Sublassing tuple works, subclassing list does not

Frank Millman frank at
Wed Mar 31 13:51:05 CEST 2010

On Mar 31, 8:49 am, "Frank Millman" <fr... at> wrote:
> Hi all

Thanks to all for the helpful replies.

Rob, you are correct, I had not realised I was adding attributes to the 
class instead of the instance. Your alternative does work correctly. Thanks.

Carl, I understand your concern about modifying attributes. In my particular 
case, this is not a problem, as the class is under my control, and an 
instance will not be modified once it is set up, but I agree one must be 
careful not to mis-use it.

My use-case is that I want to create a number of objects, I want to store 
them in a tuple/list so that I can retrieve them sequentially, and I also 
want to retrieve them individually by name. Normally I would create a tuple 
and a dictionary to serve the two purposes, but I thought this might be a 
convenient way to get both behaviours from the same structure.

Regarding adding elements after instantiation, I would subclass 'list', as 
suggested by others, and then add an 'add' method, like this -

    def add(self, name, value):
        setattr(self, name, value)

I tested this and it behaves as I want.

Having said all of this, I have realised that what I probably want is an 
ordered dict. I will play with the one in PyPi, hopefully it will render 
this entire discussion redundant. It was interesting, though, and I have 
learned a lot.

Thanks again


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