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Chris Proctor
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q_learning.py
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q_learning.py
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"""Q-learning agent for BabySnake.
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This file contains starter code for implementing a Q-learning agent.
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You need to fill in two functions:
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- choose_action: select an action using an epsilon-greedy policy
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- update_q: update the Q-table using the Bellman equation
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Run this file to train the agent:
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python q_learning.py
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After training, run this to watch it play:
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python -c "from q_learning import watch; watch()"
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"""
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import random
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import babysnake
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from retro.input import ProgrammaticInput
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from retro.views.headless import HeadlessView
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# The four actions the agent can take.
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ACTIONS = ["KEY_RIGHT", "KEY_DOWN", "KEY_LEFT", "KEY_UP"]
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# ---------------------------------------------------------------------------
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# Environment wrapper
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# ---------------------------------------------------------------------------
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class BabySnakeEnv:
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"""A simple wrapper that lets us step through BabySnake programmatically.
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Usage:
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env = BabySnakeEnv()
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state = env.reset() # start a new episode
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next_state, reward, done = env.step("KEY_RIGHT")
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"""
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def reset(self):
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"""Start a new episode. Returns the initial state tuple."""
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self._inp = ProgrammaticInput()
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self.game = babysnake.create_game()
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self.game.input_source = self._inp
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self.game.view = HeadlessView()
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self.game.start()
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self._prev_reward = 0.0
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return self._get_state()
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def step(self, action):
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"""Take one action. Returns (next_state, reward, done).
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Arguments:
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action (str): One of ACTIONS, or None for no-op.
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Returns:
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next_state (tuple): The state after the action.
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reward (float): The reward received this step.
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done (bool): True if the episode has ended.
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"""
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self._inp.press(action)
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self.game.step()
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next_state = self._get_state()
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reward = self.game.state['reward'] - self._prev_reward
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self._prev_reward = self.game.state['reward']
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done = not self.game.playing
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return next_state, reward, done
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def _get_state(self):
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"""Return the current state as a tuple of four integers."""
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s = self.game.state
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return (int(s['agent_x']), int(s['agent_y']),
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int(s['food_x']), int(s['food_y']))
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# ---------------------------------------------------------------------------
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# Q-learning functions — fill these in!
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# ---------------------------------------------------------------------------
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def choose_action(q_table, state, epsilon):
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"""Choose an action using an epsilon-greedy policy.
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With probability `epsilon`, return a random action from ACTIONS.
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Otherwise, return the action with the highest Q-value in `q_table`
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for the given `state`. If a (state, action) pair has not been seen
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before, treat its Q-value as 0.
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Arguments:
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q_table (dict): Maps (state, action) -> Q-value.
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state (tuple): The current state, e.g. (1, 2, 3, 0).
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epsilon (float): Exploration rate, between 0.0 and 1.0.
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Returns:
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str: One action from ACTIONS.
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Hint: random.random() returns a float in [0.0, 1.0).
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random.choice(ACTIONS) returns a random action.
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q_table.get(key, default) is handy for missing entries.
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"""
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raise NotImplementedError("Fill in choose_action")
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def update_q(q_table, state, action, reward, next_state, alpha, gamma):
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"""Update one entry of the Q-table using the Bellman equation.
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The update rule is:
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Q(s, a) <- Q(s, a) + alpha * (r + gamma * max_a' Q(s', a') - Q(s, a))
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where:
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s, a — the state we were in and the action we took
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r — the reward we received
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s' — the state we ended up in
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max_a' ... — the best possible Q-value from the new state
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Arguments:
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q_table (dict): Maps (state, action) -> Q-value (modified in place).
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state (tuple): The state before the action.
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action (str): The action taken.
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reward (float): The reward received.
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next_state (tuple): The state after the action.
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alpha (float): Learning rate (how much to update).
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gamma (float): Discount factor (how much to value future rewards).
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Returns:
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None — modifies q_table in place.
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Hint: Q-values for unseen (state, action) pairs start at 0.
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"""
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raise NotImplementedError("Fill in update_q")
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# ---------------------------------------------------------------------------
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# Training loop
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# ---------------------------------------------------------------------------
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def train(
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episodes=1000,
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alpha=0.1,
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gamma=0.95,
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epsilon=1.0,
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epsilon_decay=0.995,
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epsilon_min=0.05,
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):
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"""Train a Q-learning agent on BabySnake.
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Arguments:
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episodes (int): How many episodes to run.
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alpha (float): Learning rate.
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gamma (float): Discount factor.
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epsilon (float): Starting exploration rate.
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epsilon_decay (float): Multiply epsilon by this each episode.
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epsilon_min (float): Epsilon never falls below this.
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Returns:
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dict: The trained Q-table.
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"""
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q_table = {}
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env = BabySnakeEnv()
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for episode in range(episodes):
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state = env.reset()
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total_reward = 0.0
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while env.game.playing:
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action = choose_action(q_table, state, epsilon)
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next_state, reward, done = env.step(action)
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update_q(q_table, state, action, reward, next_state, alpha, gamma)
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state = next_state
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total_reward += reward
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epsilon = max(epsilon_min, epsilon * epsilon_decay)
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if (episode + 1) % 100 == 0:
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print(
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f"Episode {episode + 1:5d} "
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f"reward={total_reward:6.1f} "
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f"score={env.game.state['score']} "
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f"epsilon={epsilon:.3f} "
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f"q_entries={len(q_table)}"
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)
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return q_table
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def watch(q_table=None):
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"""Watch the trained agent play in the terminal.
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Arguments:
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q_table (dict | None): A trained Q-table. If None, trains first.
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"""
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import babysnake
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from retro.input import ProgrammaticInput
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if q_table is None:
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print("Training first...")
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q_table = train()
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_inp = ProgrammaticInput()
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class PolicyInput:
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"""An input source that picks actions from the Q-table."""
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def collect(self):
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s = game.state
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state = (int(s['agent_x']), int(s['agent_y']),
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int(s['food_x']), int(s['food_y']))
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q_values = [q_table.get((state, a), 0.0) for a in ACTIONS]
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best = ACTIONS[q_values.index(max(q_values))]
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_inp.press(best)
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return _inp.collect()
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game = babysnake.create_game()
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game.play(input_source=PolicyInput())
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if __name__ == '__main__':
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print("Training Q-learning agent on BabySnake...")
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q_table = train()
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print(f"\nDone. Q-table has {len(q_table)} entries.")
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print("\nWatching trained agent (press Enter or Escape to quit)...")
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watch(q_table)
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