F77 function

First try (typical attempt by a Fortran/C programmer):

      subroutine gridloop1(a, xcoor, ycoor, nx, ny, func1)
      integer nx, ny
      real*8 a(0:nx-1,0:ny-1), xcoor(0:nx-1), ycoor(0:ny-1)
      real*8 func1
      external func1

      integer i,j
      real*8 x, y
      do j = 0, ny-1
         y = ycoor(j)
         do i = 0, nx-1
            x = xcoor(i)
            a(i,j) = func1(x, y) 
         end do
      end do
      return
      end

Note: float type in NumPy array must match real*8 or double precision in Fortran! (Otherwise F2PY will take a copy of the array a so the type matches that in the F77 code)