I found a mistake in the check_winner() function.

It was working only with three or less X or O signs.
I have changed it to work for all combinations

ttt/game.py

@@ -61,6 +61,8 @@ class TTTGame:
         winning_combinations=[[0,1,2],[3,4,5],[6,7,8],[0,3,6],[1,4,7],[2,5,8],[0,4,8],[2,4,6]]
         board=state["board"]
         combination=[index for index in range(9) if board[index]==symbol]
-        if combination in winning_combinations:
-            return True
+        for combo in winning_combinations:
+            tot = sum([board[index] == symbol for index in combo])
+            if tot == 3:
+                return True
         return False

ttt/player.py

@@ -1,5 +1,6 @@
 from click import Choice, prompt
 from strategy.random_strategy import RandomStrategy
+from strategy.lookahead_strategy import LookaheadStrategy
 from ttt.game import TTTGame
 import random
 
@@ -24,7 +25,7 @@ class TTTComputerPlayer:
     def __init__(self, name):
         "Sets up the player."
         self.name = name
-        self.strategy = RandomStrategy(TTTGame())
+        self.strategy = LookaheadStrategy(TTTGame())
 
     def choose_action(self, state):
         "Chooses a random move from the moves available."