Signature file#
The interface definition file (.pyf) is how you can fine-tune the interface
between Python and Fortran. The syntax specification for signature files
(.pyf
files) is modeled on 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
some extensions to the Fortran 90/95 language specification that help in the
design of the Fortran to Python interface, making it more “Pythonic”.
Signature files may contain arbitrary Fortran code so that any Fortran 90/95 codes can be treated as signature files. F2PY silently ignores Fortran constructs that are irrelevant for creating the interface. However, this also means that syntax errors are not caught by F2PY and will only be caught when the library is built.
Note
Currently, F2PY may fail with valid Fortran constructs, such as intrinsic modules. If this happens, you can check the NumPy GitHub issue tracker for possible workarounds or work-in-progress ideas.
In general, the contents of the signature files are case-sensitive. When
scanning Fortran codes to generate a signature file, F2PY lowers all cases
automatically except in multi-line blocks or when the --no-lower
option is
used.
The syntax of signature files is presented below.
Signature files syntax#
Python module block#
A signature file may contain one (recommended) or more python
module
blocks. The python module
block describes the contents of
a Python/C extension module <modulename>module.c
that F2PY
generates.
Warning
Exception: if <modulename>
contains a substring __user__
, then the
corresponding python module
block describes the signatures of call-back
functions (see 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 section, dots ...
indicate one or
more of a previous section. So, []...
is to be read as zero or more of a
previous section.
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 forinteger
type specifications. In such casesinteger*<negintlen>
represents unsigned C integers;
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 similar to the
<argument/variable type declaration>
, but without <typespec>
.
An attribute statement cannot contain other attributes, and <entitydecl>
can
be only a list of names. See Attributes for more details on the
attributes that can be used by F2PY.
Use statements#
The definition of the
<use statement>
part isuse <modulename> [ , <rename_list> | , ONLY : <only_list> ]
where
<rename_list> := <local_name> => <use_name> [ , <rename_list> ]
Currently F2PY uses
use
statements only for linking call-back modules andexternal
arguments (call-back functions). See Call-back arguments.
Common block statements#
The definition of the
<common block statement>
part iscommon / <common name> / <shortentitydecl>
where
<shortentitydecl> := <name> [ ( <arrayspec> ) ] [ , <shortentitydecl> ]
If a
python module
block contains two or morecommon
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 these need not be specified 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 the following:call
statements and function calls ofexternal
arguments (see more details on external arguments);include
statementsinclude '<filename>' include "<filename>"
If a file
<filename>
does not exist, theinclude
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
statementsimplicit 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 rules are given by:implicit real (a-h,o-z,$_), integer (i-m)
entry
statementsentry <entry name> [([<arguments>])]
F2PY generates wrappers for all entry names using the signature of the routine block.
Note
The
entry
statement can be used to describe the signature of an arbitrary subroutine or function 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
andcallprotoargument
can be used only if they are valid for all entry routines, etc.
F2PY statements#
In addition, F2PY introduces the following statements:
threadsafe
Uses a
Py_BEGIN_ALLOW_THREADS .. Py_END_ALLOW_THREADS
block around the call to Fortran/C function.callstatement <C-expr|multi-line block>
Replaces the 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 the
callstatement
statement is used, F2PY may not generate proper prototypes for Fortran/C functions (because<C-expr>
may contain 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>]
F2PY allows for the use of an arbitrary
<routine name>
for a given Fortran/C function. Then this statement is used for the<actual Fortran/C routine name>
.If
fortranname
statement is used without<actual Fortran/C routine name>
then a dummy wrapper is generated.usercode <multi-line block>
When this is used inside a
python module
block, the 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 for the initialization of optional arguments.
