Usage

Quick overview

To use f90wrap to wrap a set of Fortran 90 source files and produce wrappers suitable for input to f2py use:

f90wrap -m MODULE F90_FILES

where MODULE is the name of the Python module you want to produce (e.g. the name of the Fortran code you are wrapping) and F90_FILES is a list of Fortran 90 source files containing the modules, types and subroutines you would like to expose via Python.

This will produce two types of output: Fortran 90 wrapper files suitable for input to f2py to produce a low-level Python extension module, and a high-level Python module desinged to be used together with the f2py-generated module to give a more Pythonic interface.

One Fortran 90 wrapper file is written for each source file, named f90wrap_F90_FILE.f90, plus possibly an extra file named f90wrap_toplevel.f90 if there are any subroutines or functions defined outside of modules in F90_FILES.

To use f2py to compile these wrappers into an extension module, use:

f2py -c -m _MODULE OBJ_FILES f90wrap_*.f90

where _MODULE is the name of the low-level extension module.

How f90wrap works

There are five steps in the process of wrapping a Fortran 90 routine to allow it to be called from Python.

  1. The Fortran source files are scanned, building up an AST (ast) which describes all the modules, types, subroutines and functions found.

  2. The AST is ~f90wrap.transform to remove nodes which should not be wrapped (e.g. private symbols in modules, routines with arguments of a derived type not defined in the project, etc.)

  3. The F90WrapperGenerator class is used to write a simplified Fortran 90 prototype for each routine, with derived type arguments replaced by integer arrays containing a representation of a pointer to the derived type, in the manner described in [Pletzer2008]. This allows opaque references to the true Fortran derived type data structures to be passed back and forth between Python and Fortran.

  4. f2py is used to combine the F90 wrappers and the original compiled functions into a Python extension module (optionally, f2py can be replaced by f2py-f90wrap, a slightly modified version which adds support for exception handling and interruption during exceution of Fortran code).

  5. The PythonWrapperGenerator class is used to write a thin object-oriented layer on top of the f2py generated wrapper functions which handles conversion between Python object instances and Fortran derived-type variables, converting arguments back and forth automatically.

Transformation of the symbol tree

Generation of Fortran 90 wrappers

All routines which accept derived types arguments are wrapped by equivalent routines which instead accept integer arrays as opaque handles. The Fortran transfer() intrinsic is used to convert these handles into pointers to derived types, as described in [Pletzer2008]. The size of the integer array required to store a pointer is determined automatically at compile-time by the f90wrap.sizeoffortran.sizeof_fortran_t() function. In this way we can access the underlying Fortran structures from Python.

Extra routines are generated to access the values of attributes within Fortran modules and derived types. For scalars a pair of get and set routines is created, whilst for arrays a single routine which returns the shape, memory location and type of the array is output. Derived type elements within modules or other derived types are also supported, so that entire hiereachies of types can be wrapped.

Fortran Types and Kinds (-k/–kind-map)

Constructors and Destructors (-C/-D)

Constructor and desctructor routines are handled specially: on initialisation, a derived type pointer is allocated before the wrapped routine is invoked, and an opaque reference to this new derived type is returned. On finalisation the underlying derived type pointer is deallocated after the wrapped routine returns.

f2py-f90wrap

Optionally, you can replace f2py with f2py-f90wrap, which is slightly modified version of f2py which introduces the following features:

  1. Allow the Fortran present() function to work correctly with optional arguments. If an argument to an f2py wrapped function is optional and is not given, replace it with NULL.

  2. Allow Fortran routines to raise a RuntimeError exception with a message by calling an external function f90wrap_error_abort(). This is implemented using a setjmp()/ longjmp() trap.

  3. Allow Fortran routines to be interrupted with Ctrl+C by installing a custom interrupt handler before the call into Fortran is made. After the Fortran routine returns, the previous interrupt handler is restored.

[Pletzer2008] (1,2)

Pletzer, A et al., Exposing Fortran Derived Types to C and Other Languages, Computing in Science and Engineering, 10, 86 (2008). http://link.aip.org/link/?CSENFA/10/86/1