==================
Signature file
==================
The syntax specification for signature files (.pyf files) is borrowed
from the Fortran 90/95 language specification. Almost all Fortran
90/95 standard constructs are understood, both in free and fixed
format (recall that Fortran 77 is a subset of Fortran 90/95). F2PY
introduces also some extensions to Fortran 90/95 language
specification that help designing Fortran to Python interface, make it
more "Pythonic".
Signature files may contain arbitrary Fortran code (so that Fortran
codes can be considered as signature files). F2PY silently ignores
Fortran constructs that are irrelevant for creating the interface.
However, this includes also syntax errors. So, be careful not making
ones;-).
In general, the contents of signature files is case-sensitive. When
scanning Fortran codes and writing a signature file, F2PY lowers all
cases automatically except in multiline blocks or when ``--no-lower``
option is used.
The syntax of signature files is presented below.
Python module block
=====================
A signature file may contain one (recommended) or more ``python
module`` blocks. ``python module`` block describes the contents of
a Python/C extension module ``<modulename>module.c`` that F2PY
generates.
Exception: if ``<modulename>`` contains a substring ``__user__``, then
the corresponding ``python module`` block describes the signatures of
so-called call-back functions (see :ref:`Call-back arguments`).
A ``python module`` block has the following structure::
python module <modulename>
[<usercode statement>]...
[
interface
<usercode statement>
<Fortran block data signatures>
<Fortran/C routine signatures>
end [interface]
]...
[
interface
module <F90 modulename>
[<F90 module data type declarations>]
[<F90 module routine signatures>]
end [module [<F90 modulename>]]
end [interface]
]...
end [python module [<modulename>]]
Here brackets ``[]`` indicate an optional part, dots ``...`` indicate
one or more of a previous part. So, ``[]...`` reads zero or more of a
previous part.
Fortran/C routine signatures
=============================
The signature of a Fortran routine has the following structure::
[<typespec>] function | subroutine <routine name> \
[ ( [<arguments>] ) ] [ result ( <entityname> ) ]
[<argument/variable type declarations>]
[<argument/variable attribute statements>]
[<use statements>]
[<common block statements>]
[<other statements>]
end [ function | subroutine [<routine name>] ]
From a Fortran routine signature F2PY generates a Python/C extension
function that has the following signature::
def <routine name>(<required arguments>[,<optional arguments>]):
...
return <return variables>
The signature of a Fortran block data has the following structure::
block data [ <block data name> ]
[<variable type declarations>]
[<variable attribute statements>]
[<use statements>]
[<common block statements>]
[<include statements>]
end [ block data [<block data name>] ]
Type declarations
-----------------
The definition of the ``<argument/variable type declaration>`` part
is
::
<typespec> [ [<attrspec>] :: ] <entitydecl>
where
::
<typespec> := byte | character [<charselector>]
| complex [<kindselector>] | real [<kindselector>]
| double complex | double precision
| integer [<kindselector>] | logical [<kindselector>]
<charselector> := * <charlen>
| ( [len=] <len> [ , [kind=] <kind>] )
| ( kind= <kind> [ , len= <len> ] )
<kindselector> := * <intlen> | ( [kind=] <kind> )
<entitydecl> := <name> [ [ * <charlen> ] [ ( <arrayspec> ) ]
| [ ( <arrayspec> ) ] * <charlen> ]
| [ / <init_expr> / | = <init_expr> ] \
[ , <entitydecl> ]
and
+ ``<attrspec>`` is a comma separated list of attributes_;
+ ``<arrayspec>`` is a comma separated list of dimension bounds;
+ ``<init_expr>`` is a `C expression`__.
+ ``<intlen>`` may be negative integer for ``integer`` type
specifications. In such cases ``integer*<negintlen>`` represents
unsigned C integers.
__ `C expressions`_
If an argument has no ``<argument type declaration>``, its type is
determined by applying ``implicit`` rules to its name.
Statements
----------
Attribute statements:
The ``<argument/variable attribute statement>`` is
``<argument/variable type declaration>`` without ``<typespec>``.
