Ron Adam wrote:
Nick Coghlan wrote:
Kristján Valur Jónsson wrote:
So, maybe I should rephrase the "idea." The idea is for example to speed up the initialization of lists (and subclasses of lists) from other lists when the source list's contents is to be discarded. The semantics of a.swap(b) are the same as a[:], b[:] = b[:], a[:] but it a) is orders of magnitude faster b) skips any special methods (such as __setslice__) and so may break class invariants of list subclasses, as pointed out on this list.
It sounds to me like what is really desired in this case is not a copy/swap operation but a type mutate operation on the data. So instead of copying the data, the data's type is mutated to another type providing that the data format is compatible.
'Compatible' is the keyword. As I understand him, Kristján's problem is initializating list subclasses with additional fields, which are *not* compatible. Class mutation is already possible for user-defined classes.