Rob Williscroft wrote: > John Machin wrote in news:1167008799.074885.250770@ > 73g2000cwn.googlegroups.com in comp.lang.python: > > > Given a = Adder(), > > a.tally = 0 > > gets AttributeError: 'Adder' object attribute 'tally' is read-only > > a.notinslots = 1 > > gets AttributeError: 'Adder' object attribute 'notinslots' is read-only > > > > So is there some magic class-fu going down here, or is this just a > > waste of memory space in the instances? > > > > Haven't you, with your 2 examples above, answered your own question ?
No. > > Clearly from your example it doesn't make any difference if you add > a class attribute to the slots, one way or another its as if you > hadn't put it in there in the first place. Clearly? Not so. It takes up memory. A list of 1 million Adder instances takes up about 68 Mb (Python 2.5 on Windows XP). With the method names removed from the __slots__, it takes only about 44 Mb. [For comparison: with no __slots__ at all, it takes about 180 Mb] > > This will give the same error, which shows its about class attributes > and not just methods: > > class Adder(object): > > __slots__ = [ > 'class_name' > ] > > class_name = 3 > > > a = Adder() > > a.class_name = 2 > > It would seem that the interpreter removes any names it finds as class > attribute names from the list it finds in __slots__ before it creates > the instance. It doesn't seem so to me. If it did that, the memory usage would not increase. > > Of course if my guessing above isn't good enough, we could look at > the documentation: > > http://docs.python.org/ref/slots.html#l2h-218 > > __slots__ are implemented at the class level by creating descriptors > (3.4.2) for each variable name. As a result, class attributes cannot be > used to set default values for instance variables defined by __slots__; > otherwise, the class attribute would overwrite the descriptor assignment. I have read that, before I posted. Asides: (1) It would be useful if it stated the empirically determined fact that the result is that the class attribute is thusly made read-only. (2) The second sentence is not a model of clarity. In any case I can't see how the paragraph gives any support for your next statement: > > So its that the __slots__ assignment makes the descriptors and then the > subsiquent method defenitions and class attribute bindings remove them. Errrmmm ... if the descriptors are removed, how is it that the behaviour is read-only? Cheers, John -- http://mail.python.org/mailman/listinfo/python-list
