Solve tic-tac-toe checker

Write a function that verifies whether or not a tic-tac-toe board is valid.

scratch/facebook/moderate/tic-tac-toe-checker.py

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+import random
+
+def print_board(board):
+    result = []
+    for row in range(len(board)):
+        r = []
+        for col in range(len(board[row])):
+            cell = board[row][col]
+            if not cell:
+                r.append("-")
+            else:
+                r.append(cell)
+        result.append(" | ".join(r))
+    print("\n---------\n".join(result))
+
+def init_board():
+    result = []
+    for row in range(3):
+        r = []
+        for col in range(3):
+            r.append(None)
+        result.append(r)
+    return result
+
+def check(board, player):
+    print_board(board)
+    print()
+    if player not in "XO":
+        raise Exception("Only checking the board for Xs or Os. You supplied {}".format(player))
+    dn, ax, ddg, udg = "DOWN", "ACROSS", "DOWN_DIAGONAL", "UP_DIAGONAL"
+    ways = [
+        [[dn, ax, ddg], [dn], [dn, udg]],
+        [[ax], [], []],
+        [[ax], [], []],
+    ]
+    for row in range(len(board)):
+        for col in range(len(board[row])):
+            if board[row][col] == player:
+                xs = ways[row][col]
+                for x in xs:
+                    if x == dn:
+                        if {player} == {board[row+1][col], board[row+2][col]}:
+                            return True
+                    if x == ax:
+                        if {player} == {board[row][col+1], board[row][col+2]}:
+                            return True
+                    if x == ddg:
+                        if {player} == {board[row+1][col+1], board[row+2][col+2]}:
+                            return True
+                    if x == udg:
+                        if {player} == {board[row+1][col-1], board[row+2][col-2]}:
+                            return True
+    return False
+
+def op(player):
+    return "X" if player == "O" else "O"
+
+dn_win = lambda p: [
+    [op(p), p, None],
+    [op(p), p, None],
+    [None,  p, None],
+]
+
+ax_win = lambda p: [
+    [p, p, p],
+    [op(p), op(p), None],
+    [None, None, None],
+]
+
+ddg_win = lambda p: [
+    [p, None, None],
+    [op(p), p, None],
+    [op(p), None, p],
+]
+
+udg_win = lambda p: [
+    [op(p), None, p],
+    [op(p), p, None],
+    [p, None, None],
+]
+
+# Down
+p = random.choice(["X", "O"])
+assert check(dn_win(p), p) == True
+assert check(dn_win(p), op(p)) == False
+# Across
+p = random.choice(["X", "O"])
+assert check(ax_win(p), p) == True
+assert check(ax_win(p), op(p)) == False
+# Down Diagonally
+p = random.choice(["X", "O"])
+assert check(ddg_win(p), p) == True
+assert check(ddg_win(p), op(p)) == False
+# Down Diagonally
+p = random.choice(["X", "O"])
+assert check(udg_win(p), p) == True
+assert check(udg_win(p), op(p)) == False
+# Success
+print("Tests pass!")