3. Case 1 - DBo Derived, Standalone

As example, we take Library. This a DBo derived class, but we choose not to export the parent classes. From Python, it will appear as a base class.

3.1 Class Associated Header File

Here is the typical content of a header file (for PyLibrary):

#ifndef  PY_LIBRARY_H
#define  PY_LIBRARY_H

#include "hurricane/isobar/PyHurricane.h"
#include "hurricane/Library.h"

namespace  Isobar {
  using namespace Hurricane;

  extern "C" {

    typedef struct {
        PyObject_HEAD
        Library* _object;
    } PyLibrary;

    extern  PyTypeObject  PyTypeLibrary;
    extern  PyMethodDef   PyLibrary_Methods[];
    extern  PyObject*     PyLibrary_Link       ( Hurricane::Library* lib );
    extern  void          PyLibrary_LinkPyType ();


#define IsPyLibrary(v) ( (v)->ob_type == &PyTypeLibrary )
#define PYLIBRARY(v)   ( (PyLibrary*)(v) )
#define PYLIBRARY_O(v) ( PYLIBRARY(v)->_object )

  }  // extern "C".
}  // Isobar namespace.

#endif  // PY_LIBRARY_H

The code is organized as follow:

  1. It must have, as the first include PyHurricane.h, which provides the complete bunch of macros needed to build the module. Then the include of the C++ class we want to wrap (Library.h).

  2. As Python is written in C, all the wrapper code has to be but inside an extern "C" namespace.

  3. Definition of the wrapped struct, PyLibrary. It is standard Python here.

    Note

    For our set of macros to work, the name of the pointer to the C++ class must always be _object, and the various functions and macros defined here must take the name of the class (either in lowercase, camel case or capitals).

  4. Declaration of the Python type PyTypeLibrary (standard).

  5. Declaration of the Python type table of methods PyLibrary_Methods (standard).

  1. Declaration of PyLibrary_Link(), helper to convert a C++ Lybrary into a PyLibrary (put in the support shared library).
  2. Declaration of PyLibrary_LinkPyType(), this function setup the class-level function of the new Python type (here, PyTypeLibrary).
  3. And, lastly, three macros to:
    • IsPylibrary(), know if a Python object is a PyLibrary
    • PYLIBRARY(), force cast (C style) of a PyObject into a PyLibrary.
    • PYLIBRARY_O(), extract the C++ object (Library*) from the Python object (PyLibrary).

3.2 Class Associated File

3.2.1 Head of the file

#include "hurricane/isobar/PyLibrary.h"
#include "hurricane/isobar/PyDataBase.h"
#include "hurricane/isobar/PyCell.h"

namespace Isobar {
  using namespace Hurricane;

  extern "C" {

#define  METHOD_HEAD(function)   GENERIC_METHOD_HEAD(Library,lib,function)

As for the header, all the code must be put inside a extern "C" namespace.

A convenience macro METHOD_HEAD() must be defined, by refining GENERIC_METHOD_HEAD(). This macro will be used in the method wrappers below to cast the _object field of the Python object into the appropriate C++ class, this is done using a C-style cast. The parameters of that macro are:

  1. The C++ encapsulated class (Library).
  2. The name of the variable that will be used to store a pointer to the C++ working object.
  3. The name of the C++ method which is to be wrapped.

3.2.2 The Python Module Part

First, we have to build all the wrappers to the C++ methods of the class. For common predicates, accessors, and mutators macros are supplied.

