newbie class-building question

[Peter Otten]

jrpfinch wrote:

> I am constructing a simple class to make sure I understand how classes
> work in Python (see below this paragraph).
> 
> It works as expected, except the __add__ redefinition.  I get the
> following in the Python interpreter:
> 
>>>> a=myListSub()
>>>> a
> []
>>>> a+[5]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> TypeError: 'str' object is not callable
>>>>
> 
> Please could you let me know where I am going wrong.  I have hacked
> around with the code and tried googling this error message but am
> having difficulty finding the source of the problem.


> class myList:
>     def __init__ (self,value=[]):
>         self.wrapped=[]
>         for x in value :
>             self.wrapped.append(x)
>     def __repr__ (self):
>         return `self.wrapped`
>     def __getattr__ (self,attrib):
>         return getattr(self.wrapped,attrib,'attribute not found')

When Python is looking for an attribute that neither exists in myList nor
the 'wrapped' list, you give back a string.
In your case this happens for the __coerce__() method, and the interpreter
ends up calling "attribute not found" which of course must fail. The
solution:

- Don't provide a bogus default for getattr(), and get a traceback that is
  easier to understand.
- Use new-style classes (i.e. class myList(object): ...) which look for
  special methods in the class, not the instance.


Peter