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",PyInt_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'
>>>