[SciPy-user] Is it possible to pass Fortran derived data types to Python via C and SWIG?

[Berthold Höllmann]

Matthieu Brucher <matthieu.brucher <at> gmail.com> writes:

> 
> 2008/11/30 David Cournapeau <david <at> ar.media.kyoto-u.ac.jp>:
> > John Salvatier wrote:
> >> I have a Fortran 90 algorithm which uses a derived data type to return
> >> data, and I would like to make a python wrapper for this algorithm. I
> >> understand that f2py cannot wrap derived data types; is it possible to
> >> do so with a C interface for the Fortran algorithm and SWIG? I would
> >> have to pass the derived data type into a C struct and then to Python.
> >
> > It is possible as long as you can pass the structure from fortran to C.
> > I don't know anything about Fortran derived data types, but if it is a
> > non trivial object (more than a set of fundamental types), I am afraid
> > it will be difficult. Does F90 supports POD data ? Otherwise, you will
> > need a scheme for marshalling your data from Fortran to C (to match
> > exactly how the structure would look like in C at the binary level).
> >
> > David
> 
> I've read an article (I don't remember where though, possibly CiSE)
> that stated that it's really not an easy task, as each Fortran
> compiler can do as it pleases it. So depending on the compiler and the
> Fortran standard, it can be possible, or not. So as there are no
> guaranties, you should write a function that transforms the Fortran
> structure in several pieces that are then passed to the C function.
> 
> Matthieu

A feasible way to achieve this would be to write a Fortran wrapper
around your routine(x) that decomposes your derived data type to
standard types and exposes these in the interface. Than you can compose
the derived data type again in the wrapper and pass it to the original
routine. ::

   module geom
     type Point
        real :: x, y
     end type Point
     type Circle
        type (Point) :: Center
        real :: Radius
     end type Circle
   end module geom
   subroutine test(c)
     use geom
     type (Circle) :: c
     print*, c%Radius
     print*, c%Center%X
     print*, c%Center%Y
   end subroutine test
   subroutine w_test(x, y, r)
     use geom
     real :: x, y, z
     type (Circle) :: C
     c%Radius = r
     c%Center%X = x
     c%Center%Y = y
     call test(c)
   end subroutine w_test

Wrapping w_test should be trivial using f2py

Regards
Berthold