For example, if
usercode
is used twice insidepython module
block then the second multi-line block is inserted after the definition of the external routines.When used inside
<routine signature>
, then the given C code will be inserted into the corresponding wrapper function just after the declaration of variables but before any C statements. So, theusercode
follow-up can contain both declarations and C statements.When used inside the first
interface
block, then the given C code will be inserted at the end of the initialization function of the extension module. This is how the extension modules dictionary can be modified and has many use-cases; for example, to define additional variables.pymethoddef <multiline block>
This is a multi-line block which will be inserted into the definition of a 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 insidepython module
block.
Attributes#
The following attributes can be 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 via<init_expr>
(see theentitydecl
definition)Note
The default value must be given as a valid C expression.
Whenever
<init_expr>
is used, theoptional
attribute is set automatically by F2PY.For an optional array argument, all its dimensions must be bounded.
required
The corresponding argument with this attribute is considered mandatory. This is the default.
required
should only be specified if there is a need to disable the automaticoptional
setting when<init_expr>
is used.If a Python
None
object is used as a required argument, the argument is treated as optional. That is, in the case of array arguments, the memory is allocated. If<init_expr>
is given, then the corresponding initialization is carried out.dimension(<arrayspec>)
The corresponding variable is considered as an array with dimensions given in
<arrayspec>
.intent(<intentspec>)
This specifies the “intention” of the corresponding argument.
<intentspec>
is a comma separated list of the following keys:in
The corresponding argument is considered to be input-only. This means that the value of the argument is passed to a Fortran/C function and that the function is expected to not change the value of this argument.
inout
The corresponding argument is marked for input/output or as an in situ output argument.
intent(inout)
arguments can be only contiguous NumPy arrays (in either the Fortran or C sense) with proper type and size. The latter coincides with the default contiguous concept used in NumPy and is effective only ifintent(c)
is used. F2PY assumes Fortran contiguous arguments by default.Note
Using
intent(inout)
is generally not recommended, as it can cause unexpected results. For example, scalar arguments usingintent(inout)
are assumed to be array objects in order to have in situ changes be effective. Useintent(in,out)
instead.See also the
intent(inplace)
attribute.
inplace
The corresponding argument is considered to be an input/output or in situ output argument.
intent(inplace)
arguments must be NumPy arrays of a proper size. If the type of an array is not “proper” or the array is non-contiguous then the array will be modified in-place to fix the type and make it contiguous.Note
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, the data pointers for the slices may point to an unallocated memory area.
out
The corresponding argument is considered to be a return variable. It is appended to the
<returned variables>
list. Usingintent(out)
setsintent(hide)
automatically, unlessintent(in)
orintent(inout)
are specified as well.By default, returned multidimensional arrays are Fortran-contiguous. If
intent(c)
attribute is used, then the returned multidimensional arrays are C-contiguous.
hide
The corresponding argument is removed from the list of required or optional arguments. Typically
intent(hide)
is used withintent(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 corresponding argument is treated as a C scalar or C array argument. For the case of a scalar argument, its value is passed to a C function as a C scalar argument (recall that Fortran scalar arguments are actually C pointer arguments). For array arguments, 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, irrespective of whether the wrapped function is in Fortran or C. 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 theintent(c)
attribute to all arguments.Also, when wrapping C functions, one must use
intent(c)
attribute for<routine name>
in order to disable Fortran specificF_FUNC(..,..)
macros.
cache
The corresponding argument is treated as junk memory. No Fortran nor C contiguity checks are carried out. Using
intent(cache)
makes sense only for array arguments, also in conjunction withintent(hide)
oroptional
attributes.
copy
Ensures that the original contents of
intent(in)
argument is preserved. Typically used with theintent(in,out)
attribute. F2PY creates an optional argumentoverwrite_<argument name>
with the default value0
.
overwrite
This indicates that the original contents of the
intent(in)
argument may be altered by the Fortran/C function. F2PY creates an optional argumentoverwrite_<argument name>
with the default value1
.
out=<new name>
Replaces the returned name with
<new name>
in the__doc__
string of the wrapper function.
callback
Constructs an external function suitable for calling Python functions from Fortran.
