python

For High Energy Physics, the go-to framework for big data analysis has been CERN’s ROOT framework. ROOT is a massive C++ library that even predates the STL in some areas. It is¹ also a JIT C++ interpreter called Cling, probably the best in the business. If you have heard of the Xeus C++ Kernel for Jupyter, that is built on top of Cling. ROOT has everything a HEP physicist could want: math, plotting, histograms, tuple and tree structures, a very powerful file format for IO, machine learning, Python bindings, and more. It also does things like dictionary generation and arbitrary class serialization (other large frameworks like Qt have similar generation tools).

You may already be guessing one of the most common problems for ROOT. It is huge and difficult to install – if you build from source, that’s a several hour task on a single core. It has gotten much better in the last 6 years, and there are several places you can find ROOT, but there are still areas where it is challenging. This is especially true for Python; ROOT is linked to your distro’s Python (both python2 and python3 if your distro supports it, as of ROOT 6.22); but the common rule for using Python is “don’t touch your system Python” - so modern Python users should be in a virtual environment, and for that ROOT requires the system site-packages option be enabled, which is not always ideal. And, if you use the Anaconda Python distribution, which is the most popular scientific distribution of Python and massively successful for ML frameworks, the general rule even for people who build ROOT themselves has been: don’t. But now, you can get a fully featured ROOT binary package for macOS or Linux, Python 2.7, 3.6, 3.7, or 3.8 from Conda-Forge, the most popular Anaconda community channel! Many more HEP recipes have now been added to Conda-Forge, as well! ROOT now also provides a conda docker image, too!

For particle physicists, ROOT is one of the most important toolkits around. It is a huge suite of tools that predates the C++ standard library, and has almost anything a particle physicist could want. It has driven developments in other areas too. ROOT’s current C++ interpreter, CLING, is the most powerful C++ interpreter available and is used by the Xeus project for Jupyter. The Python work has helped PyPy, with CPPYY also coming from ROOT. However, due to the size, complexity, and age of some parts of ROOT, it can be a bit challenging to install; and it is even more challenging when you want it to talk to Python. I would like to point to the brand-new Conda-Forge ROOT package for Linux and macOS, and point out a few other options for macOS installs. Note for Windows users: Due to the fact that ROOT expects the type long to match the system pointer size, 64-bit Windows cannot be supported for quite some time. While you can use it in 32 bit form, this is generally impossible to connect to Python, which usually will be a 64-bit build.

These are some illustrations and videos that I have made over the years.

Let’s compare several ways of making Histograms. I’m going to assume you would like to end up with a nice OO histogram interface, so all the 2D methods will fill a Physt histogram. We will be using a 2 x 1,000,000 element array and filling a 2D histogram, or 10,000,000 elemends in a 1D histogram. Binnings are regular.

1D 10,000,000 item histogram

2D 1,000,000 item histogram

Let’s try a non-trivial example of a binding: Minuit2 (6.14.0 standalone edition).

This was originally given as a PyHEP 2018 talk, It is designed to be interactive, and can be run in SWAN if you have a CERN account. If you want to run it manually, just download the repository: github.com/henryiii/pybindings_cc. It is easy to run in Anaconda.