error in tutorial for 3.0, section 9.3.3

[Benjamin Kaplan]

On Sat, May 23, 2009 at 10:46 AM, Vincent Davis <vincent at vincentdavis.net>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'
>

That's how it would work in almost any language other than Python. In
Python, anything declared in that scope is a member of the class. For
something immutable like an int, it doesn't matter. If you can change it
however, you get problems. This is one of the biggest sources of problems
for python beginners (you get the same behavior with default method
arguments btw)

>>> class Foo :
...    lst = []
...
>>> a = Foo()
>>> b = Foo()
>>> b.lst
[]
>>> a.lst.append(1)
>>> b.lst
[1]



In Python, if you want something to be a part of the instance, you have to
add it to the instance. That's what the "self" parameter in the method
argument list is. x.f() is just syntax sugar for MyClass.f(x). Dynamically
adding variables can get very confusing, so people usually declare
everything (or almost everything) they're going to use in the __init__
method discussed in section 9.3.2.


>
>

> 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
>>>
>>>
>>
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>> http://mail.python.org/mailman/listinfo/python-list
>>
>>
>
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