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# 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`

    !    -*- f90 -*-
    python module spam
        usercode '''
      static char doc_spam_system[] = "Execute a shell command.";
      static PyObject *spam_system(PyObject *self, PyObject *args)
      {
        char *command;
        int sts;

        if (!PyArg_ParseTuple(args, "s", &command))
            return NULL;
        sts = system(command);
        return Py_BuildValue("i", sts);
      }
        '''
        pymethoddef '''
        {"system",  spam_system, METH_VARARGS, doc_spam_system},
        '''
    end python module spam

wraps the C library function `system()`:

    f2py -c spam.pyf

In Python this can then be used as:

    >>> import spam
    >>> status = spam.system('whoami')
    pearu
    >>> status = spam.system('blah')
    sh: line 1: blah: command not found

## 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`):

    !    -*- f90 -*-
    python module var
      usercode '''
        int BAR = 5;
      '''
      interface
        usercode '''
          PyDict_SetItemString(d,"BAR",PyLong_FromLong(BAR));
        '''
      end interface
    end python module

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:

    >>> import var
    >>> var.BAR
    5

## 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:

    double
    float
    long_double
    char
    signed_char
    unsigned_char
    short
    unsigned_short
    int
    long
    long_long
    unsigned
    complex_float
    complex_double
    complex_long_double
    string

For example, for a Fortran file `func1.f` containing:

          subroutine func1(n, x, res)
            use, intrinsic :: iso_fortran_env, only: int64, real64
            implicit none
            integer(int64), intent(in) :: n
            real(real64), intent(in) :: x(n)
            real(real64), intent(out) :: res
    Cf2py   intent(hide) :: n
            res = sum(x)
          end

In order to convert `int64` and `real64` to valid `C` data types, a `.f2py_f2cmap` file with the following content can be created in the current directory:

    dict(real=dict(real64='double'), integer=dict(int64='long long'))

and create the module as usual. F2PY checks if a `.f2py_f2cmap` file is present in the current directory and will use it to map `KIND` specifiers to `C` data types.

    f2py -c func1.f -m func1

Alternatively, the mapping file can be saved with any other name, for example `mapfile.txt`, and this information can be passed to F2PY by using the `--f2cmap` option.

    f2py -c func1.f -m func1 --f2cmap mapfile.txt

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

## Character strings

### Assumed length character strings

In Fortran, assumed length character string arguments are declared as `character*(*)` or `character(len=*)`, that is, the length of such arguments are determined by the actual string arguments at runtime. For `intent(in)` arguments, this lack of length information poses no problems for f2py to construct functional wrapper functions. However, for `intent(out)` arguments, the lack of length information is problematic for f2py generated wrappers because there is no size information available for creating memory buffers for such arguments and F2PY assumes the length is 0. Depending on how the length of assumed length character strings are specified, there exist ways to workaround this problem, as exemplified below.

If the length of the `character*(*)` output argument is determined by the state of other input arguments, the required connection can be established in a signature file or within a f2py-comment by adding an extra declaration for the corresponding argument that specifies the length in character selector part. For example, consider a Fortran file `asterisk1.f90`:

    subroutine foo1(s)
      character*(*), intent(out) :: s
      !f2py character(f2py_len=12) s
      s = "123456789A12"
    end subroutine foo1

Compile it with `f2py -c asterisk1.f90 -m asterisk1` and then in Python:

    >>> import asterisk1
    >>> asterisk1.foo1()
    b'123456789A12'

Notice that the extra declaration `character(f2py_len=12) s` is interpreted only by f2py and in the `f2py_len=` specification one can use C-expressions as a length value.

In the following example:

    subroutine foo2(s, n)
      character(len=*), intent(out) :: s
      integer, intent(in) :: n
      !f2py character(f2py_len=n), depend(n) :: s
      s = "123456789A123456789B"(1:n)
    end subroutine foo2

the length of the output assumed length string depends on an input argument `n`, after wrapping with F2PY, in Python:

    >>> import asterisk
    >>> asterisk.foo2(2)
    b'12'
    >>> asterisk.foo2(12)
    b'123456789A12'
    >>>