w chun wrote: > On 4/4/06, Kent Johnson <[EMAIL PROTECTED]> wrote: >> Mike Hansen wrote: >>>>> - you can use __slots__ to restrict arbirtrary creation of >>>> (dynamic) >>>>> instrance attributes >>>> You can do this, but it is generally considered a misuse of >>>> __slots__ and potentially problematic. > > what specific problems were you thinking of? i would say that's it's > clumsy to use during development because you're constantly changing > __dict__ by adding new instance attributes, etc., so i only add a > __slots__ when i'm finally done with the coding and want to prevent > others from creating (dynamically) more instance attributes. it's too > bad, because it sort goes against Zen#19 (namespaces are a honking > good idea).
The main problem seems to be that it can be broken by inheritance. A base or derived class can have a __dict__ which then allows additional attributes to be added even if the derived or base class uses __slots__. I admit to arguing from authority here but the consistent message on c.l.py from those who should know is, don't use __slots__ to restrict assignment, use __setattr__ instead; __slots__ should be used as a memory optimization only. Search c.l.py for discussion. >>> I'll bite. What is the proper/intended use of __slots__? Does it have >>> something to do with memory? >> Yes, it is intended specifically to reduce memory consumption of objects >> that are going to be instantiated a lot. I'm not sure how many counts as >> a lot, but in the thousands at least. Using __slots__ saves the cost of >> the attribute dict for each instance. > > right, a class will either have __slots__ or __dict__ but not both. As noted above, that is not necessarily true when inheritance is involved. Kent _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
