[Tutor] subtyping builtin type

[spir]

On 12/31/2013 06:53 PM, eryksun wrote:
> On Tue, Dec 31, 2013 at 11:21 AM, Mark Lawrence <breamoreboy at 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