Op 2004-12-16, Max M schreef <[EMAIL PROTECTED]>: > Antoon Pardon wrote: > >> Well IMO there are two sides in this argument. The first is whether >> or not python allows mutable keys. The second is whether or not >> limiting keys to immutables in dictionaries provides a performance >> gain. > > > The problem is that you don't understand what dicts are typically used > for. Because of the nonliniarity in dict lookups, dicts are used for > optimisation. > > I actually think it's the most important tool for optimising Python code. > > If dicts allowed mutable keys, a dict would need to run code that > corresponds to:: > > def has_key(key): > for key in self.keys(): > if a_key == key: > return True > return False
No you just would have to advise the programmer that he shouldn't mutated objects that are keys in a dictionary, because if he does things will break. Just as people shouldn't mutate objects that are in a sorted list on which one wants to do searches based on the fact that the list is sorted and just as people shouldn't mutate objects that are in a heapqueue. The fact that the elements in certain kind of containers should be have some kind of invariant is not a good argument to limit those elements to immutable objects. And as it turns out to be, python allows mutable objects as keys just fine. > Using immutable keys, a code can be generated for the key, that can be > efficiently stored in something like a binary tree. Just as it can with a mutable key, as long as the programmer takes care not to mutate the objects that are currently used as a key. > This makes lookup *very much* faster, and is the speed concern that > Python programmers care about. > > Not the time taken to convert a list into a tuple. You shouldn't speak for others. Others do have argued about speed loss because of the need to copy a key before it was entered in a dictionary. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