intent(callback)
must be specified before the correspondingexternal
statement. If the ‘argument’ is not in the argument list then it will be added to Python wrapper but only by initializing an external function.Note
Use
intent(callback)
in situations where the Fortran/C code assumes that the user implemented a function with a given prototype and linked it to an executable. Don’t useintent(callback)
if the function appears in the argument list of a Fortran routine.With
intent(hide)
oroptional
attributes specified and using a wrapper function without specifying the callback argument in the argument list; then the call-back function is assumed to be found in the namespace of the F2PY generated extension module where it can be set as a module attribute by a user.
aux
Defines an auxiliary C variable in the F2PY generated wrapper function. Useful to save parameter values so that they can be accessed in initialization expressions for other variables.
Note
intent(aux)
silently impliesintent(c)
.
The following rules apply:
If none of
intent(in | inout | out | hide)
are specified,intent(in)
is assumed.intent(in,inout)
isintent(in)
;intent(in,hide)
orintent(inout,hide)
isintent(hide)
;intent(out)
isintent(out,hide)
unlessintent(in)
orintent(inout)
is specified.
If
intent(copy)
orintent(overwrite)
is used, then an additional optional argument is introduced with a nameoverwrite_<argument name>
and a default value 0 or 1, respectively.intent(inout,inplace)
isintent(inplace)
;intent(in,inplace)
isintent(inplace)
;intent(hide)
disablesoptional
andrequired
.
check([<C-booleanexpr>])
Performs a consistency check on the arguments by evaluating
<C-booleanexpr>
; if<C-booleanexpr>
returns 0, an exception is raised.Note
If
check(..)
is not used then F2PY automatically generates a few standard checks (e.g. in a case of an array argument, it checks for the proper shape and size). Usecheck()
to disable checks generated by F2PY.depend([<names>])
This declares that the corresponding argument depends on the values of variables in the
<names>
list. For example,<init_expr>
may use the values of other arguments. Using information given bydepend(..)
attributes, F2PY ensures that arguments are initialized in a proper order. If thedepend(..)
attribute is not used then F2PY determines dependence relations automatically. Usedepend()
to disable the 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 for 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 a Fortran 90 allocatable array defined as Fortran 90 module data.
external
The corresponding argument is a function provided by user. The signature of this 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 the
intent(callback)
attribute.parameter
This indicates that 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 F2PY directives allow using F2PY signature file constructs in Fortran 77/90 source codes. With this feature one can (almost) completely skip the intermediate signature file generation and apply F2PY directly to Fortran source codes.
F2PY directives have 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 non-comment Fortran line:
Note
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,
the F2PY directives can appear anywhere in a file.
C expressions#
C expressions are used in the following parts of signature files:
<init_expr>
for variable initialization;<C-booleanexpr>
of thecheck
attribute;<arrayspec>
of thedimension
attribute;callstatement
statement, here also a C multi-line block can be used.
A C expression may contain:
standard C constructs;
functions from
math.h
andPython.h
;variables from the argument list, presumably initialized before according to given dependence relations;
the following CPP macros:
f2py_rank(<name>)
Returns the rank of an array
<name>
.f2py_shape(<name>, <n>)
Returns the
<n>
-th dimension of an array<name>
.f2py_len(<name>)
Returns the length of an array
<name>
.f2py_size(<name>)
Returns the size of an array
<name>
.f2py_itemsize(<name>)
Returns the itemsize of an array
<name>
.f2py_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).
Multi-line blocks#
A multi-line block starts with '''
(triple single-quotes) and ends
with '''
in some strictly subsequent line. Multi-line blocks can
be used only within .pyf files. The contents of a multi-line block can
be arbitrary (except that it cannot contain '''
) and no
transformations (e.g. lowering cases) are applied to it.
Currently, multi-line 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.
Extended char-selector#
F2PY extends char-selector specification, usable within a signature file or a F2PY directive, as follows:
<extended-charselector> := <charselector>
| (f2py_len= <len>)
See Character strings for usage.