On 12/31/2013 06:53 PM, eryksun wrote:
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.
Oh, that's close to what i first thought when reading about the trickiness of
using __slots__: that it may relate to subtyping supertypes with __slots__
(right, I should try myself, but here it's 21.28, on dec 31, time to move my
ass... ;-)
Denis
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