Subclassing C++ in Python with SIP

I use SIP in MapsEvolved to generate bindings for interfacing Python with C++. I really like SIP due to its straight-forward syntax that mostly allows just copying class definitions over from C++. Further, it's really well thought out and contains support for a number of advanced use cases.

One such feature is implementing a C++ interface in Python. The resulting class can then even be passed back to C++, and any methods called on it will be forwarded to the Python implementation. Sweet!

Here is an example I originally wrote for this Stack Overflow question. It illustrates how ridiculously easy it is to get this working:

visitor.h:

class EXPORT Node {
public:
    int getN() const;
    ...
};
struct EXPORT NodeVisitor {
    virtual void OnNode(Node *n) = 0;
};
void visit_graph_nodes(NodeVisitor *nv);

visitor.sip:

%Module pyvisit

%ModuleHeaderCode
#include "visitor.h"
%End

class Node {
public:
    int getN() const;
    ...
};

struct NodeVisitor {
    virtual void OnNode(Node* n) = 0;
};

void visit_graph_nodes(NodeVisitor *nv);

Using it from Python:

>>> import pyvisit
>>> class PyNodeVisitor(pyvisit.NodeVisitor):
>>>     def OnNode(self, node):
>>>         print(node.getN())
>>> pnv = PyNodeVisitor()
>>> visit_graph_nodes(pnv)
1
2
3
...

Here, the C++ function visit_graph_nodes() calls the Python method pnv.OnNode() for every node in its (internal) graph. A zip file with the full working source code of this example can be downloaded here.

The subclassing capabilities of SIP don't stop at interfaces, either. It's possible to derive from any C++ class, abstract or not, inheriting (or overriding) existing method implementations as needed. This gives a lot of flexibility and makes it easy to have classes with some parts implemented in C++, and others being in Python.