fill_genevent_from_hepevt not adding particles

[janTOffermann]

Hi there! Thanks for creating this easy-to-install alternative to building HepMC3.

I'm currently trying to use fill_genevent_from_hepevt to fill a blank GenEvent with some particles (I'm using numpythia to generate some events to be written to a HepMC3 file for Delphes usage, but I want to do some filtering before writing since HepMC3 files are very large and I'm not interested in most of the particles in each event).

However, I can't seem to get the filling of the GenEvent to work. Here's a minimal working example demonstrating the issue:

import numpy as np
import pyhepmc_ng as hep

evt = hep.GenEvent()
evt.clear()

print(evt) # Check how things look after calling "clear".

# Get some dummy data -- here, just a single particle.
event_number = 1
p = np.array([1.,1.,1.,4.]).reshape(1,-1)
m = np.array([1.])
v = np.array([1.,1.,1.,0.]).reshape(1,-1)
pid = np.array([5])
parents = np.array([1,1]).reshape(1,-1)
status = np.array([1])

hep.fill_genevent_from_hepevt(evt,
                              event_number = event_number,
                              p = p,
                              m = m,
                              v = v,
                              pid = pid,
                              parents = parents,
                              status = status
                             )

print(evt) # See if anything changed.

And this is what I get as output:

GenEvent: #0
 Momentum units: GEV Position units: MM
 Entries in this event: 0 vertices, 0 particles, 0 weights.
 Position offset: 0, 0, 0, 0
                                    GenParticle Legend
         ID    PDG ID   ( px,       py,       pz,     E )   Stat ProdVtx
________________________________________________________________________
________________________________________________________________________

________________________________________________________________________
GenEvent: #1
 Momentum units: GEV Position units: MM
 Entries in this event: 0 vertices, 0 particles, 0 weights.
 Position offset: 0, 0, 0, 0
                                    GenParticle Legend
         ID    PDG ID   ( px,       py,       pz,     E )   Stat ProdVtx
________________________________________________________________________
________________________________________________________________________

It looks like I am modifying the event_number (trying different numbers works as well), but there doesn't appear to be any information on the particle that I've tried to add.

I think the issue might lie with HEPEVT.to_genevent(), which is called at the end of fill_genevent_from_hepevt, as I tried calling this on a HEPEVT that I created (mimicking the global buffer) and similarly didn't see any particles added to the GenEvent I passed as an argument.

I see that GenEvent has methods for explicitly adding particles and vertices, which might let me circumvent this issue for now, but it would certainly be nice to have a vectorized approach instead of adding them on the Python side of things one-by-one (I also see that there is a 2nd unused version of fill_genevent_from_hepevt that does not call HEPEVT.to_genevent).

[janTOffermann]

I forgot to include some version information in the above post, here it is in case it's helpful:

OS: Ubuntu 18.04.5 LTS
Python: 3.8.10
pyhepmc: 0.4.3 (via pypi)

[HDembinski]

Hi, this seems to be a bug that was introduced at some point in HepMC3. I am currently preparing a new version with the latest HepMC3 where this issue is fixed.