Defining class methods on C code

**Diez B. Roggisch** · Jul 18 '05, 08:12 AM

Re: Defining *class* methods on C code

> How can I attach *class* methods to a type in C code, when I have a[color=blue]
> function
>
> PyObject *func(PyObject *type, PyObject *arg);
>
> with the METH_O calling convention? The type is already created, I
> cannot insert it into the tp_methods array anymore.
>
> Something like this:
>
> class X(object):
> pass
>
> def func(cls, arg):
> ....
>
> X.func = classmethod(fun c)[/color]

Try METH_STATIC as flag for PyMethodDef.

--

Regards,

Diez B. Roggisch

**Michael Hudson** · Jul 18 '05, 08:13 AM

Re: Defining *class* methods on C code

Thomas Heller <theller@python .net> writes:
[color=blue]
> I once knew how to do it, but I cannot find or remember it anymore:
>
> How can I attach *class* methods to a type in C code, when I have a
> function
>
> PyObject *func(PyObject *type, PyObject *arg);
>
> with the METH_O calling convention? The type is already created, I
> cannot insert it into the tp_methods array anymore.
>
> Something like this:
>
> class X(object):
> pass
>
> def func(cls, arg):
> ....
>
> X.func = classmethod(fun c)[/color]

Just stuff it into tp_dict? You'll need to make the method object
yourself, but that's easy.

I think you probably have to hope that the name of the class method
isn't a special method name (you'd want to call
typeobject.c:up date_slots then, but I don't think you can arrange for
that to happen from outside typeobject.c as all the juicy symbols are
static).

Or just stick TP_HEAPTYPE into tp_flags, use PyObject_SetAtt r and take
TP_HEAPTYPE out again (that's very sick, though).

Cheers,
mwh

--[color=blue]
> Well, as an American citizen I hope that the EU tells the MPAA
> and RIAA to shove it where the Sun don't shine.[/color]
Actually they already did. Only first they bent over and dropped
their trousers. -- Shmuel (Seymour J.) Metz & Toni Lassila, asr

**Thomas Heller** · Jul 18 '05, 08:13 AM

Re: Defining *class* methods on C code

Michael Hudson <mwh@python.net > writes:
[color=blue]
> Thomas Heller <theller@python .net> writes:
>[color=green]
>> I once knew how to do it, but I cannot find or remember it anymore:
>>
>> How can I attach *class* methods to a type in C code, when I have a
>> function
>>
>> PyObject *func(PyObject *type, PyObject *arg);
>>
>> with the METH_O calling convention? The type is already created, I
>> cannot insert it into the tp_methods array anymore.
>>
>> Something like this:
>>
>> class X(object):
>> pass
>>
>> def func(cls, arg):
>> ....
>>
>> X.func = classmethod(fun c)[/color]
>
> Just stuff it into tp_dict? You'll need to make the method object
> yourself, but that's easy.
>
> I think you probably have to hope that the name of the class method
> isn't a special method name (you'd want to call
> typeobject.c:up date_slots then, but I don't think you can arrange for
> that to happen from outside typeobject.c as all the juicy symbols are
> static).[/color]

No special names. Here's the code:

static PyObject *my_method(PyOb ject *self, PyObject *arg)
{
Py_INCREF(arg);
return arg;
}

static PyMethodDef my_methods[] = {
{ "my_method" , my_method, METH_O },
{ NULL, NULL },
};

and then ('type' is the type where I want to create the class method on):

if (somecondition) {
PyObject *func;
PyObject *meth;
PyMethodDef *ml = my_methods;

for (; ml->ml_name; ++ml) {
func = PyCFunction_New (ml, NULL);
if (!func)
return NULL;
meth = PyClassMethod_N ew(func);
if (!meth)
return NULL;
if (-1 == PyDict_SetItemS tring(result->tp_dict,
ml->ml_name,
meth))
return NULL;
}
}

I know that there are refcount leaks in this snippet, but that's not the
point.

