[Python-ideas] Put default setstate and getstate on object for use in coöperative inheritance.

[Nick Coghlan]

On 7 June 2014 16:05, Neil Girdhar <mistersheik at gmail.com> wrote:
> I use cooperative multiple inheritance throughout my (large-ish) project,
> and I find it very comfortable and powerful.  I am currently using the class
> below to serve as an anchor point.  The thing is that this behavior is
> already implemented somewhere in Python (where?) since it is the default
> behaviour if getstate or setstate don't exist.  Why not explicitly make it
> available to call super?

There is fallback behaviour in the pickle and copy modules that
doesn't rely on the getstate/setstate APIs. Those fallbacks are
defined by the protocols, not by the object model.

https://docs.python.org/3/library/pickle.html#pickle-inst covers the
available protocols for instance pickling.
https://docs.python.org/3/library/copy.html covers (towards the end)
some of the options for making class instances copyable

https://docs.python.org/3/library/copyreg.html is an additional
registry that allows third parties to make instances of classes
defined elsewhere support pickling and copying without relying on
monkeypatching.

> I think I saw or got an email from Guido that I can't seem to find that
> rightly points out that object doesn't have __dict__ so this can't be done.
> I'm curious why object doesn't have __dict__?  Where does the __dict__ comes
> into existence?  I assume that objects of type object and instantiated
> objects of other types have the same metaclass; does the metaclass treat
> them differently?

Types defined in C extensions and those defined dynamically on the
heap share a metaclass at runtime, but their initialisation code is
different. You can also define Python level types without a __dict__
by declaring a __slots__ attribute with no __dict__ entry (for
example, collections.namedtuple uses that to ensure namedtuple
instances are exactly the same size as ordinary tuples - the mapping
from field names to tuple indices is maintained on the class).

Cheers,
Nick.

P.S. Posting through Google Groups doesn't work properly - it messes
up the reply headers completely. gmane does a better job of
interoperating with the mailing list software (as far as I am aware,
Google just don't care whether or not interaction with non-Google
lists actually works)

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia