Implementation

If you have a cool new idea for a better method, and you want to optimise it, you can easily plug it in the current package. This section explains how the package is setup internally, and how you can extend it. Most of this concerns C++, and Python only comes in when exposing the classes from C++.

The core of the Leiden algorithm is implemented in C++ in the package libleidenalg, available from https://github.com/vtraag/libleidenalg. This C++ library is used as the core for the Python package, which is just an interface to the underlying C++ library. New methods need to be added to the C++ library first, and then exposed to the Python interface. For more information, please see the contributing guide of the libleidenalg package at https://github.com/vtraag/libleidenalg/blob/main/CONTRIBUTING.md

Python

Exposing the method to python takes a bit more effort. There are various places in which you need to change/add things. In the following, we assume you created a new class called CoolVertexPartition in libleidenalg. Please then follow the following steps:

1. Your own new VertexPartition class should add some specific methods. In particular, you need to ensure you create a method

   CoolVertexPartition* CoolVertexPartition::create(Graph* graph)
   {
     return new CoolVertexPartition(graph);
   }
   

   and

   CoolVertexPartition* CoolVertexPartition::create(Graph* graph, vector<size_t> const& membership)
   {
     return new CoolVertexPartition(graph, membership);
   }
   

   These methods ensure that based on a current partition, we can create a new partition (without knowing its type).

2. In python_partition_interface.cpp some methods need to be added. In particular

   PyObject* _new_CoolVertexPartition(PyObject *self, PyObject *args, PyObject *keywds)
   

   You should be able to simply copy an existing method, and adapt it to your own needs.

3. These methods need to be exposed in pynterface.h. In particular, you need to add the method you created in step (2) to leiden_funcs[]. Again, you should be able to simply copy an existing line.

4. You can then finally create the Python class in VertexPartition.py. The base class derives from the VertexClustering from igraph, so that it is compatible with all operations in igraph. You should add the method as follows:

   class CoolVertexPartition(MutableVertexPartition):
   
     def __init__(self, ... ):
       ...
   

   Again, you should be able to copy the outline for another class and adapt it to your own needs. Don’t forget to change to docstring to update the documentation so that everybody knows how your new cool method works.

5. Expose your newly created python class directly in __init__.py by importing it:

   from .VertexPartition import CoolVertexPartition
   

That’s it! You’re done and should now be able to find communities using your new CoolVertexPartition:

>>> la.find_partition(G, la.CoolVertexPartition);