[Python-Dev] Global __slots__ pondering

[François Pinard]

Hi, people.

Since a few days, I'm working at a Python rewrite of a difficult and
complex C application, written by a "power" programmer years ago, who
left the company before I came in.  The rewrite goes extremely well,
Python legibility and facilities are such that I could improve the angle
of attack for the problem, making it appear much, much easier.

The C code mallocs a lot, and the Python rewrite is memory intensive.
Since it handles hundreds of thousands of objects, I thought it was
a good case for using `__slots__' and indeed, the memory savings are
interesting.  Deriving from built-in types for a good amount of objects
(for a limited amount of types), whenever appropriate, is also welcome.

In one module, sub-classing is frequent, and if I understand well how
it works, a `__dict__' is used in sub-types unless `__slots__ = ()'
is explicitely included in the sub-type, and I decided to spare that
`__dict__' whenever possible in that particular module.

So, just after the global `__metaclass = type' at the beginning of the
module, I added a global `__slots__ = ()', with the clear intent of
overriding that `__slots__' assignment only for those particular types
needing slots.  In this module of the application, I do not mind having
to restrain all types so none uses a `__dict__'.

But it does not seem to work that way.  Could it be pondered that, the
same as `__metaclass__' may have a global value, `__slots__' may also
have?  One objection I see is that, while __metaclass__ may be redefined
to the classic class metatype for a specific type, a global `__slots__'
could not be arbitrarily defeated in a specific type.  Nevertheless, in
the precise case I have, it would have been a bit useful.

I do not mind much having to repeat `__slots__' in each and every class
and sub-class for this module, yet I thought about sharing the above
thoughts with Python developers, in case they would like to level the
said difference.  It would be less surprising if `__metaclass__' and
`__slots__' could equally be implied globally.  Or maybe? :-)

-- 
François Pinard   http://www.iro.umontreal.ca/~pinard