Re: How to create an instance of a python class from C++

2014-03-12 Thread Stefan Behnel 
Barry Scott, 11.03.2014 22:37:
 On 5 Mar 2014, at 00:14, Bill wrote:
 I can't figure out how to create an instance
 of a python class from 'C++':
 
 Why not use pycxx from http://sourceforge.net/projects/cxx/?
 
 This lib does all the heavy lifting for you for both python2 and python3.
 Has docs and examples.

Yes, tool support definitely helps here. I was going to suggest Cython
(also for obvious reasons), where the code that the OP posted would look
like this:

  def RegisterClass(class_decl):
  an = type(class_decl)()
  print(an.description())
  return 0

Clearly substantially simpler than the posted C code (and certainly safer,
faster and more correct) - although that doesn't really help me much with
understanding what the intention of this code is, looks rather weird...

Stefan


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Re: How to create an instance of a python class from C++

2014-03-11 Thread Barry Scott 
On 5 Mar 2014, at 00:14, Bill galaxyblu...@gmail.com wrote:

 Hello:
 
 I can't figure out how to create an instance
 of a python class from 'C++':
 

Why not use pycxx from http://sourceforge.net/projects/cxx/?

This lib does all the heavy lifting for you for both python2 and python3.
Has docs and examples.

Barry
PyCXX maintainer.



 ( I am relatively new to Python so excuse some of
  the following. )
 
 In a .py file I create an ABC and then specialize it:
 
from MyMod import *
from abc import ABCMeta, abstractmethod
 
# Declare an abstract base class.
class Base(metaclass=ABCMeta):
Base class.
@abstractmethod
def description(self):
return From the base class.
 
# Declare a class that inerits from the base.
class Derived(Base):
Derived class.
def description(self):
return From the Derived.
 
# Register the derived class.
RegisterClass(Derived)
 
 Then from 'C++' (my implementation of RegisterClass)
 I try to create an instance
 
static PyObject *
RegisterClass( PyObject *, PyObject *args ) {   // This gets called ok.
 
PyObject *class_decl;
if( ! PyArg_ParseTuple(args, O, class_decl) )
return NULL;
Py_INCREF(class_decl);
 
PyTypeObject *typ = class_decl-ob_type;
 
// Tried this.
// PyObject *an = _PyObject_New(class_decl-ob_type); assert(an);
// PyObject *desc = PyObject_CallMethod(an,description,NULL); 
 assert(desc);
 
// Tried this.
// PyObject *an = PyType_GenericNew((PyTypeObject 
 *)class_decl-ob_type, NULL, NULL); assert(an);
// assert(class_decl); assert(class_decl-ob_type); 
 assert(class_decl-ob_type-tp_new);
 
// This returns something.
assert(class_decl); assert(class_decl-ob_type); 
 assert(class_decl-ob_type-tp_new);
PyObject *an_inst = 
 class_decl-ob_type-tp_new(class_decl-ob_type,NULL, NULL); assert(an_inst);
assert(class_decl-ob_type-tp_init);
 
// This crashes.
int ret = class_decl-ob_type-tp_init(an_inst,NULL, NULL); assert(ret 
 == 0);
// PyObject_CallMethod(an_inst,__init__,NULL);
// PyObject *an_inst = PyObject_CallMethod(class_decl,__new__,NULL); 
 assert(an_inst);
 
// Never get here.
PyObject *desc = PyObject_CallMethod(an_inst,description,NULL); 
 assert(desc);
char *cp = _PyUnicode_AsString(desc);
cerr  Description:  cp  endl;
 
return PyLong_FromLong(0);
}
 
static PyMethodDef MyModMethods[] = {
{ RegisterClass, RegisterClass, METH_VARARGS, Register class. },
{  NULL,   NULL,  0, NULL }
};
 
static struct PyModuleDef MyModMod = {
   PyModuleDef_HEAD_INIT,
   MyMod,// name of module
   NULL,// module documentation, may be NULL
   -1,
   MyModMethods,
   NULL,
   NULL,
   NULL,
   NULL
};
 
PyMODINIT_FUNC
PyInit_MyMod( void ) {
PyObject* m = PyModule_Create(MyModMod);
if( m == NULL )
return NULL;
return m;
}
 
int
main( int, char ** ) {
 
PyImport_AppendInittab( MyMod, PyInit_MyMod );
 
Py_Initialize();
 
const char *file_name = z.py;
FILE *fp = fopen(file_name,r);
if( fp ) {
PyRun_SimpleFileExFlags(fp,file_name,1,0);
}
 
Py_Finalize();
 
return 0;
}
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Re: How to create an instance of a python class from C++

2014-03-05 Thread Bill 
 
 So far, so good.  The object that was passed in was the Derived
 class object.  Since you presumably only want class objects to be
 passed in, you might want to check that here using PyType_Check.

Yes. Will do.
 
  PyTypeObject *typ = class_decl-ob_type;
 
 In Python, you instantiate a class by calling it.  You should do the 
 same in C, using PyObject_CallFunction.  But as above, note that you
 want to call class_decl, not class_decl-ob_type.
 

Of course. That works.

Thanks.

