Game — Tic-Tac-Toe · Part 2 (Win Detection)

By the end of this lesson you can:

• List the 8 winning lines of a 3×3 board.
• Write winner(board) that returns "X", "O" or None.
• Detect a draw — full board with no winner.
• End the move loop the instant the game finishes — no extra moves played.

How do you win at tic-tac-toe? Three of your marks on the same row, column, or diagonal. That's 3 + 3 + 2 = 8 winning lines.

Rows         (0,0) (0,1) (0,2)
             (1,0) (1,1) (1,2)
             (2,0) (2,1) (2,2)

Columns      (0,0) (1,0) (2,0)
             (0,1) (1,1) (2,1)
             (0,2) (1,2) (2,2)

Diagonals    (0,0) (1,1) (2,2)
             (0,2) (1,1) (2,0)

Store these as a list of triples. Then check each one.

Listing the lines

LINES = [
    # rows
    [(0, 0), (0, 1), (0, 2)],
    [(1, 0), (1, 1), (1, 2)],
    [(2, 0), (2, 1), (2, 2)],
    # columns
    [(0, 0), (1, 0), (2, 0)],
    [(0, 1), (1, 1), (2, 1)],
    [(0, 2), (1, 2), (2, 2)],
    # diagonals
    [(0, 0), (1, 1), (2, 2)],
    [(0, 2), (1, 1), (2, 0)],
]

Eight lines, three cells each. Hand-typed for clarity. (You could generate them with a loop, but a literal list is easier to read.)

The winner check

def winner(board):
    for line in LINES:
        (r1, c1), (r2, c2), (r3, c3) = line
        a, b, c = board[r1][c1], board[r2][c2], board[r3][c3]
        if a == b == c and a != ".":
            return a            # "X" or "O"
    return None

For each line, read the three cells. If all three are the same and not blank, we've found the winner. Otherwise, return None.

The chain comparison a == b == c

That's Python letting you chain. It's the same as a == b and b == c. Reads naturally; works on any two-step comparison.

Draw detection

def is_full(board):
    for row in board:
        if "." in row:
            return False
    return True

If any row still has a ".", the board isn't full. A draw = full board AND no winner.

Putting it together

def game_over(board):
    w = winner(board)
    if w:
        return (True, w)              # someone won
    if is_full(board):
        return (True, None)            # draw
    return (False, None)               # keep playing

Returns a tuple — (done?, winner_or_None). The caller can unpack and react.

Update the main loop

Yesterday's loop ran exactly 9 moves. Today, we check after every move:

while True:
    show(board)
    r, c = ask_move(turn)
    if not place(board, r, c, turn):
        print("  ! Invalid move."); continue

    done, w = game_over(board)
    if done:
        show(board)
        if w:
            print(f"\n🏆 {w} wins!")
        else:
            print("\nIt's a draw.")
        break

    turn = "O" if turn == "X" else "X"

Notice break — we exit the loop the instant we detect a win or draw.

Save as ttt2.py:

# ttt2.py — tic-tac-toe with full win + draw detection

def new_board():
    return [["." for _ in range(3)] for _ in range(3)]

def show(board):
    print()
    print("   0   1   2")
    print("  ───┬───┬───")
    for r in range(3):
        print(f"{r}  {board[r][0]} │ {board[r][1]} │ {board[r][2]}")
        if r < 2:
            print("  ───┼───┼───")
    print()

LINES = [
    [(0, 0), (0, 1), (0, 2)],
    [(1, 0), (1, 1), (1, 2)],
    [(2, 0), (2, 1), (2, 2)],
    [(0, 0), (1, 0), (2, 0)],
    [(0, 1), (1, 1), (2, 1)],
    [(0, 2), (1, 2), (2, 2)],
    [(0, 0), (1, 1), (2, 2)],
    [(0, 2), (1, 1), (2, 0)],
]

def winner(board):
    for line in LINES:
        a = board[line[0][0]][line[0][1]]
        b = board[line[1][0]][line[1][1]]
        c = board[line[2][0]][line[2][1]]
        if a == b == c and a != ".":
            return a
    return None

def is_full(board):
    return all(cell != "." for row in board for cell in row)

def game_over(board):
    w = winner(board)
    if w:
        return (True, w)
    if is_full(board):
        return (True, None)
    return (False, None)

def place(board, r, c, piece):
    if not (0 <= r <= 2 and 0 <= c <= 2):
        return False
    if board[r][c] != ".":
        return False
    board[r][c] = piece
    return True

def ask_move(turn):
    while True:
        raw = input(f"{turn}'s move — row col: ").strip().split()
        if len(raw) != 2:
            print("  ! Two numbers please."); continue
        try:
            return int(raw[0]), int(raw[1])
        except ValueError:
            print("  ! Numbers only.")

# --- main ---

board = new_board()
turn  = "X"

while True:
    show(board)
    while True:
        r, c = ask_move(turn)
        if place(board, r, c, turn):
            break
        print("  ! Square taken or off-board.")

    done, w = game_over(board)
    if done:
        show(board)
        if w:
            print(f"🏆 {w} wins!")
        else:
            print("It's a draw.")
        break

    turn = "O" if turn == "X" else "X"

Sample win

X's move — row col: 1 1
   0   1   2
  ───┬───┬───
0  . │ . │ .
  ───┼───┼───
1  . │ X │ .
  ───┼───┼───
2  . │ . │ .

O's move — row col: 0 0
... (a few moves later) ...

X's move — row col: 2 2
   0   1   2
  ───┬───┬───
0  X │ O │ .
  ───┼───┼───
1  . │ X │ O
  ───┼───┼───
2  . │ . │ X

🏆 X wins!

Read the diff

Three new functions — winner, is_full, game_over — and the main loop now breaks the moment done is True. is_full uses the one-liner trick all(cell != "." for row in board for cell in row) — same shape as PY-L2-39's empties homework.

Eight winning lines. One function. Check each line; if all three cells are the same non-empty piece, that piece won. Draw = full board with no winner. game_over(board) returns a (done, winner) tuple; the main loop breaks the instant done is True. Tomorrow we add an AI opponent and a play-again loop.