Pickling a callback function with access to the LAMMPS instance itself

[raynol-dsouza]

in.simple.txt

As discussed with @jan-janssen, LammpsLibrary at the moment cannot be used as follows:

from pylammpsmpi import LammpsLibrary

def callback(caller, ntimestep, nlocal, tag, x, f):
    lmp = caller[0]
    print(f"energy_pot = {lmp.get_thermo('pe')}")

lmp = LammpsLibrary(cores=4,
                    oversubscribe=False,
                    working_directory=".",
                    client=None,
                    cmdargs=None)

lmp.file("in.simple")
lmp.fix(f"test all external pf/callback 1 1")
lmp.set_fix_external_callback("test", callback, [lmp])
lmp.run(0)

The objective here is to use LAMMPS' fix external command to do some math in the callback function using properties from the LAMMPS instance.

It works without any issues with lammps.lammps.

[jan-janssen]

@raynol-dsouza create a branch https://github.com/pyiron/pylammpsmpi/tree/fix_exernal_callback

[raynol-dsouza]

@jan-janssen I believe I figured it out! I've pushed my changes to https://github.com/pyiron/pylammpsmpi/tree/fix_exernal_callback

Here is my new working example:

from pylammpsmpi import LammpsLibrary

class testing:
    def __init__(self, this_works: str = 'This_works.'):
        self.this_works = this_works
    
    def print_this(self):
        print(f"{self.this_works}")

def my_callback(caller, ntimestep, nlocal, tag, x, f):
    lmp, a_class_instance, a_number = caller
    print("Hello.")
    print(a_class_instance.print_this())
    print(f'Took me only {a_number} hours!')
    print(f"energy_pot = {lmp.get_thermo('pe')}")

lmp = LammpsLibrary(cores=2,
                    oversubscribe=False,
                    working_directory=".",
                    client=None,
                    cmdargs=None)

lmp.file("in.simple")
lmp.fix(f"test all external pf/callback 1 1")
lmp.set_fix_external_callback('test', my_callback, [lmp, testing(), 2])
lmp.run(0)
lmp.close()

Let me know if this good enough to make a pull request!