error in tutorial for 3.0, section 9.3.3

[Dave Angel]

Vincent Davis wrote:
> Thank you that makes sense to me. Much more clear then the tutorial, I think
> so anyway. If you are learning about classes that you kinda expect MyClass
> to have counter in it. I might be nice to show that x.counter = 1 creates an
> instance that would look like (is this correct?)
>
> class MyClass:
>     """A simple example class"""
>     i = 12345
>     counter = 1
>     def f(self):
>         return 'hello world'
>
> Thanks again
>
> Vincent Davis
>
>
> On Sat, May 23, 2009 at 8:24 AM, Benjamin Kaplan
> <benjamin.kaplan at case.edu>wrote:
>
>   
>> On Sat, May 23, 2009 at 9:13 AM, Vincent Davis <vincent at vincentdavis.net>wrote:
>>
>>     
>>> let me add that I see that this could be right if x.counter = 1 and
>>> counter need not have anything to do with MyClass but this could be more
>>> clear.
>>> Thanks
>>> Vincent Davis
>>> 720-301-3003
>>>
>>>
>>> On Sat, May 23, 2009 at 7:08 AM, Vincent Davis <vincent at vincentdavis.net>wrote:
>>>
>>>       
>>>> Section 9.3.3 says that given,
>>>> class MyClass:
>>>>     """A simple example class"""
>>>>     i = 12345
>>>>     def f(self):
>>>>         return 'hello world'
>>>>
>>>> and x = MyClass()
>>>> then this
>>>>
>>>> x.counter = 1
>>>> while x.counter < 10:
>>>>     x.counter = x.counter * 2
>>>> print(x.counter)
>>>> del x.counter
>>>>
>>>> will print 16
>>>>
>>>> link,
>>>> http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes
>>>>
>>>> I am reading this section so to learn about classes but if this is right
>>>> I think I need to start over.
>>>>
>>>>
>>>>         
>> The code given is correct, though the description in the tutorial could be
>> clearer. Basically, a class in Python is represented by a dict with strings
>> mapping to other stuff. Internally, x.counter = 1 is just a shortcut for
>> x.__dict__['counter'] = 1. This appears in the code as dynamically adding
>> the variable "counter" to the instance of MyClass. Unlike in static
>> languages, an instance variable in python doesn't need to be declared inside
>> the class for you to use it. It also doesn't need to appear in every
>> instance of the class.
>>
>> The last line in the code (del x.counter) removes the "counter" key from x
>> so that the instance variable disappears. That's how the code works "without
>> leaving a trace".
>>
>>
>>
>>
>>     
>>>> Thanks
>>>> Vincent Davis
>>>> 720-301-3003
>>>>
>>>>         
>>> --
>>> http://mail.python.org/mailman/listinfo/python-list
>>>
>>>
>>>       
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
>>
>>     
(You accidentally top-posted.  That makes the context much harder for 
others to follow.)

No, the attribute 'counter' you demonstrated is an attribute of the 
class.  To duplicate what  x.counter=1 does, you have to add it to the 
instance.  Normally, this would be done in the __init__() method, but 
that would put it in each instance of the class, as it's being created.  
But this one is only in the 'x' instance.