[Python-ideas] Proposal for extending the collections module - bags / multisets, ordered sets / unique lists

[Steven D'Aprano]

On Sun, 19 Jul 2009 06:45:46 am Mathias Panzenböck wrote:
> I don't understand what's so bad about such a collection factory. As
> a programmer I don't care what class an object is. I just want an
> object that behaves a certain way. E.g. I want to say: I need a
> random accessible collection, or I need a collection with unique
> entries, or I need a collection with a fast (near to O(1)) way to
> append elements, or all of the above. What class this object will
> have I don't care.
>
> Where is the problem?


The problem is that you *do* need to care what the class is, because 
different classes have different interfaces:


>>> def selector(code):
...     return {0: list, 1: tuple, 2: set}[code]
...
>>> k = selector(0)
>>> instance1 = k([1, 2, 3, 4])
>>> k = selector(2)
>>> instance2 = k([1, 2, 3, 4])

So far so good. But now:

>>> instance1.append(5)
>>> instance2.append(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'set' object has no attribute 'append'


Unless you can guarantee identical interfaces, you can't use the 
different collection classes as drop-in replacements for each other. 
And you can't have identical interfaces once you have mutable and 
immutable types, so the whole idea is still-born.




-- 
Steven D'Aprano