Trying out the method:

c:\sf\ctypes>py 23 -c "from ctypes import *; print c_int.my_method ()"
<class 'ctypes.c_int'>

c:\sf\ctypes>py 23 -c "from ctypes import *; print c_int.my_method (0)"
Traceback (most recent call last):
File "<string>", line 1, in ?
TypeError: my_method() takes exactly one argument (2 given)
c:\sf\ctypes>

Works *nearly*, but I would have expected that the method accepts one
parameter, pass it as 'arg' to the C function, plus the type itself as
'self'.
[color=blue]
> Or just stick TP_HEAPTYPE into tp_flags, use PyObject_SetAtt r and take
> TP_HEAPTYPE out again (that's very sick, though).[/color]

Um, why that?

Thanks,

Thomas

**Thomas Heller** · Jul 18 '05, 08:13 AM

Re: Defining *class* methods on C code

Thomas Heller <theller@python .net> writes:

[followup to myself, I found it][color=blue]
> Michael Hudson <mwh@python.net > writes:
>[color=green]
>> Thomas Heller <theller@python .net> writes:
>>[color=darkred]
>>> I once knew how to do it, but I cannot find or remember it anymore:
>>>
>>> How can I attach *class* methods to a type in C code, when I have a
>>> function
>>>
>>> PyObject *func(PyObject *type, PyObject *arg);
>>>
>>> with the METH_O calling convention? The type is already created, I
>>> cannot insert it into the tp_methods array anymore.
>>>
>>> Something like this:
>>>
>>> class X(object):
>>> pass
>>>
>>> def func(cls, arg):
>>> ....
>>>
>>> X.func = classmethod(fun c)[/color]
>>
>> Just stuff it into tp_dict? You'll need to make the method object
>> yourself, but that's easy.
>>
>> I think you probably have to hope that the name of the class method
>> isn't a special method name (you'd want to call
>> typeobject.c:up date_slots then, but I don't think you can arrange for
>> that to happen from outside typeobject.c as all the juicy symbols are
>> static).[/color]
>
> No special names. Here's the code:
>
> static PyObject *my_method(PyOb ject *self, PyObject *arg)
> {
> Py_INCREF(arg);
> return arg;
> }
>
> static PyMethodDef my_methods[] = {
> { "my_method" , my_method, METH_O },
> { NULL, NULL },
> };
>
> and then ('type' is the type where I want to create the class method on):
>[/color]
[snipped non-working code]

I have to call PyDescr_NewClas sMethodType:

if (somecondition) {
PyObject *meth;
PyMethodDef *ml = my_methods;

for (; ml->ml_name; ++ml) {
meth = PyDescr_NewClas sMethod(type, ml);
if (!meth)
return NULL;
if (-1 == PyDict_SetItemS tring(type->tp_dict,
ml->ml_name,
meth))
return NULL;
}
}
[color=blue]
> I know that there are refcount leaks in this snippet, but that's not the
> point.[/color]

Thomas

**Michael Hudson** · Jul 18 '05, 08:13 AM

Re: Defining *class* methods on C code

Thomas Heller <theller@python .net> writes:
[color=blue]
> Michael Hudson <mwh@python.net > writes:
>[color=green]
> > Thomas Heller <theller@python .net> writes:
> >[color=darkred]
> >> I once knew how to do it, but I cannot find or remember it anymore:
> >>
> >> How can I attach *class* methods to a type in C code, when I have a
> >> function
> >>
> >> PyObject *func(PyObject *type, PyObject *arg);
> >>
> >> with the METH_O calling convention? The type is already created, I
> >> cannot insert it into the tp_methods array anymore.
> >>
> >> Something like this:
> >>
> >> class X(object):
> >> pass
> >>
> >> def func(cls, arg):
> >> ....
> >>
> >> X.func = classmethod(fun c)[/color]
> >
> > Just stuff it into tp_dict? You'll need to make the method object
> > yourself, but that's easy.
> >
> > I think you probably have to hope that the name of the class method
> > isn't a special method name (you'd want to call
> > typeobject.c:up date_slots then, but I don't think you can arrange for
> > that to happen from outside typeobject.c as all the juicy symbols are
> > static).[/color]
>
> No special names. Here's the code:
>
> static PyObject *my_method(PyOb ject *self, PyObject *arg)
> {
> Py_INCREF(arg);
> return arg;
> }
>
> static PyMethodDef my_methods[] = {
> { "my_method" , my_method, METH_O },
> { NULL, NULL },
> };
>
> and then ('type' is the type where I want to create the class method on):
>
> if (somecondition) {
> PyObject *func;
> PyObject *meth;
> PyMethodDef *ml = my_methods;
>
> for (; ml->ml_name; ++ml) {
> func = PyCFunction_New (ml, NULL);
> if (!func)
> return NULL;
> meth = PyClassMethod_N ew(func);[/color]

