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If the input hepmc3 file ends with a partial event, the final partial event is dropped but contrary to log messages, no END_EVENT_LISTING is appended.

WARNING: File does not end with END_EVENT_LISTING, appending
WARNING: Invalid particle count for event: E 305 8 20 @ -7.4609364157173369e-02 -7.7988299819229476e-03 9.6855910264790026e+00 -7.2251504307264325e+00
 --> skipping event
WARNING: Skipped invalid event E 305 8 20 @ -7.4609364157173369e-02 -7.7988299819229476e-03 9.6855910264790026e+00 -7.2251504307264325e+00

At the end of the file, when going into flush_buffer, the buffer has the partial event with the END_EVENT_LISTING appended:

    if not end_reached:
        warn("File does not end with END_EVENT_LISTING, appending")
        buffer.append('HepMC::Asciiv3-END_EVENT_LISTING\n')
    # final buffer flush at the end
    flush_buffer(header, buffer)

However, flush_buffer drops the entire buffer, including the END_EVENT_LISTING and returns:

def flush_buffer(header, buffer):
    if header:
        event_record = header.get_record()
        if not event_record:
            warn('Skipped invalid event', header.raw)
            return
        sys.stdout.write(header.get_record())
    for line in buffer:
        sys.stdout.write(line)

A likely fix may be to flush the buffer before if not end_reached:, then append the END_EVENT_LISTING again, and flush the buffer again.

That leaves as a case that will result in a missing END_EVENT_LISTING an input file that ends with END_EVENT_LISTING after an actual incomplete event, in which case the final buffer includes the END_EVENT_LISTING, is dropped, and doesn't trigger the if not end_reached:.

Maybe we can also just always drop END_EVENT_LISTING from the input, and always end by writing END_EVENT_LISTING to the output.