Add functions Player.get_diplo_state(player) and Player.set_diplo_state(player, state) where Player.get_diplo_state(player) gets the diplomatic state between Player and player, returning a string like "War", "Peace", or "Ceasefire", and Player.set_diplo_state(player, state) sets the diplomatic state between the two players to the state argument's value
I think the API should also support other things that are supported by "DiplRel" requirement (like "Gives Shared Vision"). Example patch I once made here.
Add functions Player.get_diplo_state(player) and Player.set_diplo_state(player, state) where Player.get_diplo_state(player) gets the diplomatic state between Player and player, returning a string like "War", "Peace", or "Ceasefire", and Player.set_diplo_state(player, state) sets the diplomatic state between the two players to the state argument's value