[Fwd: Re: managed lists?]

[Gabriel Genellina]

En Tue, 22 May 2007 04:13:38 -0300, Jorgen Bodde  
<jorgen.maillist at gmail.com> escribió:

> Thanks. I agree that it is only 'me' that is the one doing it wrong.
> But consider this scenario:
>
> - Somewhere in my app I add a wrong type to an open regular list
> - The app continues like it should
> - After a (long) while I need to perform some search on the list, or  
> whatever
> - Exception occurs
>
> It makes it hard to test this way. For example I can do something to
> that list and not know about it much later .. If it blows up in my
> face, I can fix it, but when that error hinders people who are using
> my application, it is much worse.

You could use something like this to make a "restricted list":

class RestrictedList(list):
     def __init__(self, items, itemclass_=None):
         self.itemclass_ = itemclass_
         for item in items:
             self.append(item)

     def check(self, item):
         if not isinstance(item, self.itemclass_):
             raise ValueError("This list can only contain %r instances" %  
self.itemclass_)

     def __setitem__(self, index, item):
         if not isinstance(index,int):
             raise NotImplementedError
         self.check(item)
         list.__setitem__(self, index, item)

     def __setslice__(self, *args):
         raise NotImplementedError

     def append(self, item):
         self.check(item)
         list.append(self, item)

     def insert(self, index, item):
         self.check(item)
         list.insert(self, index, item)

     def extend(self, items):
         for item in items:
             self.append(item)

I think I'm catching all the ways someone could add a new item to this  
list.
You may change the check() method to test for another type of conditions  
instead of isinstance; by example, to ensure that all items have a "write"  
method (and could be used as file-like objects to output data):

     def check(self, item):
         try: item.write
         except AttributeError:
             raise ValueError("blah blah...")

-- 
Gabriel Genellina