How about "meth = PyDescr_NewClas sMethod(result, ml);" here?
[color=blue]
> if (!meth)
> return NULL;
> if (-1 == PyDict_SetItemS tring(result->tp_dict,
> ml->ml_name,
> meth))
> return NULL;
> }
> }
>
> I know that there are refcount leaks in this snippet, but that's not the
> point.
>
> Trying out the method:
>
> c:\sf\ctypes>py 23 -c "from ctypes import *; print c_int.my_method ()"
> <class 'ctypes.c_int'>
>
> c:\sf\ctypes>py 23 -c "from ctypes import *; print c_int.my_method (0)"
> Traceback (most recent call last):
> File "<string>", line 1, in ?
> TypeError: my_method() takes exactly one argument (2 given)
> c:\sf\ctypes>
>
> Works *nearly*, but I would have expected that the method accepts one
> parameter, pass it as 'arg' to the C function, plus the type itself as
> 'self'.[/color]

Oh... I wouldn't have :-)
[color=blue][color=green]
> > Or just stick TP_HEAPTYPE into tp_flags, use PyObject_SetAtt r and take
> > TP_HEAPTYPE out again (that's very sick, though).[/color]
>
> Um, why that?[/color]

Well, if "my_method" was spelt "__add__", say, you'd want to call
typeobject.c:up date_slots() to get the function into the
tp_as_number.nb _add fields of the type and all the subclasses. The
hack I outline is one way of doing that.

Cheers,
mwh

--
Every now and then, Google doesn't throw up what I need so I start
checking Altavista, Yahoo, etc. In almost every single case, I am
brutally reminded why I use Google in the first place.
-- John Riddoch, asr

**Thomas Heller** · Jul 18 '05, 08:14 AM

Re: Defining *class* methods on C code

[color=blue][color=green]
>> if (somecondition) {
>> PyObject *func;
>> PyObject *meth;
>> PyMethodDef *ml = my_methods;
>>
>> for (; ml->ml_name; ++ml) {
>> func = PyCFunction_New (ml, NULL);
>> if (!func)
>> return NULL;
>> meth = PyClassMethod_N ew(func);[/color]
>
> How about "meth = PyDescr_NewClas sMethod(result, ml);" here?
>[/color]

Yes, that works. But it's not in Python 2.2, which is required ;-(

Thanks,

Thomas

**Andrew MacIntyre** · Jul 18 '05, 08:14 AM

Re: Defining *class* methods on C code

On Fri, 6 Feb 2004, Thomas Heller wrote:
[color=blue]
> I have to call PyDescr_NewClas sMethodType:[/color]

File a doc bug? I can't find this in the 2.3.3 docs at all...

--
Andrew I MacIntyre "These thoughts are mine alone..."
E-mail: andymac@bullsey e.apana.org.au (pref) | Snail: PO Box 370
andymac@pcug.or g.au (alt) | Belconnen ACT 2616
Web: http://www.andymac.org/ | Australia

**Michael Hudson** · Jul 18 '05, 08:15 AM

Re: Defining *class* methods on C code

Thomas Heller <theller@python .net> writes:
[color=blue][color=green][color=darkred]
> >> if (somecondition) {
> >> PyObject *func;
> >> PyObject *meth;
> >> PyMethodDef *ml = my_methods;
> >>
> >> for (; ml->ml_name; ++ml) {
> >> func = PyCFunction_New (ml, NULL);
> >> if (!func)
> >> return NULL;
> >> meth = PyClassMethod_N ew(func);[/color]
> >
> > How about "meth = PyDescr_NewClas sMethod(result, ml);" here?
> >[/color]
>
> Yes, that works. But it's not in Python 2.2, which is required ;-([/color]

Arghl. I guess you can just steal the relavent code... wasn't it your
patch in the first place that implemented all this?

