Re: [C++-sig] Trying to implement subscriptable type in c++.

Dec. 23, 2009


      In Python C API you cannot define a __getitem__ method; instead define a
tp_as_mapping slot.  See:

http://docs.python.org/c-api/typeobj.html#tp_as_mapping


2009/12/23 Anita A <anitaa1981@gmail.com>
...
Hi all,
I am trying to implement a subscriptable type in c++ (equivalent to python
dict), but I get a 'type is unsubscriptable' error. Here is a minimal
code sample I tried:
--------------------------------------------------------------------------
#include <Python.h>
#include <iostream>
#include "pyerror.hpp"
/////////////////////////////////////////////////////////
PyObject* getitem(PyObject* self, PyObject* args)
{
   std::cout << "getitem" << std::endl;
   return PyString_FromString("this is an item");
}
PyMethodDef method[] = {
    {"__getitem__", &getitem, METH_VARARGS, ""},
    {0, 0, 0, 0}
};
static PyMethodDef module_methods[] = {
    { 0, 0, 0, 0 }
};
struct MyObject
{
   PyObject_HEAD
};
extern const char* code;
int main(int argc, char *argv[])
{
   Py_InitializeEx(0);
PyObject* myModule = Py_InitModule3("test", module_methods, "test
module");
   assert(myModule);
PyTypeObject myType;
   memset(&myType, 0, sizeof(PyTypeObject));
myType.tp_name = "myType";
   myType.tp_basicsize = sizeof(MyObject);
   myType.tp_methods = method;
   if (PyType_Ready(&myType) < 0)
   {
       std::cout << "type error" << std::endl;
       return 1;
   }
PyModule_AddObject(myModule, myType.tp_name, (PyObject*)&myType);
PyObject* m = PyImport_AddModule("__main__");
   PyObject* d = PyModule_GetDict(m);
   MyObject* odef = PyObject_New(MyObject, &myType);
   PyDict_SetItemString(d, "obj", (PyObject*)odef);
PyObject* rv = PyRun_String(code, Py_file_input, d, d);
   if(rv == 0)
   {
       std::cout << "run error" << std::endl;
       printException();
       return 1;
   }
Py_Finalize();
   std::cout << "Done.\n";
return 0;
}
const char* code =
"print dir(obj)\n"
"x = obj.__getitem__(1)\n"
"print 'direct call:', x\n"
"x = obj[1]\n"
;
--------------------------------------------------------------------------
Compile and run session.
--------------------------------------------------------------------------
$ g++ -I/usr/include/python2.6 pymin.cpp -lpython2.6
$ ./a.out
['__class__', '__delattr__', '__doc__', '__format__',
'__getattribute__', '__getitem__', '__hash__', '__init__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__',
'__sizeof__', '__str__', '__subclasshook__']
getitem
direct call: this is an item
run error
Error in execution:
Traceback (most recent call last):
line 4, in ??
<type 'exceptions.TypeError'>: 'myType' object is unsubscriptable
--------------------------------------------------------------------------
As you can see, when I call __getitem__() directly, it works, but when I
call it as a subscriptable object, I get an error. What am I missing?
A simple python equivalent class works as expected:
--------------------------------------------------------------------------
class Testi:
   def __getitem__(self, idx):
       return 10
if __name__ == '__main__':
   x = Testi()
   print dir(x)
   print x['a']
--------------------------------------------------------------------------
Run session.
--------------------------------------------------------------------------
$ python tst.py
['__doc__', '__getitem__', '__module__']
10
--------------------------------------------------------------------------
I had tried using the PyMappingMethods structure as documented, but there
is apparently no way to implement the __contains__() method, that is, to
check if a key is present in the dict.
Any help would be appreciated.
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-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert

Re: [C++-sig] Trying to implement subscriptable type in c++.

Gustavo Carneiro