Tic Tac Toe

environment gif

This environment is part of the classic environments. Please read that page first for general information.

Import from pettingzoo.classic import tictactoe_v3
Actions Discrete
Parallel API Yes
Manual Control No
Agents agents= ['player_1', 'player_2']
Agents 2
Action Shape (1)
Action Values [0, 8]
Observation Shape (3, 3, 2)
Observation Values [0,1]

Agent Environment Cycle

environment aec diagram

Tic Tac Toe

Tic-tac-toe is a simple turn based strategy game where 2 players, X and O, take turns marking spaces on a 3 x 3 grid. The first player to place 3 of their marks in a horizontal, vertical, or diagonal line is the winner.

Observation Space

The observation is a dictionary which contains an 'obs' element which is the usual RL observation described below, and an 'action_mask' which holds the legal moves, described in the Legal Actions Mask section.

The main observation is 2 planes of the 3x3 board. For player_1, the first plane represents the placement of Xs, and the second plane shows the placement of Os. The possible values for each cell are 0 or 1; in the first plane, 1 indicates that an X has been placed in that cell, and 0 indicates that X is not in that cell. Similarly, in the second plane, 1 indicates that an O has been placed in that cell, while 0 indicates that an O has not been placed. For player_2, the observation is the same, but Xs and Os swap positions, so Os are encoded in plane 1 and Xs in plane 2. This allows for self-play.

The legal moves available to the current agent are found in the action_mask element of the dictionary observation. The action_mask is a binary vector where each index of the vector represents whether the action is legal or not. The action_mask will be all zeros for any agent except the one whose turn it is. Taking an illegal move ends the game with a reward of -1 for the illegally moving agent and a reward of 0 for all other agents.

Action Space

Each action from 0 to 8 represents placing either an X or O in the corresponding cell. The cells are indexed as follows:

0 | 3 | 6

1 | 4 | 7

2 | 5 | 8


Winner Loser
+1 -1

If the game ends in a draw, both players will receive a reward of 0.

Version History

  • v3: Fixed bug in arbitrary calls to observe() (1.8.0)
  • v2: Legal action mask in observation replaced illegal move list in infos (1.5.0)
  • v1: Bumped version of all environments due to adoption of new agent iteration scheme where all agents are iterated over after they are done (1.4.0)
  • v0: Initial versions release (1.0.0)