Bill
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Re: How to create an instance of a python class from C++

2014-03-05 Thread Grant Edwards 
On 2014-03-05, Ian Kelly ian.g.ke...@gmail.com wrote:
 On Tue, Mar 4, 2014 at 5:14 PM, Bill galaxyblu...@gmail.com wrote:
 Hello:

 I can't figure out how to create an instance
 of a python class from 'C++':

 ( I am relatively new to Python so excuse some of the following. )

 In a .py file I create an ABC and then specialize it:

 Why are you creating an ABC?

Because it was the first binary computer that did calculations with
electronic switching elements (gates), and it would be really cool to
have one! The ABC also pioneered the use of capciators as regenerative
storage elements (it's how DRAM still works today).

http://en.wikipedia.org/wiki/Atanasoff%E2%80%93Berry_Computer

It predated ENIAC, and it's clear that some of the features of ENIAC
were inspired by the ABC after John Mauchly visited Iowa State and saw
the ABC.

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Re: How to create an instance of a python class from C++

2014-03-05 Thread Alister 
On Wed, 05 Mar 2014 16:08:00 +, Grant Edwards wrote:

 On 2014-03-05, Ian Kelly ian.g.ke...@gmail.com wrote:
 On Tue, Mar 4, 2014 at 5:14 PM, Bill galaxyblu...@gmail.com wrote:
 Hello:

 I can't figure out how to create an instance of a python class from
 'C++':

 ( I am relatively new to Python so excuse some of the following. )

 In a .py file I create an ABC and then specialize it:

 Why are you creating an ABC?
 
 Because it was the first binary computer that did calculations with
 electronic switching elements (gates), and it would be really cool to
 have one! The ABC also pioneered the use of capciators as regenerative
 storage elements (it's how DRAM still works today).
 
 http://en.wikipedia.org/wiki/Atanasoff%E2%80%93Berry_Computer
 
 It predated ENIAC, and it's clear that some of the features of ENIAC
 were inspired by the ABC after John Mauchly visited Iowa State and saw
 the ABC.

But it was not programmable

the first programmable electronic computer was 'Colossus'
which was developed during WWII but remained classified by the UK govt 
for many years afterwards 

http://en.wikipedia.org/wiki/Colossus_computer





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Re: How to create an instance of a python class from C++

2014-03-05 Thread Grant Edwards 
On 2014-03-05, Alister alister.w...@ntlworld.com wrote:

 Why are you creating an ABC?
 
 Because it was the first binary computer that did calculations with
 electronic switching elements (gates), and it would be really cool to
 have one! The ABC also pioneered the use of capciators as regenerative
 storage elements (it's how DRAM still works today).
 
 http://en.wikipedia.org/wiki/Atanasoff%E2%80%93Berry_Computer
 
 It predated ENIAC, and it's clear that some of the features of ENIAC
 were inspired by the ABC after John Mauchly visited Iowa State and saw
 the ABC.

 But it was not programmable

True.  It had only one program that was hard-wired into it when it was
built as opposed to the external patch-cords and switches that were
used on machines like Colossus and ENIAC to alter the wiring.

 the first programmable electronic computer was 'Colossus' which was
 developed during WWII but remained classified by the UK govt for many
 years afterwards 

 http://en.wikipedia.org/wiki/Colossus_computer

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  at   and a cross-eyed guy were
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Re: How to create an instance of a python class from C++

2014-03-05 Thread Gene Heskett 
On Wednesday 05 March 2014 17:09:53 Grant Edwards did opine:

 On 2014-03-05, Alister alister.w...@ntlworld.com wrote:
  Why are you creating an ABC?
  
  Because it was the first binary computer that did calculations with
  electronic switching elements (gates), and it would be really cool to
  have one! The ABC also pioneered the use of capciators as
  regenerative storage elements (it's how DRAM still works today).
  
  http://en.wikipedia.org/wiki/Atanasoff%E2%80%93Berry_Computer
  
  It predated ENIAC, and it's clear that some of the features of ENIAC
  were inspired by the ABC after John Mauchly visited Iowa State and
  saw the ABC.
  
  But it was not programmable
 
 True.  It had only one program that was hard-wired into it when it was
 built as opposed to the external patch-cords and switches that were
 used on machines like Colossus and ENIAC to alter the wiring.
 
  the first programmable electronic computer was 'Colossus' which was
  developed during WWII but remained classified by the UK govt for many
  years afterwards
  
  http://en.wikipedia.org/wiki/Colossus_computer

What machine was it that had about 12,000 12AU7 vacuum tubes in it for 
logic?  They had one of those, adapted to read the output of a bed of 
photocells installed in a Harris sheet fed press on the SUI campus in the 
later 1950's.  I saw it running once, grading the test score sheets from 
the Iowa Tests that were being used in lieu of the high price per seat S-B 
IQ test in the Iowa schools.  It was IIRC a somewhat difficult test when 
they threw it at me in the 7th grade a decade earlier, they claimed the 
test score were interchangeable with the S-B scores, but I somehow managed 
a 147 on it at the time.