Wrapping of the Library::getCell() method:

static PyObject* PyLibrary_getCell ( PyLibrary* self, PyObject* args )
{
  Cell* cell = NULL;

  HTRY
    METHOD_HEAD( "Library.getCell()" )
    char* name = NULL;
    if (PyArg_ParseTuple(args,"s:Library.getCell", &name)) {
      cell = lib->getCell( Name(name) );
    } else {
      PyErr_SetString( ConstructorError
                     , "invalid number of parameters for Library::getCell." );
      return NULL;
    }
  HCATCH

  return PyCell_Link(cell);
}

Key points about this method wrapper:

  1. The HTRY / HCATCH macros provides an insulation from the C++ exceptions. If one is emitted, it will be catched and transformed in a Python one. This way, the Python program will be cleanly interrupted and the usual stack trace displayed.
  2. The returned value of this method is of type Cell*, we have to transform it into a Python one. This is done with PyCell_Link(). This macro is supplied by the PyCell.h header and this is why it must be included.

Wrapping of the Library::create() method:

static PyObject* PyLibrary_create( PyObject*, PyObject* args )
{
  PyObject* arg0;
  PyObject* arg1;
  Library* library = NULL;

  HTRY
    __cs.init( "Library.create" );                          // Step (1).
    if (not PyArg_ParseTuple( args, "O&O&:Library.create"
                            , Converter, &arg0
                            , Converter, &arg1 )) {         // Step (2).
      PyErr_SetString( ConstructorError
                     , "invalid number of parameters for Library constructor." );
      return NULL;
    }
    if (__cs.getObjectIds() == ":db:string") {              // Step (3.a)
      DataBase* db = PYDATABASE_O(arg0);
      library = Library::create( db, Name(PyString_AsString(arg1)) );
    } else if (__cs.getObjectIds() == ":library:string") {  // Step (3.b)
      Library* masterLibrary = PYLIBRARY_O(arg0);
      library = Library::create( masterLibrary, Name(PyString_AsString(arg1)) );
    } else {
      PyErr_SetString( ConstructorError
                     , "invalid number of parameters for Library constructor." );
      return NULL;
    }
  HCATCH

  return PyLibrary_Link( library );
}

Key point about this constructor:

  1. We want the Python interface to mimic as closely as possible the C++ API. As such, Python object will be created using a static .create() method. So we do not use the usual Python allocation mechanism.
  2. As it is a static method, there is no first argument.
  3. Python do not allow function overload like C++. To emulate that behavior we use the __cs object (which is a global variable).
    1. Init/reset the __cs object: see step (1).
    2. Call PyArg_ParseTuple(), read every mandatory or optional argument as a Python object ("O&") and use Converter on each one. Converter will determine the real type of the Python object given as argument by looking at the encapsulated C++ class. It then update the __cs object. Done in step (2)
    3. After the call to PyArg_ParseTuple(), the function __cs.getObjectIds() will return the signature of the various arguments. In our case, the valid signatures will be ":db:string" (step (3.a)*a) and ``”:library:string”`` (*step (3.b)).
    4. Call the C++ method after extracting the C++ objects from the Python arguments. Note the use of the PYLIBRARY_O() and PYDATABSE_O() macros to perform the conversion.
  4. Return the result, encapsulated through a call to PyLibrary_Link().

Wrapping of the Library::destroy() method:

DBoDestroyAttribute(PyLibrary_destroy, PyLibrary)

For C++ classes that are derived from DBo, the destroy method wrapper must be defined using the macro DBoDestroyAttribute(). This macro implements the bi-directional communication mechanism using Hurricane::Property. It must not be used for non DBo derived classes.

Defining the method table of the PyLibrary type:

PyMethodDef PyLibrary_Methods[] =
  { { "create"    , (PyCFunction)PyLibrary_create , METH_VARARGS|METH_STATIC
                  , "Creates a new library." }
  , { "getCell"   , (PyCFunction)PyLibrary_getCell, METH_VARARGS
                  , "Get the cell of name <name>" }
  , { "destroy"   , (PyCFunction)PyLibrary_destroy, METH_NOARGS
                  , "Destroy associated hurricane object The python object remains." }
  , {NULL, NULL, 0, NULL}           /* sentinel */
  };

This is standard Python/C API. The name of the PyMethodDef table must be named from the class: PyLibrary_Methods.

