[Python-Dev] Python's C interface for types (original) (raw)

Nick Maclaren nmm1 at cus.cam.ac.uk
Fri Jan 26 14:51:04 CET 2007

I have a fair amount of my binary floating-point model written, though even of what I have done only some is debugged (and none has been rigorously tested). But I have hit some things that I can't work out, and one query reduced comp.lang.python to a stunned silence :-)

Note that I am not intending to do all the following, at least for now, but I have had to restructure half a dozen times to match my implementation requirements to the C interface (as I have learnt more about Python!) and designing to avoid that is always good.

Any pointers appreciated.

I can't find any detailed description of the methods that I need to provide. Specifically:

Does Python use classic division (nb_divide) and inversion (nb_invert) or are they entirely historical? Note that I can very easily provide the latter.

Is there any documentation on the coercion function (nb_coerce)? It seems to have unusual properties.

How critical is the 'numeric' property of the nb_hash function? I can certainly honour it, but is it worth it?

I assume that Python will call nb_richcompare if defined and nb_compare if not. Is that right?

Are the inplace methods used and, if so, what is their specification?

I assume that I can ignore all of the allocation, deallocation and attribute handling functions, as the default for a VAR object is fine. That seems to work.

Except for one thing! My base type is static, but I create some space for every derivation (and it can ONLY be used in derived form). The space creation is donein C but the derivation in Python. I assume that I need a class (not instance) destructor, but what should it do to free the space? Call C to Py_DECREF it?

I assume that a class structure will never go away until after all instances have gone away (unless I use Py_DECREF), so a C pointer from an instance to something owned by the class is OK.

Is there any documentation on how to support marshalling/pickling and the converse from C types?

I would quite like to provide some attributes. They are 'simple' but need code executing to return them. I assume that means that they aren't simple enough, and have to be provided as methods (like conjugate). That's what I have done, anyway.

Is there any obvious place for a reduction method to be hooked in? That is a method that takes a sequence, all members of which must be convertible to a single class, and returns a member of that class. Note that it specifically does NOT make sense on a single value of that class.

Sorry about the length of this!

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

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