[Tutor] subclass question

[Rich Lovely]

2009/12/20 David Perlman <dperlman at wisc.edu>:
> If I make a subclass of a built-in class, like this:
>
> class mylist(list):
>    def __init__(self):
>        list.__init__(self)
>
> Then it is valid for me to do this:
>
>>>> x=mylist()
>>>> x.hello=3
>>>>
>
> But I can't do this:
>
>>>> y=list()
>>>> y.hello=3
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> AttributeError: 'list' object has no attribute 'hello'
>>>>
>
> What is it that is special about built-in classes that prevents you from
> adding methods or, uh, whatever the generic term is for sub-variables?  Is
> there a way to make your own classes restricted like that?
>
> OK thanks!
>
>
> --
> -dave----------------------------------------------------------------
> Unfortunately, as soon as they graduate, our people return
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>
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You can use the __slots__ attribute to get the same behaviour:
>>> class Demo(object):
...     __slots__ = []
...
>>> d = Demo()
>>> d.a=1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Demo' object has no attribute 'a'

But beware:  You need to add every attribute you want your class to
have to __slots__, if they're not defined on the class (i.e. instance
variables, defined within methods):

>>> class Demo(object):
...    def __init__(self):
...        self.a = 1
...    __slots__ = []
...
>>> d = Demo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __init__
AttributeError: 'Demo' object has no attribute 'a'
>>> class Demo(object):
...    def __init__(self):
...        self.a = 1
...    __slots__ = ['a']
...
>>> d = Demo()
>>> d.a
1

This will need updating every time you add a new instance variable to
a class, but it does help detect typos.

There is also the option of using __getattr__, __setattr__ and so on,
but that's a little more complicated.

I will however say, that the original behaviour is entirely
intentional.  Python was designed to be a language used by consenting
adults who should know better than doing things that are likely to
break stuff.  For instance, there is nothing to stop you from
overwriting any of the builtin functions or types.

See http://docs.python.org/reference/datamodel.html#slots for some of
the other side effects.

-- 
Rich "Roadie Rich" Lovely

There are 10 types of people in the world: those who know binary,
those who do not, and those who are off by one.