3.2.3 Python Type Linking

Defining the PyTypeLibrary class methods and the type linking function.

Those are the functions for the Python object itself to work, not the wrapped method from the C++ class.

Note

At this point we do not define the PyTypeLibrary itself. Only it’s functions and a function to set them up once the type will be defined.

DBoDeleteMethod(Library)
PyTypeObjectLinkPyType(Library)

The macro DBoDeleteMethod() define the function to delete a PyLibrary Python object. Again, do not mistake it for the deletion of the C++ class (implemented by DBoDestroyAttribute()). Here again, DBoDeleteMethod() is specially tailored for DBo derived classes.

To define PyLibrary_LinkPyType(), use the PyTypeObjectLinkPyType() macro. This macro is specific for DBo derived classes that are seen as base classes under Python (i.e. we don’t bother exposing the base class under Python). PyLibrary_LinkPyType() setup the class functions in the PyTypeLibrary type object, it must be called in the Python module this class is part of (in this case: PyHurricane.cpp). This particular flavor of the macro will define and setup the following class functions:

  • PyTypeLibrary.tp_compare (defined by the macro).
  • PyTypeLibrary.tp_repr (defined by the macro).
  • PyTypeLibrary.tp_str (defined by the macro).
  • PyTypeLibrary.tp_hash (defined by the macro).
  • PyTypeLibrary.tp_methods sets to the previously defined PyLibrary_Methods table.
  • PyTypeLibrary.tp_dealloc is set to a function that must be named PyLibrary_DeAlloc, this is what DBoDeleteMethod does. It is not done by PyTypeObjectLinkPyType.

Defining the PyTypeLibrary type:

3.2.4 The Shared Library Part

This part will be put in a separate supporting shared library, allowing other Python module to link against it (and make use of its symbols).

DBoLinkCreateMethod(Library)
PyTypeObjectDefinitions(Library)

To define PyTypeLibrary, use the PyTypeObjectDefinitions() macro. This macro is specific for classes that, as exposed by Python, are neither derived classes nor base classes for others. That is, they are standalone from the inheritance point of view.

The DBoLinkCreateMethod() macro will define the PyLibrary_Link() function which is responsible for encapsulating a C++ Library object into a Python PyLibrary one.

3.3 Python Module (C++ namespace)

We use the Python module to replicate the C++ namespace. Thus, for the Hurricane namespace we create a Python Hurricane module which is defined in the PyHurricane.cpp file, then we add into that module dictionary all the Python types encapsulating the C++ classes of that namespace.

DL_EXPORT(void) initHurricane ()
{
  PyLibrary_LinkPyType();  // step 1.

  PYTYPE_READY( Library )  // step 2.

  __cs.addType( "library", &PyTypeLibrary, "<Library>", false ); // step 3.

  PyObject* module = Py_InitModule( "Hurricane", PyHurricane_Methods );
  if (module == NULL) {
    cerr << "[ERROR]\n"
         << "  Failed to initialize Hurricane module." << endl;
    return;
  }

  Py_INCREF( &PyTypeLibrary );                                        // step 4.
  PyModule_AddObject( module, "Library", (PyObject*)&PyTypeLibrary ); // step 4.
}

The initHurricane() initialisation function shown above has been scrubbed of everything not relevant to the PyLibrary class. The integration of the PyLibrary class into the module needs four steps:

  1. A call to PyLibrary_LinkPyType() to hook the Python type functions in the Python type object.

  2. A call to the PYTYPE_READY() macro (standard Python).

  3. Registering the type into the __cs object, with addType(). The arguments are self explanatory, save for the last which is a boolean to tell if this is a derived class or not.

  4. Adding the type object (PyTypeLibrary) into the dictionnary of the module itself. This allow to mimic closely the C++ syntax:

    import Hurricane
lib = Hurricane.Library.create( db, 'root' )