I’d like to call an externally compiled Fortran routine, passing input and output variables, from python in two steps.

1. an initial call, which is very slow, it will read a lot of data files and allocates variable structures, etc., in the Fortran program

2. millions of subsequent calls which only take milli-seconds to do the actual computations.

subroutine crunch(input, output)
  implicit none
  real,intent(in) :: input
  real,intent(out) :: output
  logical,save :: firstCall=.true.
  real,save :: data(10000) 
  if (firstCall) then
    !--- initialisation: reads data from files ---
    firstCall = .false.
  endif
  !--- the actual computations, which depend on input and data ---
  output = ...
end subroutine

So the variable structures (here called “data”), resulting from call (1) must stay in the memory of the Fortran routine.

Thanks, your help is appreciated

I could only find examples of single calls where all initialisations of the Fortran routine are erased on exit