How to get python slicing to work with my c++ array class using SWIG

Question

I have an an array class, Array1D, defined in c++ which essentially wraps the STL vector class. I extended this class so that I can display individual elements of the array vector. Here is the relevant code in my SWIG interface file:

namespace std{
    %template(dblVector) vector<double>;
}

%extend Array1D{
    double __getitem__(int index) {
        return (*self)[index];
    }
}

This allows me to access individual elements of the array in python:

>>> a = Array1D(10) # creates a c++ vector of length 10 with zeros
>>> a[0]
>>> 0

I want to be able to call a[1:3] for example, however, I get a TypeError when I try this:

TypeError: in method 'Array1D___getitem__', argument 2 of type 'int'

Answer 1

The problem is that python passes a Slice object when calling the slice variant of the getitem and your function definition is expecting an int. You will need to write a version of getitem that takes PyObject* as a parameter and then you'd have to implement the slicing of the vector there.

I am writing this without being setup to actually test it, so take it with a grain of salt. But I would do something like the following.

%extend Array1D
{
    Array1D* __getitem__(PyObject *param) 
    {
        if (PySlice_Check(param))
        {
            /* Py_ssize_t might be needed here instead of ints */
            int len = 0, start = 0, stop = 0, step = 0, slicelength = 0;

            len = this->size(); /* Or however you get the size of a vector */

            PySlice_GetIndicesEx((PySliceObject*)param, len, &start, &stop, &step, &slicelength);

            /* Here do stuff in order to return an Array1D that is the proper slice
               given the start/stop/step defined above */
        }

        /* Unexpected parameter, probably should throw an exception here */
    }
}