Cheers, Gene
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 soap, ballot, jury, and ammo. Please use in that order.
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Genes Web page http://geneslinuxbox.net:6309/gene

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How to create an instance of a python class from C++

2014-03-04 Thread Bill 
Hello:

I can't figure out how to create an instance
of a python class from 'C++':

( I am relatively new to Python so excuse some of
  the following. )

In a .py file I create an ABC and then specialize it:

from MyMod import *
from abc import ABCMeta, abstractmethod

# Declare an abstract base class.
class Base(metaclass=ABCMeta):
Base class.
@abstractmethod
def description(self):
return From the base class.

# Declare a class that inerits from the base.
class Derived(Base):
Derived class.
def description(self):
return From the Derived.

# Register the derived class.
RegisterClass(Derived)

Then from 'C++' (my implementation of RegisterClass)
I try to create an instance

static PyObject *
RegisterClass( PyObject *, PyObject *args ) {   // This gets called ok.

PyObject *class_decl;
if( ! PyArg_ParseTuple(args, O, class_decl) )
return NULL;
Py_INCREF(class_decl);

PyTypeObject *typ = class_decl-ob_type;

// Tried this.
// PyObject *an = _PyObject_New(class_decl-ob_type); assert(an);
// PyObject *desc = PyObject_CallMethod(an,description,NULL); 
assert(desc);

// Tried this.
// PyObject *an = PyType_GenericNew((PyTypeObject 
*)class_decl-ob_type, NULL, NULL); assert(an);
// assert(class_decl); assert(class_decl-ob_type); 
assert(class_decl-ob_type-tp_new);

// This returns something.
assert(class_decl); assert(class_decl-ob_type); 
assert(class_decl-ob_type-tp_new);
PyObject *an_inst = 
class_decl-ob_type-tp_new(class_decl-ob_type,NULL, NULL); assert(an_inst);
assert(class_decl-ob_type-tp_init);

// This crashes.
int ret = class_decl-ob_type-tp_init(an_inst,NULL, NULL); assert(ret 
== 0);
// PyObject_CallMethod(an_inst,__init__,NULL);
// PyObject *an_inst = PyObject_CallMethod(class_decl,__new__,NULL); 
assert(an_inst);

// Never get here.
PyObject *desc = PyObject_CallMethod(an_inst,description,NULL); 
assert(desc);
char *cp = _PyUnicode_AsString(desc);
cerr  Description:  cp  endl;

return PyLong_FromLong(0);
}

static PyMethodDef MyModMethods[] = {
{ RegisterClass, RegisterClass, METH_VARARGS, Register class. },
{  NULL,   NULL,  0, NULL }
};

static struct PyModuleDef MyModMod = {
   PyModuleDef_HEAD_INIT,
   MyMod,// name of module
   NULL,// module documentation, may be NULL
   -1,
   MyModMethods,
   NULL,
   NULL,
   NULL,
   NULL
};

PyMODINIT_FUNC
PyInit_MyMod( void ) {
PyObject* m = PyModule_Create(MyModMod);
if( m == NULL )
return NULL;
return m;
}

int
main( int, char ** ) {

PyImport_AppendInittab( MyMod, PyInit_MyMod );

Py_Initialize();

const char *file_name = z.py;
FILE *fp = fopen(file_name,r);
if( fp ) {
PyRun_SimpleFileExFlags(fp,file_name,1,0);
}

Py_Finalize();

return 0;
}
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Re: How to create an instance of a python class from C++

2014-03-04 Thread Ian Kelly 
On Tue, Mar 4, 2014 at 5:14 PM, Bill galaxyblu...@gmail.com wrote:
 Hello:

 I can't figure out how to create an instance
 of a python class from 'C++':

 ( I am relatively new to Python so excuse some of
   the following. )

 In a .py file I create an ABC and then specialize it:

Why are you creating an ABC?  Most Python classes do not use them.
Maybe you have a reason for it, but it's irrelevant to what you're
currently trying to do.

 Then from 'C++' (my implementation of RegisterClass)
 I try to create an instance

 static PyObject *
 RegisterClass( PyObject *, PyObject *args ) {   // This gets called 
 ok.

 PyObject *class_decl;
 if( ! PyArg_ParseTuple(args, O, class_decl) )
 return NULL;
 Py_INCREF(class_decl);

So far, so good.  The object that was passed in was the Derived
class object.  Since you presumably only want class objects to be
passed in, you might want to check that here using PyType_Check.

 PyTypeObject *typ = class_decl-ob_type;

Okay, now if class_decl is the class object that was passed in, then
class_decl-ob_type is the *type* of that class object -- the
metaclass, which in this case would be ABCMeta.  You probably don't
need this, because you want to instantiate Derived, not ABCMeta.

 // Tried this.
 // PyObject *an = _PyObject_New(class_decl-ob_type); assert(an);
 // PyObject *desc = PyObject_CallMethod(an,description,NULL); 
 assert(desc);

In Python, you instantiate a class by calling it.  You should do the
same in C, using PyObject_CallFunction.  But as above, note that you
want to call class_decl, not class_decl-ob_type.

PyObject_New doesn't do any initialization and is, I believe, meant to
be used when implementing types in C.
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