In addition, in an attribute statement one cannot use other
attributes, also ``<entitydecl>`` can be only a list of names.
Use statements:
The definition of the ``<use statement>`` part is
::
use <modulename> [ , <rename_list> | , ONLY : <only_list> ]
where
::
<rename_list> := <local_name> => <use_name> [ , <rename_list> ]
Currently F2PY uses ``use`` statement only for linking call-back
modules and ``external`` arguments (call-back functions), see
:ref:`Call-back arguments`.
Common block statements:
The definition of the ``<common block statement>`` part is
::
common / <common name> / <shortentitydecl>
where
::
<shortentitydecl> := <name> [ ( <arrayspec> ) ] [ , <shortentitydecl> ]
If a ``python module`` block contains two or more ``common`` blocks
with the same name, the variables from the additional declarations
are appended. The types of variables in ``<shortentitydecl>`` are
defined using ``<argument type declarations>``. Note that the
corresponding ``<argument type declarations>`` may contain array
specifications; then you don't need to specify these in
``<shortentitydecl>``.
Other statements:
The ``<other statement>`` part refers to any other Fortran language
constructs that are not described above. F2PY ignores most of them
except
+ ``call`` statements and function calls of ``external`` arguments
(`more details`__?);
__ external_
+ ``include`` statements
::
include '<filename>'
include "<filename>"
If a file ``<filename>`` does not exist, the ``include``
statement is ignored. Otherwise, the file ``<filename>`` is
included to a signature file. ``include`` statements can be used
in any part of a signature file, also outside the Fortran/C
routine signature blocks.
+ ``implicit`` statements
::
implicit none
implicit <list of implicit maps>
where
::
<implicit map> := <typespec> ( <list of letters or range of letters> )
Implicit rules are used to determine the type specification of
a variable (from the first-letter of its name) if the variable
is not defined using ``<variable type declaration>``. Default
implicit rule is given by
::
implicit real (a-h,o-z,$_), integer (i-m)
+ ``entry`` statements
::
entry <entry name> [([<arguments>])]
F2PY generates wrappers to all entry names using the signature
of the routine block.
Tip: ``entry`` statement can be used to describe the signature
of an arbitrary routine allowing F2PY to generate a number of
wrappers from only one routine block signature. There are few
restrictions while doing this: ``fortranname`` cannot be used,
``callstatement`` and ``callprotoargument`` can be used only if
they are valid for all entry routines, etc.
In addition, F2PY introduces the following statements:
+ ``threadsafe``
Use ``Py_BEGIN_ALLOW_THREADS .. Py_END_ALLOW_THREADS`` block
around the call to Fortran/C function.
+ ``callstatement <C-expr|multi-line block>``
Replace F2PY generated call statement to Fortran/C function with
``<C-expr|multi-line block>``. The wrapped Fortran/C function
is available as ``(*f2py_func)``. To raise an exception, set
``f2py_success = 0`` in ``<C-expr|multi-line block>``.
+ ``callprotoargument <C-typespecs>``
When ``callstatement`` statement is used then F2PY may not
generate proper prototypes for Fortran/C functions (because
``<C-expr>`` may contain any function calls and F2PY has no way
to determine what should be the proper prototype). With this
statement you can explicitly specify the arguments of the
corresponding prototype::
extern <return type> FUNC_F(<routine name>,<ROUTINE NAME>)(<callprotoargument>);
+ ``fortranname [<actual Fortran/C routine name>]``
You can use arbitrary ``<routine name>`` for a given Fortran/C
function. Then you have to specify
``<actual Fortran/C routine name>`` with this statement.
If ``fortranname`` statement is used without
``<actual Fortran/C routine name>`` then a dummy wrapper is
generated.
+ ``usercode <multi-line block>``
When used inside ``python module`` block, then given C code
will be inserted to generated C/API source just before
wrapper function definitions. Here you can define arbitrary
C functions to be used in initialization of optional arguments,
for example. If ``usercode`` is used twice inside ``python
module`` block then the second multiline block is inserted
after the definition of external routines.
