[Python] Simple Tic-Tac-Toe in python 2.7

I'm trying to create a simple tic-tac-toe game for a class project, but I'm not exactly sure how to go about finishing the rest of the code. I'd really appreciate any insight or input as to how I can finish the rest of this off. The code will technically work at the moment, but all it does is display a board filled with none and "The game was a tie.".

PLAYERS = ["X", "O"]

def display_board(board):
print board[0][0:3]
print board[1][0:3]
print board[2][0:3]

def create_empty_board():
return [[None, None, None], [None, None, None], [None, None, None]]

def board_is_full(board):
for row in board:
if None not in board:
return True


def winner(board):
if board[0][0] == board[0][1] == board[0][2] != None:
return board[0][0]
elif board[1][0] == board[1][1] == board[1][2] != None:
return board[1][0]
elif board[2][0] == board[2][1] == board[2][2] != None:
return board[2][0]
elif board[0][0] == board[1][0] == board[2][0] != None:
return board[0][0]
elif board[0][1] == board[1][1] == board[2][1] != None:
return board[0][1]
elif board[0][2] == board[1][2] == board[2][2] != None:
return board[0][2]
elif board[0][0] == board[1][1] == board[2][2] != None:
return board[0][0]
else:
return None

def game_over(board):
if board_is_full(board) == True:
return True
def player_turn(board, playerid):
""" Ask the player to select a coordinates for their next move. The player needs to select a row and a column. If the coordinates the player selects are outside of the board, or are already occupied, they need to be prompted to select coordinates again, until their input is valid."""
return 1, 1 # by default, return row 1, column 1 as the player's desired location on the board; you need to implement this

def play():
""" This is the main function that implements a hot seat version of Tic Tac Toe."""
# the code below is just an example of how you could structure your play() function
# if you implement all the functions above correctly, this function will work
# however, feel free to change it if you want to organize your code differently
board = create_empty_board()
display_board(board)
current_player = 0
while not game_over(board):
board = player_turn(board, PLAYERS[current_player])
current_player = (current_player + 1) % len(PLAYERS)
display_board(board)
who_won = winner(board)
if who_won is None:
print "The game was a tie."
else:
print "The winning player is", who_won

if __name__ == "__main__":
play()

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