The WhatsHap source code is on GitHub. WhatsHap is developed in Python 3, Cython and C++.

Development installation

For development, make sure that you install Cython and tox. We also recommend using a virtualenv. This sequence of commands should work (use as URL if you do not have a GitHub account):

git clone
cd whatshap
python3 -m venv .venv
source .venv/bin/activate
pip install -e .[dev]

The last command installs also all the development dependencies. Omit the [dev] to leave them out.

Install also pre-commit and run pre-commit install.

Development installation when using Conda

If you are familiar with Conda, you can also use a Conda environment for developing WhatsHap. We recommend that you use Conda only to install Python itself and let the rest of the dependencies be handled by pip:

conda create -n whatshap-dev python=3.10
conda activate whatshap-dev
pip install -e .[dev]

Running tests

While in the virtual (or Conda) environment, you can run the tests for the current Python version like this:


Whenever you change any Cython code (.pyx files), you need to re-run the pip install -e . step in order to compile it.

Optionally, to run tests for all supported Python versions, you can run tox. It creates separate virtual environments for each Python version, installs WhatsHap into it and then runs the tests. It also tests documentation generation with sphinx. Run it like this:


If tox is installed on the system, you do not need to be inside a virtual environment for this. Run tox --skip-missing-interpreters if you do not have all tested Python versions installed. See one way below for how to install them on Ubuntu.

Code style

Python code needs to be formatted with Black. Either run black whatshap tests manually before you commit or use the pre-commit framework to automate this.

To check other style issues, run

tox -e flake8

Installing other Python versions in Ubuntu

Ubuntu comes with one default Python 3 version, and in order to test WhatsHap with older or newer Python versions, follow the instructions for enabling the “deadsnakes” repository. After you have done so, ensure you have the following packages:

sudo apt install build-essential python-software-properties

Then get and install the desired Python versions. Make sure you install the -dev package. For example, for Python 3.10:

sudo apt update
sudo apt install python3.10-dev


Here is one way to get a backtrace from gdb (assuming the bug occurs while running the tests):

$ gdb python3
(gdb) run -m pytest

After you get a SIGSEGV, let gdb print a backtrace:

(gdb) bt

Another way is to set PYTHONFAULTHANDLER=1:

PYTHONFAULTHANDLER=1 pytest -vxs tests/

Wrapping C++ classes

The WhatsHap phasing algorithm is written in C++, as are many of the core data structures such as the “Read” class. To make the C++ classes usable from Python, we use Cython to wrap the classes. All these definitions are spread across multiple files. To add new attributes or methods to an existing class or to add a new class, changes need to be made in different places.

Let us look at the “Read” class. The following places in the code may need to be changed if the Read class is changed or extended:

  • src/read.cpp: Implementation of the class (C++).

  • src/read.h: Header with the class declaration (also normal C++).

  • whatshap/cpp.pxd: Cython declarations of the class. This repeats – using the Cython syntax this time – a subset of the information from the src/read.h file. This duplication is required because Cython cannot read .h files (it would need a full C++ parser for that).

    Note that the cpp.pxd file contains definitions for all the .h headers. (It would be cleaner to have them in separate .pxd files, but this leads to problems when linking the compiled files.)

  • whatshap/core.pxd: This contains declarations of all Cython classes wrapping C++ classes. Note that the class Read in this file has the same name as the C++ class, but that it is not the same as the C++ one! The distinction is made by prefixing the C++ class with cpp., which is the name of the module in which it is declared in (that is, the C++ class Read is declared in cpp.pxd). The wrapping (Cython) class Read stores the C++ class in an attribute named thisptr. If you add a new class, it needs to be added to this file. If you only modify an existing one, you probably do not need to change this file.

  • whatshap/core.pyx: The Cython implementation of the wrapper classes. Again, the name Read by itself is the Python wrapper class and cpp.Read is the name for the C++ class.

Before adding yet more C++ code, which then requires extra code for wrapping it, consider writing an implementation in Cython instead. See readselect.pyx, for example, which started out as a Python module and was then transferred to Cython to make it faster. Here, the Cython code is not merely a wrapper, but contains the implementation itself.

Writing documentation

Documentation is located in the doc/ subdirectory. It is written in reStructuredText format and is translated by Sphinx into HTML format.

Documentation is hosted on Read the Docs. It is built automatically whenever a commit is made. The documentation in the master branch should be visible at and documentation for the most recent released version should be visible at

To generate documentation locally, ensure that you installed sphinx and the add-ons necessary to build documentation (running pip install -e .[docs] will take care of this). Then go into the doc/ directory and run make. You can then open doc/_build/html/index.html in your browser. The theme that is used is a bit different from the one used on Read the Docs.

Making a release

  1. Update CHANGES.rst: Set the correct version number and ensure that all nontrivial, user-visible changes are listed.

  2. Ensure you have no uncommitted changes in the working copy.

  3. Run tox.

  4. Tag the current commit with the version number (there must be a v prefix):

    git tag -a -m "Version 0.1" v0.1

    To release a development version, use an rc version number such as v0.17rc1. Users will only get these when they use pip install --pre.

  5. Push the tag:

    git push --tags
  6. Wait for the GitHub Action to finish. It will deploy the sdist and wheels to PyPI if everything worked correctly.

  7. To update the Bioconda recipe, wait for the Bioconda bot to open a PR (in the bioconda-recipes repository). Ensure that the list of dependencies (the requirements: section in the recipe) is in sync with the file. If changes are necessary to the bot-generated PR, just add your own commits to that PR.

If something went wrong, fix the problem and follow the above instructions again, but with an incremented revision in the version number. That is, go from version x.y to x.y.1. PyPI will not allow you to change a version that has already been uploaded.

Adding a new subcommand

Use one of the modules in whatshap/cli/ as a template. All modules in that directory are automatically used as subcommands.

Download count statistics

Some statistics for the PyPI package are available at

Here is a query for Google BigQuery that shows download counts (from PyPI) since a given date, broken down by version

    COUNT(*) as total_downloads,
    file.project = 'whatshap'
    file.project, file.version

Statistics for the Conda package are available on the WhatsHap package detail page.