Question

I am developing Python bindings for a C++ class which extends a std::vector instantiation. To get a Python subscript operator working for this class I added __getitem__ function looking as follows (I cut irrelevant code and error handling):

class Column;

typedef vector<Column*> MetaDataBase;

class MetaData : public MetaDataBase {
public:

#ifdef SWIGPYTHON
Column* __getitem__(int i) { return (*this)[i]; }
#endif
};

The above works all right in Python for accessing single elements, but it does not work for slices.

OK, so I understand I need to change type of the function’s parameter to PyObject * and use PySlice_Check to see if the function is supposed to return a PyList.

This is fine, no problem with that. But because I have to return PyList from the function sometimes, the type of the __getitem__ return value has to be PyObject* as well and I cannot rely on SWIG to convert my C++ type (Column *) to a wrapper class. Also, when creating a slice, I need to ‘manually’ convert Column* to PyObject* before inserting it into the PyList.

How can I do it?

Answer 1

I think there’s a much simpler solution to this for std::vector with SWIG+Python. SWIG’s Python code generation already has support for doing quite a good job wrapping some STL containers.

If you add to the beginning of your module interface:

%include "pyabc.i"
%include "std_vector.i" // Assuming you don't already

and somewhere, if you haven’t done so:

%template(MetaDataBase) std::vector<Column*>;

then this will cause the wrapped std::vector to meet the requirements of Python’s MutableSequence. (I think this should be sufficient to achieve what you’re looking for on the Python side, you might need to call SWIG with -extranative as well).

Also worth noting possibly – for your current __getitem__ you can declare and define it in the SWIG interface file using something like:

%extend MetaData {
   Column* __getitem__(int i) { return (*self)[i]; }
};

Which allows you to do this without “polluting” your “normal” header files with SWIG+Python specific code.