F2Py: Working with allocatable arrays in Fortran being invoked through Python

Question

Using F2Py to compile Fortran routines being suitable to be used within Python, the following piece of code is successfully compiled configured gfortran as the compiler while using F2Py, however, at the time of invoking in Python it raises a runtime error!
Any comments and solutions?

function select(x) result(y)
   implicit none
   integer,intent(in):: x(:) 
   integer:: i,j,temp(size(x))
   integer,allocatable:: y(:)
   j = 0
   do i=1,size(x)
      if (x(i)/=0) then
         j = j+1
         temp(j) = x(i)
      endif
   enddo
   allocate(y(j))
   y = temp(:j)
end function select

A similar StackOverflow post can be found here.

The f2py list would also be a good place to ask, the responses are usually quite fast. — bdforbes
– bdforbes, Commented Dec 14, 2011 at 21:25
@VladimirF The code I used to invoke it in Python is: import test; print test.select([0,1,2,9,5]) assuming that I compiled it already as test.pyd. — Developer
– Developer, Commented Dec 15, 2011 at 8:21
@bdforbes Thanks for the hint. I submitted and encouraged the list having a look on this page and giving me some solution ideas. — Developer
– Developer, Commented Dec 15, 2011 at 8:25

Vladimir F Героям слава · Accepted Answer · 2011-12-15 09:42:32Z

0

Take a look at this article http://www.shocksolution.com/2009/09/f2py-binding-fortran-python/ , especially at the example and the meaning of

!f2py depend(len_a) a, bar

However, the author does not touch of the problem of generating a an array of different size.

answered Dec 15, 2011 at 9:42

Vladimir F Героям слава

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1 Comment

Developer Over a year ago

Thanks for the link. However the problem is we don't know how much the size of output array is!

Francois Jacq · Accepted Answer · 2014-03-13 01:54:15Z

-2

Your function should be declared :

function select(n,x) result(y)
   implicit none
   integer,intent(in) :: n
   integer,intent(in) :: x(n) 
   integer :: y(n) ! in maximizing the size of y
   ...

Indeed, Python is written in C and your Fortran routine must follow the rules of the Iso_C_binding. In particular, assumed shape arrays are forbidden.

In any case, I would prefer a subroutine :

  subroutine select(nx,y,ny,y)
     implicit none
     integer,intent(in) :: nx,x(nx)
     integer,intent(out) :: ny,y(nx)

ny being the size really used for y (ny <= nx)

edited Mar 13, 2014 at 1:54

answered Mar 13, 2014 at 1:02

Francois Jacq

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2 Comments

Vladimir F Героям слава Over a year ago

No, f2py handles assumed shape arrays just right.

Francois Jacq Over a year ago

OK. Nice to learn. But I don't understand how it can do that in a portable manner...

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