On Fri, 26 Jun 2009 08:37:36 am Kristján Valur Jónsson wrote:
Since I started uncovering list bad-boys, I think perhaps list.pop() might be the evilest of the bunch in this context, since it does change the list length and so might crash unwary C extensions with some invariants.
True, the caller can shoot himself in the foot by bypassing your class and calling list.pop(instance) instead of instance.pop(). That's a bad thing, but probably unavoidable given Python's OO design. However, it's also a fairly unusual thing to do: like calling "private" double-underscore methods, anyone calling superclass.method(instance) instead of instance.method() will know that they're doing something unsupported and potentially dangerous. Since we agree it's a bad thing, why do you want to copy it and create another method which is even more dangerous, by design? As I see it, Python already has a perfectly good solution for swapping the contents of arbitrary mutable sequences, one which doesn't break any invariants: a[:], b[:] = b[:], a[:] What you really want is a fast way of initialising a list, given another list. That was your use-case, after all. Swapping the contents is just one particular implementation of that fast-initialise. I believe that the correct place for this is inside a list constructor, not as an external method which magically transports the internal data from one list into another. I suggested adding extra functionality to list.__new__, and Nick suggested a new constructor list.from_list(). Nick suggested making from_list destructive, but I don't see the advantage of doing so. I think this is a probably a more productive direction to take than a swap() method. -- Steven D'Aprano