How to get array return from callback function in F2PY?

Question

I'm trying to write a little interface from Python to Fortran with F2PY, where an array gets passed to a callback function in Python and the resulting array gets passed back to Fortran. I have the following Fortran code:

      Subroutine myscript(x,fun,o,n)
      external fun
      integer n
      real*8 x(n)
cf2py intent(in,copy) x
cf2py intent(out) o
cf2py integer intent(hide),depend(x) :: n=shape(x,0)
cf2py function fun(n,x) result o
cf2py integer intent(in,hide) :: n
cf2py intent(in), Dimension(n), depend(n) :: x
cf2py end function fun
      o = fun(n,x)
      write(*,*) o
      end

where fun is a callback function in Python which looks like this:

def f(x):
    print(x)
    return x

Now when I wrap the Fortran code with F2PY and run it from Python, eg like this:

myscript.myscript(numpy.array([1,2,3]),f)

I get the following result:

[1. 2. 3.]
1.00000000

So apparently the array gets passed through to the callback function f but then when it gets passed back only the first entry is preserved. What do I need to do to get the whole array back? ie get the variable o in the Fortran Code to contain the array [1.,2.,3.] instead of just 1.?

Answer 1

Okay, I finally figured it out. As pointed out, o has to be declared and then o has to be put into the function fun too. The function then has to be called with Fortran's call statement (as opposed to o = fun(n,x) ). Apparently, one can also get rid of most of the cf2py statements. Interestingly, fun does not have to explicitly be declared as a function with array return. The following code works for me:

      Subroutine myscript(x,fun,o,n)
      external fun
      integer n
      real*8 x(n)
      real*8 o(n)
cf2py intent(in,copy), depend(n) :: x
cf2py intent(hide) :: n
cf2py intent(out), depend(n) :: o
      call fun(n,x,o)
      write(*,*) o
      end

It returns

[1. 2. 3.]
1.00000000 2.00000000 3.00000000