[Python-Dev] Python's C interface for types

[Nick Maclaren]

Thanks very much!  That answers most things.  Yes, I had got many
of my answers from searching the source, but there is clearly some
history there, and it isn't always clear what is current.  Here are
a few responses to the areas of confusion:

> nb_invert is used for bitwise inversion (~) and PyNumber_Invert(). It's not
> historical, it's actual.

Ah!  So it's NOT 1/x!  No relevant to floating-point, then.

> I don't recall ever seeing useful documentation on coerce() and nb_coerce.
> I suggest not to use it; it's gone in Python 3.0 anyway.

Excellent!  Task completed :-)

> Which numeric property? the fact that it returns a C long? Or that, for
> natural numbers, it *seems* to return self? 

The latter.  hash(123) == hash(123.0) for example.  It is a real
pain for advanced formats.  Making it the same for things that compare
equal isn't a problem.

> [inplace ] I assume your floating-point type is
> immutable, so you won't have to implement them.

I haven't done anything special to flag it as such, but it is.

> Where do you allocate this space, and how do you allocate it? If it's space
> you malloc() and store somewhere in the type struct, yecchh. You should not
> just allocate stuff at the end of the type struct, as the type struct's
> layout is not under your control (we actually extend the type struct as
> needed, which is why newer features end up in less logical places at the end
> of the struct ;) I would suggest using attributes of the type instead, with
> the normal Python refcounting. That means the 'extra space' has to be an
> actual Python object, though.

PyMem_Malloc.  I can certainly make it an attribute, as the overhead
isn't large for a per-class object.  It is just a block of mutable
memory, opaque to the Python layer, and NOT containing any pointers!

> I don't you can make your own type marshallable. For pickle it's more or
> less the same as for Python types. The pickle docs (and maybe
> http://www.python.org/dev/peps/pep-0307/) probably cover what you want to
> know. You can also look at one of the complexer builtin types that support
> pickling, like the datetime types.

The only documentation I have found is how to do it in Python.  Is
that what you mean?  I will look at the datetime types.

> You can use PyGetSetDef to get 'easy' attributes with getters and setters.
> http://docs.python.org/api/type-structs.html#l2h-1020

I was put off by some of the warnings.  I will revisit it.

> There's nothing I can think of that is a natural match for that in standard
> Python methods. I would suggest just making it a classmethod.
> (dict.fromkeysis a good example of a classmethod in C.)

Thanks.  That is a useful reference.  Reductions are a problem in
many languages.


Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679