Cheers,
mwh

--
$ head -n 2 src/bash/bash-2.04/unwind_prot.c
/* I can't stand it anymore! Please can't we just write the
whole Unix system in lisp or something? */
-- spotted by Rich van der Hoff

**Thomas Heller** · Jul 18 '05, 08:17 AM

Re: Defining *class* methods on C code

Michael Hudson <mwh@python.net > writes:
[color=blue]
> Thomas Heller <theller@python .net> writes:
>[color=green][color=darkred]
>> >> if (somecondition) {
>> >> PyObject *func;
>> >> PyObject *meth;
>> >> PyMethodDef *ml = my_methods;
>> >>
>> >> for (; ml->ml_name; ++ml) {
>> >> func = PyCFunction_New (ml, NULL);
>> >> if (!func)
>> >> return NULL;
>> >> meth = PyClassMethod_N ew(func);
>> >
>> > How about "meth = PyDescr_NewClas sMethod(result, ml);" here?
>> >[/color]
>>
>> Yes, that works. But it's not in Python 2.2, which is required ;-([/color]
>
> Arghl. I guess you can just steal the relavent code... wasn't it your
> patch in the first place that implemented all this?[/color]

No, it was my feature request. IIRC, it was implemented by Fred Drake.

Thomas

**Thomas Heller** · Jul 18 '05, 08:17 AM

Re: Defining *class* methods on C code

Thomas Heller <theller@python .net> writes:
[color=blue]
> Michael Hudson <mwh@python.net > writes:
>[color=green]
>> Thomas Heller <theller@python .net> writes:
>>[color=darkred]
>>> >> if (somecondition) {
>>> >> PyObject *func;
>>> >> PyObject *meth;
>>> >> PyMethodDef *ml = my_methods;
>>> >>
>>> >> for (; ml->ml_name; ++ml) {
>>> >> func = PyCFunction_New (ml, NULL);
>>> >> if (!func)
>>> >> return NULL;
>>> >> meth = PyClassMethod_N ew(func);
>>> >
>>> > How about "meth = PyDescr_NewClas sMethod(result, ml);" here?
>>> >
>>>
>>> Yes, that works. But it's not in Python 2.2, which is required ;-([/color]
>>
>> Arghl. I guess you can just steal the relavent code... wasn't it your
>> patch in the first place that implemented all this?[/color]
>
> No, it was my feature request. IIRC, it was implemented by Fred Drake.[/color]

Guess I'm getting old. It *was* my patch ;-)

Thomas

**Thomas Heller** · Jul 18 '05, 08:17 AM

Re: Defining *class* methods on C code

Andrew MacIntyre <andymac@bullse ye.apana.org.au > writes:
[color=blue]
> On Fri, 6 Feb 2004, Thomas Heller wrote:
>[color=green]
>> I have to call PyDescr_NewClas sMethodType:[/color]
>
> File a doc bug? I can't find this in the 2.3.3 docs at all...[/color]

I've checked it in directly, but only the function prototype.

Thomas

**Michael Hudson** · Jul 18 '05, 08:17 AM

Re: Defining *class* methods on C code

Thomas Heller <theller@python .net> writes:
[color=blue]
> Thomas Heller <theller@python .net> writes:
>[color=green]
> > Michael Hudson <mwh@python.net > writes:
> >[color=darkred]
> >> Arghl. I guess you can just steal the relavent code... wasn't it your
> >> patch in the first place that implemented all this?[/color]
> >
> > No, it was my feature request. IIRC, it was implemented by Fred Drake.[/color]
>
> Guess I'm getting old. It *was* my patch ;-)[/color]

:-)

Cheers,
mwh

--
at any rate, I'm satisfied that not only do they know which end of
the pointy thing to hold, but where to poke it for maximum effect.
-- Eric The Read, asr, on google.com

Defining class methods on C code

Defining class methods on C code

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