allow python subclasses of pyspiel.Policy

[VitamintK]

It would be nice if we could subclass pyspiel.Policy, and then pass those policies to pyspiel functions which expect pyspiel.Policys.

So something like this:

import pyspiel
class PolicySubclass(pyspiel.Policy):
    def __init__(self, game):
        pyspiel.Policy.__init__(self)
   def get_state_policy_from_state(self, state, player):
       ...
   def get_state_policy_from_infostate_string(self, infostate_string):
      ...

Currently, trying to do so throws an error __init__ must be called when overriding __init__ (pybind/pybind11#2152), presumably because Policy has no constructor defined in policy.h. It looks like Game had the same issue, so a "trampoline class" was defined in open_spiel/python/pybind11/python_games.h. Would a similar solution should work for Policy?

[lanctot]

From a quick read of the thread it seems like the trampoline class was needed because Game is not default constructible.

I wonder if we can just add an empty constructor in this case. @elkhrt, do you know?

[elkhrt]

This would be a great contribution! You'll still need a trampoline class, but the overall setup should be a lot simpler than with Game. There's an overview of the necessary setup here: https://pybind11.readthedocs.io/en/stable/advanced/classes.html

[lanctot]

This is quite old, seems abandoned. @VitamintK please feel free to re-open if you ever get to this.