========================
Advanced F2PY use cases
========================

Adding user-defined functions to F2PY generated modules
=========================================================

User-defined Python C/API functions can be defined inside
signature files using ``usercode`` and ``pymethoddef`` statements
(they must be used inside the ``python module`` block). For
example, the following signature file ``spam.pyf``

.. include:: ./code/spam.pyf
   :literal:

wraps the C library function ``system()``::

  f2py -c spam.pyf

In Python this can then be used as:

.. literalinclude:: ./code/results/spam_session.dat
   :language: python

Adding user-defined variables
==============================

The following example illustrates how to add user-defined variables to a F2PY
generated extension module by modifying the dictionary of a F2PY generated
module. Consider the following signature file (compiled with ``f2py -c var.pyf``):

.. literalinclude:: ./code/var.pyf
  :language: fortran

Notice that the second ``usercode`` statement must be defined inside
an ``interface`` block and the module dictionary is available through
the variable ``d`` (see ``varmodule.c`` generated by ``f2py var.pyf`` for
additional details).

Usage in Python:

.. literalinclude:: ./code/results/var_session.dat
  :language: python


Dealing with KIND specifiers
============================

Currently, F2PY can handle only ``<type spec>(kind=<kindselector>)``
declarations where ``<kindselector>`` is a numeric integer (e.g. 1, 2,
4,...), but not a function call ``KIND(..)`` or any other
expression. F2PY needs to know what would be the corresponding C type
and a general solution for that would be too complicated to implement.

However, F2PY provides a hook to overcome this difficulty, namely,
users can define their own <Fortran type> to <C type> maps. For
example, if Fortran 90 code contains::

    REAL(kind=KIND(0.0D0)) ...

then create a mapping file containing a Python dictionary::

    {'real': {'KIND(0.0D0)': 'double'}}

for instance.

Use the ``--f2cmap`` command-line option to pass the file name to F2PY.
By default, F2PY assumes file name is ``.f2py_f2cmap`` in the current
working directory.

More generally, the f2cmap file must contain a dictionary
with items::

    <Fortran typespec> : {<selector_expr>:<C type>}

that defines mapping between Fortran type::

    <Fortran typespec>([kind=]<selector_expr>)

and the corresponding <C type>. The <C type> can be one of the following::

    char
    signed_char
    short
    int
    long_long
    float
    double
    long_double
    complex_float
    complex_double
    complex_long_double
    string

For more information, see the F2Py source code ``numpy/f2py/capi_maps.py``.