30, 2012 4:30:59 PM UTC+2, Dave Angel wrote:
>> On 08/30/2012 10:11 AM, Marco Nawijn wrote:
>>> On Thursday, August 30, 2012 3:25:52 PM UTC+2, Hans Mulder wrote:
>>>> <snip>
>>> Learned my lesson today. Don't assume you know something. Test it first ;).
A very important lesson.
Next week's lesson will be: if you test it first, then
paste it into a message for this forum, then tweak just
one unimportant detail, you'll need to test it again.
>>> I have done quite some programming in Python, but did not know that
class
>>> attributes are still local to the instances.
>> They're not. They're just visible to the instances, except where the
>> instance has an instance attribute of the same name. Don't be confused
>> by dir(), which shows both instance and class attributes.
>>
>> Please show me an example where you think you observe each instance
>> getting a copy of the class attribute. There's probably some other
>> explanation.
>
> I don't have an example. It was just what I thought would happen.
> Consider the following. In a class declaration like this:
>
> class A(object):
> attr_1 = 10
>
> def __init__(self):
> self.attr_2 = 20
>
> If I instantiated it twice:
>
> obj_1 = A()
> obj_2 = A()
>
> For both obj_1 and obj_2 attr_1 equals 10. What I thought would happen after
> the following statement:
>
> obj_1.attr_1 = 12
>
> is that obj_2.attr_1 also equals 12. This is what surprised me a little,
> that's all.
The trick is to look at obj_1.__dict__ to see what is defined locally:
>>> obj_1 = A()
>>> obj_1.__dict__
{'attr_2': 20}
>>> obj_1.attr_1 = 12
>>> obj_1.__dict__
{'attr_2': 20, 'attr_1': 12}
Hope this helps,
-- HansM
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