[Cython] Cython and complex floats

[Stefan Behnel]

Hi,

Dag Sverre Seljebotn wrote:
> Buffers and Python has the complex datatype, while I'm reasonably sure I 
> haven't found anything about it in Cython (?).

You can use Python's complex types in Cython by simply declaring them as an
extension type:

    cdef extern from "complexobject.h":

        struct Py_complex:
            double real
            double imag

        ctypedef class __builtin__.complex [object PyComplexObject]:
            cdef Py_complex cval

    # A function which uses the above type
    def spam(complex c):
        print "Real:", c.cval.real
        print "Imag:", c.cval.imag

That's already done in Cython/Includes/python2.5.pxd.


> 1) Complex numbers can be pulled out into a struct consisting of two 
> floats. Like this:
> 
> cdef struct cdouble:
>    double real
>    double imag
> 
> def f(object[cdouble] buf):
>    ...

I'm not sure we need such syntactic sugar. All you gain is one ".cval" less.

Or do I misunderstand your proposal here?


> Operations on the fields must happen manually.
> 
> 2) In addition, Cython gets support for the C99 "complex" float modifier.
> 
> def complex double sum(object[complex double] buf):
>    cdef complex double result = 0 + 0j
>    ...
> 
> Here, operations between complex datatypes are done natively by the 
> compiler. However the code generated is illegal on non-C99-compilers.

I think it's fine for a specific feature to break compiler compatibility as
long as it is clearly stated in the docs and it does not impact code that does
not use it.

Stefan