fffi is a tool to transparently use shared libraries generated by Fortran in Python with NumPy arrays. It is based on CFFI and currently assumes the use of gfortran or Intel Fortran.

The focus of fffi is dynamical automatic generation of interfaces directly within Python with a minimum of extra code generation and files to allow for the simplest possible workflow for fast prototyping of Python/Fortran codes. In this sense it complements F2x which instead supplies interfaces based on generated code and ISO C bindings. Both tools aim to be more powerful and flexible alternatives to F2Py.

The following example can be found in tests/01_arrays directory and shows basic usage using CFFI (fast) API mode with a shared library libtest_arrays.so containing a module mod_arrays:

1. Import fffi and initialize fortran_module object mod_arrays

from fffi import FortranModule
mod_arrays = FortranModule('test_arrays', 'mod_arrays')

2. Define and generate Python extension (only on first run or if library routines have been added/changed)

mod_arrays.fdef("""
  subroutine test_vector(vec)
    double precision, dimension(:) :: vec
  end

  subroutine test_array_2d(arr)
    double precision, dimension(:,:) :: arr
  end
""")
mod_arrays.compile()

Internally, C function headers are generated for CFFI with the required representation of array descriptors by the used Fortran compiler.

3. Load interface module to library

  mod_arrays.load()

4. Calling of a subroutine test_vector in mod_arrays is as simple as

  vec = np.ones(15)
  mod_arrays.test_vector(vec)

The general workflow to extend such a test is:

1. Possibly add new testing routines to mod_arrays.f90 and test_arrays.f90
2. Run make. This generates a shared library libtest_arrays.so and a Fortran executable test_arrays.x for reference output
3. Edit and run test1_arrays.py

Current status:

• Code generation and invocation is transparent by encapsulation in Python class
• CFFI API mode is used (pre-generate C extension module as a wrapper)
• Subroutine signature definition in Fortran via fdef like CFFI cdef
• Array descriptor structs are working with gfortran 5, 6, 7, 8, 9 and Intel Fortran 18

Next steps:

• Add flexibility with regard to types, floating-point precision, etc.
• More testing, in particular with Intel compiler
• Test numpy.array views on existing data and memory management behavior
• Allow for ABI mode in addition to API mode, and static library API mode
• Add support and PGI compiler
• Test in real world applications