Shared variables between Cython and Fortran

[javier delgado]

Hi,

I am trying to write a package that combines Python and Fortran. I'm currently struggling to figure out how to share a module variable. Specifically, I want to be able to modify the variable in the Python code and have the change reflected in Fortran and vice versa. The following are the contents of the source files:

m.f90

       MODULE m

         USE ISO_C_BINDING

         integer(C_INT), bind(C, name="py_int") :: fort_int

       END MODULE

f.f90 - simply sets variable to 99

       subroutine f() bind(C, name="f")

         use m, only : fort_int

         use ISO_C_BINDING

         fort_int = 99

       END subroutine f

test.h

extern int py_int;

void f(void) ;

lib.pyx

def doit():

    cdef extern from "test.h":

        extern int py_int

        void f()

    py_int = 5

    print "Before f(): ", py_int

    f()

    print "after f(): ", py_int

Compilation script:

cython lib.pyx

ifort -c m.f90  -fPIC

ifort -c f.f90 -fPIC

INCLUDES="-I/usr/include/python2.7"

LIBS="-lpthread -ldl -lutil -lm -lpython2.7"

eval icc -c lib.c -fPIC $INCLUDES $LIBS

eval icc -shared f.o m.o  lib.o -o lib.so -fPIC $INCLUDES $LIBS

run.py  - The program that runs it

from lib import doit

doit()

Output

Before f():  5

after f():  5 !! <- should be 99, since f() modifies the variable

My understanding is that by declaring "py_int" within a  "cdef extern" block, Cython would make it an "extern int" that would ultimately point to the variable declared in m.f90. But apparently that is not happening. 

Can someone point out the flaw in my understanding?

Thanks!

[Pierre Complex]

The line "py_int = 5" makes the variable become local, whereas you wish to use the globally defined variable.

From Python's documentation: "A special quirk of Python is that – if no global statement is in effect – assignments to names always go into the innermost scope. Assignments do not copy data — they just bind names to objects." (see https://docs.python.org/3/tutorial/classes.html#python-scopes-and-name-spaces )

Simply add "global py_int" at the start of the function should solve the issue.

Regards,

Pierre

[javier delgado]

Thanks, Pierre!

In addition to making it global, I had to put the extern statement outside of the python function. 

Here is the final version of "lib.pyx" in case anyone runs into this in the future:

cdef extern from "test.h":

    extern int py_int

    void f()

def doit():

    global py_int

    py_int = 5

    print "Before f(): ", py_int

    f()

    print "after f(): ", py_int

[Pierre Complex]

Glad that it works! In my tests, I ended up moving the extern part as one of the steps
but did not mention it.

Regards,

Pierre