[Pyrex] pyrexc error: Python object cannot be converted to __mpz_struct (?)

[Heiko Wundram]

Hi all!

I'm trying to wrap the Gnu Multiprecision Integer library for code I'm 
creating, in an effort to maximize performance especially for public/private 
key encryption. (the mpz module of Python doesn't export a third of the 
functionality GMP actually has to offer, such as _fast_ miller-rabin testing, 
which is available as of GMP 4.0, etc.)

I'm trying to write the wrapper using Pyrex, and at the moment I'm stuck at 
the following:

I'm trying to implement the __add__ method, which returns a new GMP object, 
which contains the two numbers, added using the mpz_* methods. For this, I 
create a new GMP object (retv = GMP()), whose private member mpznumber 
contains the appropriate MPZ structure. Now, when I try to compile the 
following code fragment, pyrexc spits out the following error messages:

/home/heiko/projects/Crypto/Utility.pyx:103:34: Cannot convert Python object 
to '__mpz_struct (*)'
/home/heiko/projects/Crypto/Utility.pyx:110:31: Cannot convert Python object 
to '__mpz_struct (*)'

From what I gather from these lines, I think pyrexc thinks that retv.mpznumber 
points to a Python object, whereas it simply is a reference to the structure 
member mpznumber of the class structure. It doesn't complain about 
_otherGMP.mpznumber, which resolves similarly, as the cdef definition for the 
two of them is pretty much equal.

The code:

cdef class GMP:
	cdef mpz_t mpznumber

	<!-- init snipped -->

	def __add__(self,other):
		cdef GMP _otherGMP
		cdef unsigned int _otherint
		cdef GMP retv

		retv = GMP()

		try:
			_otherint = other
			mpz_add_ui(retv.mpznumber,self.mpznumber,_otherint)
			return retv
		except:
			pass

		try:
			_otherGMP = other
			mpz_add(retv.mpznumber,self.mpznumber,
			        _otherGMP.mpznumber)
			return retv
		except:
			raise TypeError, "Invalid type to add."

What is wrong here?

Oh, and by the way, I've read that e.g. you shouldn't call PyInt_ToLong() 
(which is how _otherint = other is resolved) before having checked the type 
of the passed in python object. I've not found a sane way (except "if 
isinstance(...)", which expands to huge amounts of runtime python code, 
whereas a simple PyInt_Check() should suffice) to check the type of an 
incoming object, when the method accepts more than a single type. Do I need 
to declare PyInt_Check() at cdef scope modulewide, and then call it myself, 
or is there a simple way to have exactly this typeswitching code generated by 
Pyrex (I've found no mention of this in the documentation)?

Thanks for any hint!

Heiko.