When used inside ``<routine signature>``, then given C code will
be inserted to the corresponding wrapper function just after
declaring variables but before any C statements. So, ``usercode``
follow-up can contain both declarations and C statements.
When used inside the first ``interface`` block, then given C
code will be inserted at the end of the initialization
function of the extension module. Here you can modify extension
modules dictionary. For example, for defining additional
variables etc.
+ ``pymethoddef <multiline block>``
Multiline block will be inserted to the definition of
module methods ``PyMethodDef``-array. It must be a
comma-separated list of C arrays (see `Extending and Embedding`__
Python documentation for details).
``pymethoddef`` statement can be used only inside
``python module`` block.
__ http://www.python.org/doc/current/ext/ext.html
Attributes
------------
The following attributes are used by F2PY:
``optional``
The corresponding argument is moved to the end of ``<optional
arguments>`` list. A default value for an optional argument can be
specified ``<init_expr>``, see ``entitydecl`` definition. Note that
the default value must be given as a valid C expression.
Note that whenever ``<init_expr>`` is used, ``optional`` attribute
is set automatically by F2PY.
For an optional array argument, all its dimensions must be bounded.
``required``
The corresponding argument is considered as a required one. This is
default. You need to specify ``required`` only if there is a need to
disable automatic ``optional`` setting when ``<init_expr>`` is used.
If Python ``None`` object is used as a required argument, the
argument is treated as optional. That is, in the case of array
argument, the memory is allocated. And if ``<init_expr>`` is given,
the corresponding initialization is carried out.
``dimension(<arrayspec>)``
The corresponding variable is considered as an array with given
dimensions in ``<arrayspec>``.
``intent(<intentspec>)``
This specifies the "intention" of the corresponding
argument. ``<intentspec>`` is a comma separated list of the
following keys:
+ ``in``
The argument is considered as an input-only argument. It means
that the value of the argument is passed to Fortran/C function and
that function is expected not to change the value of an argument.
+ ``inout``
The argument is considered as an input/output or *in situ*
output argument. ``intent(inout)`` arguments can be only
"contiguous" NumPy arrays with proper type and size. Here
"contiguous" can be either in Fortran or C sense. The latter one
coincides with the contiguous concept used in NumPy and is
effective only if ``intent(c)`` is used. Fortran contiguity
is assumed by default.
Using ``intent(inout)`` is generally not recommended, use
``intent(in,out)`` instead. See also ``intent(inplace)`` attribute.
+ ``inplace``
The argument is considered as an input/output or *in situ*
output argument. ``intent(inplace)`` arguments must be
NumPy arrays with proper size. If the type of an array is
not "proper" or the array is non-contiguous then the array
will be changed in-place to fix the type and make it contiguous.
Using ``intent(inplace)`` is generally not recommended either.
For example, when slices have been taken from an
``intent(inplace)`` argument then after in-place changes,
slices data pointers may point to unallocated memory area.
+ ``out``
The argument is considered as a return variable. It is appended
to the ``<returned variables>`` list. Using ``intent(out)``
sets ``intent(hide)`` automatically, unless also
``intent(in)`` or ``intent(inout)`` were used.
By default, returned multidimensional arrays are
Fortran-contiguous. If ``intent(c)`` is used, then returned
multidimensional arrays are C-contiguous.
+ ``hide``
The argument is removed from the list of required or optional
arguments. Typically ``intent(hide)`` is used with ``intent(out)``
or when ``<init_expr>`` completely determines the value of the
argument like in the following example::
integer intent(hide),depend(a) :: n = len(a)
real intent(in),dimension(n) :: a
+ ``c``
The argument is treated as a C scalar or C array argument. In
the case of a scalar argument, its value is passed to C function
as a C scalar argument (recall that Fortran scalar arguments are
actually C pointer arguments). In the case of an array
argument, the wrapper function is assumed to treat
multidimensional arrays as C-contiguous arrays.
There is no need to use ``intent(c)`` for one-dimensional
arrays, no matter if the wrapped function is either a Fortran or
a C function. This is because the concepts of Fortran- and
C contiguity overlap in one-dimensional cases.
