On Tue, Dec 31, 2013 at 11:21 AM, Mark Lawrence <breamore...@yahoo.co.uk> wrote: > The glossary entry for __slots__ states "A declaration inside a class that > saves memory by pre-declaring space for instance attributes and eliminating > instance dictionaries. Though popular, the technique is somewhat tricky to > get right and is best reserved for rare cases where there are large numbers > of instances in a memory-critical application." I'll admit that I really > don't understand what's tricky about it, can someone explain please.
Refer to the language reference: http://docs.python.org/3/reference/datamodel.html#notes-on-using-slots Minor correction: It says str requires empty __slots__, but that's a bug in the docs. It's referring to 2.x str. Else this thread wouldn't exist. In 3.x, str is basically the unicode type from 2.x. Its __itemsize__ is 0 because the character array is allocated separately. Actually in CPython 3.3 there's a compact string object (i.e. PyCompactUnicodeObject), but that's not used for a subclass. Appending new slots to an instance poses no problem for a subclass of str. Regarding __class__ assignment, I'll add that it also breaks if the types include a __dict__ or __weakref__ slot in addition to other slots. For example, this works fine: class C: __slots__ = '__weakref__', class D: __slots__ = '__weakref__', >>> C().__class__ = D But adding another slot pushes __weakref__ out of its expected position in the layout, so the test for a compatible layout fails: class C: __slots__ = '__weakref__', 'a' class D: __slots__ = '__weakref__', 'a' >>> C().__class__ = D Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: __class__ assignment: 'C' object layout differs from 'D' The layout is identical, but the test it uses can't see that. _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor