Creating a simple max damage player

The corresponding complete source code can be found here.

Note

A similar example using gen 7 mechanics is available here.

The goal of this example is to explain how to create a first custom agent. This agent will follow simple rules:

If the active pokemon can attack, it will attack and use the move with the highest base power
Otherwise, it will perform a random switch

Creating a player

The player that we are going to implement does not need to be trained: we can therefore directly inherit from the Player class.

Let’s create the base class:

    from poke_env.player import Player

class MaxDamagePlayer(Player):
    pass

Player’s has one abstract method, choose_move. Once implemented, we will be able to instantiate and use our player.

Creating a `choose_move` method

Method signature

The signature of choose_move is choose_move(self, battle: Battle) -> str: it takes a Battle object representing the game state as argument, and returns a move order encoded as a string. This move order must be formatted according to the showdown protocol. Fortunately, poke-env provides utility functions allowing us to directly format such orders from Pokemon and Move objects.

We therefore have to take care of two things: first, reading the information we need from the battle parameter. Then, we have to return a properly formatted response, corresponding to our move order.

Selecting a move

The battle parameter is an object of type Battle which encodes the agent’s current knowledge of the game state. It offers several properties that make accessing the game state easy. Some of the most notable are active_pokemon, available_moves, available_switches, opponent_active_pokemon, opponent_team and team.

In this example, we are going to use available_moves: it returns a list of Move objects which are available this turn.

We can therefore test if at least one move can be used with if battle.available_moves:. We are interested in the base power of available_moves, which can be accessed with the base_power property of Move objects.

class MaxDamagePlayer(Player):
    def choose_move(self, battle):
        # If the player can attack, it will
        if battle.available_moves:
            # Finds the best move among available ones
            best_move = max(battle.available_moves, key=lambda move: move.base_power)

Returning a choice

Now that we have selected a move, we need to return a corresponding order, which takes the form of a string. Fortunately, Player provides a method designed to craft such strings directly: create_order. It takes a Pokemon (for switches) or Move object as argument, and returns a string corresponding to the order. Additionally, you can use its mega, z_move and dynamax parameters to mega evolve, use a z-move, dynamax or gigantamax, if possible this turn.

We also have to return an order corresponding to a random switch if the player cannot attack. Player objects incorporate a choose_random_move method, which we will use if no attacking move is available.

class MaxDamagePlayer(Player):
    def choose_move(self, battle):
        # If the player can attack, it will
        if battle.available_moves:
            # Finds the best move among available ones
            best_move = max(battle.available_moves, key=lambda move: move.base_power)
            return self.create_order(best_move)

        # If no attack is available, a random switch will be made
        else:
            return self.choose_random_move(battle)

Running and testing our agent

We can now test our agent by making it battle a random agent. The complete code is:

    import asyncio
import time

from poke_env.player import Player, RandomPlayer


class MaxDamagePlayer(Player):
    def choose_move(self, battle):
        # If the player can attack, it will
        if battle.available_moves:
            # Finds the best move among available ones
            best_move = max(battle.available_moves, key=lambda move: move.base_power)
            return self.create_order(best_move)

        # If no attack is available, a random switch will be made
        else:
            return self.choose_random_move(battle)


async def main():
    start = time.time()

    # We create two players.
    random_player = RandomPlayer(
        battle_format="gen8randombattle",
    )
    max_damage_player = MaxDamagePlayer(
        battle_format="gen8randombattle",
    )

    # Now, let's evaluate our player
    await max_damage_player.battle_against(random_player, n_battles=100)

    print(
        "Max damage player won %d / 100 battles [this took %f seconds]"
        % (
            max_damage_player.n_won_battles, time.time() - start
        )
    )


if __name__ == "__main__":
    asyncio.get_event_loop().run_until_complete(main())

Running it should take a couple of seconds and print something similar to this:

Max damage player won 92 / 100 battles [this took 6.320682 seconds]

If you want to use Reinforcement Learning, take a look at the Reinforcement learning with the OpenAI Gym wrapper example.