If ``intent(c)`` is used as a statement but without an entity
declaration list, then F2PY adds the ``intent(c)`` attribute to all
arguments.
Also, when wrapping C functions, one must use ``intent(c)``
attribute for ``<routine name>`` in order to disable Fortran
specific ``F_FUNC(..,..)`` macros.
+ ``cache``
The argument is treated as a junk of memory. No Fortran nor C
contiguity checks are carried out. Using ``intent(cache)``
makes sense only for array arguments, also in connection with
``intent(hide)`` or ``optional`` attributes.
+ ``copy``
Ensure that the original contents of ``intent(in)`` argument is
preserved. Typically used in connection with ``intent(in,out)``
attribute. F2PY creates an optional argument
``overwrite_<argument name>`` with the default value ``0``.
+ ``overwrite``
The original contents of the ``intent(in)`` argument may be
altered by the Fortran/C function. F2PY creates an optional
argument ``overwrite_<argument name>`` with the default value
``1``.
+ ``out=<new name>``
Replace the return name with ``<new name>`` in the ``__doc__``
string of a wrapper function.
+ ``callback``
Construct an external function suitable for calling Python function
from Fortran. ``intent(callback)`` must be specified before the
corresponding ``external`` statement. If 'argument' is not in
argument list then it will be added to Python wrapper but only
initializing external function.
Use ``intent(callback)`` in situations where a Fortran/C code
assumes that a user implements a function with given prototype
and links it to an executable. Don't use ``intent(callback)``
if function appears in the argument list of a Fortran routine.
With ``intent(hide)`` or ``optional`` attributes specified and
using a wrapper function without specifying the callback argument
in argument list then call-back function is looked in the
namespace of F2PY generated extension module where it can be
set as a module attribute by a user.
+ ``aux``
Define auxiliary C variable in F2PY generated wrapper function.
Useful to save parameter values so that they can be accessed
in initialization expression of other variables. Note that
``intent(aux)`` silently implies ``intent(c)``.
The following rules apply:
+ If no ``intent(in | inout | out | hide)`` is specified,
``intent(in)`` is assumed.
+ ``intent(in,inout)`` is ``intent(in)``.
+ ``intent(in,hide)`` or ``intent(inout,hide)`` is
``intent(hide)``.
+ ``intent(out)`` is ``intent(out,hide)`` unless ``intent(in)`` or
``intent(inout)`` is specified.
+ If ``intent(copy)`` or ``intent(overwrite)`` is used, then an
additional optional argument is introduced with a name
``overwrite_<argument name>`` and a default value 0 or 1, respectively.
+ ``intent(inout,inplace)`` is ``intent(inplace)``.
+ ``intent(in,inplace)`` is ``intent(inplace)``.
+ ``intent(hide)`` disables ``optional`` and ``required``.
``check([<C-booleanexpr>])``
Perform consistency check of arguments by evaluating
``<C-booleanexpr>``; if ``<C-booleanexpr>`` returns 0, an exception
is raised.
If ``check(..)`` is not used then F2PY generates few standard checks
(e.g. in a case of an array argument, check for the proper shape
and size) automatically. Use ``check()`` to disable checks generated
by F2PY.
``depend([<names>])``
This declares that the corresponding argument depends on the values
of variables in the list ``<names>``. For example, ``<init_expr>``
may use the values of other arguments. Using information given by
``depend(..)`` attributes, F2PY ensures that arguments are
initialized in a proper order. If ``depend(..)`` attribute is not
used then F2PY determines dependence relations automatically. Use
``depend()`` to disable dependence relations generated by F2PY.
When you edit dependence relations that were initially generated by
F2PY, be careful not to break the dependence relations of other
relevant variables. Another thing to watch out is cyclic
dependencies. F2PY is able to detect cyclic dependencies
when constructing wrappers and it complains if any are found.
``allocatable``
The corresponding variable is Fortran 90 allocatable array defined
as Fortran 90 module data.
