Python Fastjet Bindings

Python bindings for fastjet. fastjet-contrib is not yet supported, but could be added depending on demand.

NOTE: These are still in development / are experimental!

Installation

Prerequisites:

0. Clone the repository, and initialize the submodule with git submodule init.

1. You need to set the environment variables:

   • FASTJET
   • CGAL_ROOT
   • GMP_ROOT

   They need to be set to the install locations of the three packages. alibuild will set the first two. (Note that alibuild used to call it $FASTJET_ROOT, but now it's known as just $FASTJET). However, GMP_ROOT unfortunately isn't seen as a fastjet dependency, so we have to load it explicitly. For convenience with AliBuild, you can use (assuming zsh):

   $ alienv modulecmd zsh load fastjet/latest GMP/latest

2. Install poetry, and install the package via poetry install in the repository root directory.

3. The tests can be via poetry run pytest -l -vv tests/.

Done. If you need a virtualenv with the package, this can be useful:

alias poetryShell='source "$(dirname $(poetry run which python))/activate"'

Usage

By way of an example from the fastjet quick start:

import pyfastjet as fj

# Create the PseudoJets
# an event with three particles:   px    py  pz      E
particles = []
particles.append(PseudoJet( 99.0,  0.1,  0, 100.0))
particles.append(PseudoJet(  4.0, -0.1,  0,   5.0))
particles.append(PseudoJet(-99.0,    0,  0,  99.0))

# choose a jet definition
R = 0.7
jet_def = fj.JetDefinition(fj.JetAlgorithm.antikt_algorithm, R)

# run the clustering, extract the jets
cs = fj.ClusterSequence(particles, jet_def)
jets = fj.sorted_by_pt(cs.inclusive_jets())

for jet in cs:
    print(f"jet: {jet}")
    for j, constituent in enumerate(jet.constituents):
        print(f"    constituent {j}: {constituent}")

See the tests for more functionality.

Alternatives

I was looking for very simple bindings, and control of the API was helpful, so I decided to write my own. But there are many other excellent binding options:

• Official bindings: Fastjet recently pushed SWIG based python bindings. You need a fairly recent version of fastjet, which isn't yet deployed for ALICE.
• pyjet: It currently (Nov 2019) doesn't have fastjet-contrib bindings. They possible could have been added, but it didn't appear as straightforward as I would have liked.
• heppy: I was unable to get this setup :-(