I was looking at the option of embedding python into fortran90 to add python functionality to my existing fortran90 code. I know that it can be done the other way around by extending python with fortran90 using the f2py from numpy. But, i want to keep my super optimized main loop in fortran and add python to do some additional tasks / evaluate further developments before I can do it in fortran, and also to ease up code maintenance. I am looking for answers for the following questions:
1) Is there a library that already exists from which I can embed python into fortran? (I am aware of f2py and it does it the other way around)
2) How do we take care of data transfer from fortran to python and back?
3) How can we have a call back functionality implemented? (Let me describe the scenario a bit....I have my main_fortran program in Fortran, that call Func1_Python module in python. Now, from this Func1_Python, I want to call another function...say Func2_Fortran in fortran)
4) What would be the impact of embedding the interpreter of python inside fortran in terms of performance....like loading time, running time, sending data (a large array in double precision) across etc.
Thanks a lot in advance for your help!!
Edit1: I want to set the direction of the discussion right by adding some more information about the work I am doing. I am into scientific computing stuff. So, I would be working a lot on huge arrays / matrices in double precision and doing floating point operations. So, there are very few options other than fortran really to do the work for me. The reason i want to include python into my code is that I can use NumPy for doing some basic computations if necessary and extend the capabilities of the code with minimal effort. For example, I can use several libraries available to link between python and some other package (say OpenFoam using PyFoam library).
There is a very easy way to do this using f2py. Write your python method and add it as an input to your Fortran subroutine. Declare it in both the cf2py hook and the type declaration as EXTERNAL and also as its return value type, e.g. REAL*8. Your Fortran code will then have a pointer to the address where the python method is stored. It will be SLOW AS MOLASSES, but for testing out algorithms it can be useful. I do this often (I port a lot of ancient spaghetti Fortran to python modules...) It's  also a great way to use things like optimised Scipy calls in legacy fortran
Embed python into fortran 90