.. _external:
``external``
The corresponding argument is a function provided by user. The
signature of this so-called call-back function can be defined
- in ``__user__`` module block,
- or by demonstrative (or real, if the signature file is a real Fortran
code) call in the ``<other statements>`` block.
For example, F2PY generates from
::
external cb_sub, cb_fun
integer n
real a(n),r
call cb_sub(a,n)
r = cb_fun(4)
the following call-back signatures::
subroutine cb_sub(a,n)
real dimension(n) :: a
integer optional,check(len(a)>=n),depend(a) :: n=len(a)
end subroutine cb_sub
function cb_fun(e_4_e) result (r)
integer :: e_4_e
real :: r
end function cb_fun
The corresponding user-provided Python function are then::
def cb_sub(a,[n]):
...
return
def cb_fun(e_4_e):
...
return r
See also ``intent(callback)`` attribute.
``parameter``
The corresponding variable is a parameter and it must have a fixed
value. F2PY replaces all parameter occurrences by their
corresponding values.
Extensions
============
F2PY directives
-----------------
The so-called F2PY directives allow using F2PY signature file
constructs also in Fortran 77/90 source codes. With this feature you
can skip (almost) completely intermediate signature file generations
and apply F2PY directly to Fortran source codes.
F2PY directive has the following form::
<comment char>f2py ...
where allowed comment characters for fixed and free format Fortran
codes are ``cC*!#`` and ``!``, respectively. Everything that follows
``<comment char>f2py`` is ignored by a compiler but read by F2PY as a
normal Fortran, non-comment line:
When F2PY finds a line with F2PY directive, the directive is first
replaced by 5 spaces and then the line is reread.
For fixed format Fortran codes, ``<comment char>`` must be at the
first column of a file, of course. For free format Fortran codes,
F2PY directives can appear anywhere in a file.
C expressions
--------------
C expressions are used in the following parts of signature files:
* ``<init_expr>`` of variable initialization;
* ``<C-booleanexpr>`` of the ``check`` attribute;
* ``<arrayspec> of the ``dimension`` attribute;
* ``callstatement`` statement, here also a C multiline block can be used.
A C expression may contain:
* standard C constructs;
* functions from ``math.h`` and ``Python.h``;
* variables from the argument list, presumably initialized before
according to given dependence relations;
* the following CPP macros:
``rank(<name>)``
Returns the rank of an array ``<name>``.
``shape(<name>,<n>)``
Returns the ``<n>``-th dimension of an array ``<name>``.
``len(<name>)``
Returns the length of an array ``<name>``.
``size(<name>)``
Returns the size of an array ``<name>``.
``slen(<name>)``
Returns the length of a string ``<name>``.
For initializing an array ``<array name>``, F2PY generates a loop over
all indices and dimensions that executes the following
pseudo-statement::
<array name>(_i[0],_i[1],...) = <init_expr>;
where ``_i[<i>]`` refers to the ``<i>``-th index value and that runs
from ``0`` to ``shape(<array name>,<i>)-1``.
For example, a function ``myrange(n)`` generated from the following
signature
::
subroutine myrange(a,n)
fortranname ! myrange is a dummy wrapper
integer intent(in) :: n
real*8 intent(c,out),dimension(n),depend(n) :: a = _i[0]
end subroutine myrange
is equivalent to ``numpy.arange(n,dtype=float)``.
.. warning::
F2PY may lower cases also in C expressions when scanning Fortran codes
(see ``--[no]-lower`` option).
Multiline blocks
------------------
A multiline block starts with ``'''`` (triple single-quotes) and ends
with ``'''`` in some *strictly* subsequent line. Multiline blocks can
be used only within .pyf files. The contents of a multiline block can
be arbitrary (except that it cannot contain ``'''``) and no
transformations (e.g. lowering cases) are applied to it.
Currently, multiline blocks can be used in the following constructs:
+ as a C expression of the ``callstatement`` statement;
+ as a C type specification of the ``callprotoargument`` statement;
+ as a C code block of the ``usercode`` statement;
+ as a list of C arrays of the ``pymethoddef`` statement;
+ as documentation string.