tictactoe with minmax

#another tic tac toe game
import random
from random import randint
#gameboard interface

gameboard = [[" "] * 3, [" "] * 3, [" "] * 3] # 2d Array
aigameboard = [[" "] * 3, [" "] * 3, [" "] * 3] # 2d Array for ai minmax sim
input_var1 = [["a1", "b1", "c1"], ["a2", "b2", "c2"], ["a3", "b3", "c3"]]
available_moves = ["a1", "b1", "c1", "a2", "b2", "c2", "a3", "b3", "c3"]
potential_moves = ["a1", "b1", "c1", "a2", "b2", "c2", "a3", "b3", "c3"] #for minmax use
rows = 3
columns = 3
player_symbols = ["_", "X", "O"]
current_player = 1

def end_game():

    global gameboard
    global aigameboard
    global potential_moves
    global available_moves
    global current_player

    print("We have a winner ", "Player ", current_player, " has won!")
    draw_gameboard()

    replay = input("Do you want to play again? Y/N: ")

    if replay == "Y" or replay == "y":
        gameboard = [[" "] * 3, [" "] * 3, [" "] * 3] # 2d Array
        aigameboard = [[" "] * 3, [" "] * 3, [" "] * 3]
        available_moves = ["a1", "b1", "c1", "a2", "b2", "c2", "a3", "b3", "c3"]
        potential_moves = ["a1", "b1", "c1", "a2", "b2", "c2", "a3", "b3", "c3"]
        current_player = 1
        main_game_loop(False)


    else:
        quit()

def draw_gameboard(): #this is our game output
    print("    A    B    C")
    for i in range(len(gameboard)):
        print(i+1, (gameboard[i]))

def draw_fake_gameboard(board): #this is debug tool
    print("    A    B    C <- fake ai gameboard")
    for i in range(len(board)):
        print(i+1, (board[i]))

def get_player_input():
    print("It's your turn, proper input example is 'a3', or 'b1'. Enter 'x' to quit")
    pos = input("Enter position you want to take:")
    if pos == "x":
        quit()
    return pos

def get_ai_input():
    global aigameboard
    global gameboard
    global potential_moves
    global available_moves
    score = []

    for i in range(len(available_moves)):
        score.append(mini_max(available_moves[i], aigameboard, True))
        aigameboard = gameboard


    print(score)
    pos = available_moves[score.index(max(score))]


    #pos = available_moves[randint(0, (len(available_moves))-1)]
    return pos

def mini_max(position, board, isMaximizing):

    global input_var1
    global potential_moves

    score = 0
    scores = []
    winner = 3
    probed_position_x = 0
    probed_position_y = 0
    player = 1
    value = 0
    position_index = 0
    internal_board = board
    next_player = not isMaximizing

    #draw_fake_gameboard(board)
    #print(position)

    if isMaximizing == True:
        player = 2
        value = 1
    else:
        player = 1
        value = -1 #we wil multiply check for winners by value to get us -1 in case of minimizing

    for x in range(len(input_var1)):
        for y in range(len(input_var1[x])):
            if input_var1[x][y] == position:
                board[x][y] = player_symbols[player]
                probed_position_x = x
                probed_position_y = y

    position_index = potential_moves.index(position)
    potential_moves.remove(position)
    winner =  check_for_winners(board, potential_moves)
    if winner == 1 or winner == 0:
        #there is a winner here
        score = winner*value
        board[probed_position_x][probed_position_y] = " "
        potential_moves.insert(position_index, position)
        return score
    else:
        for i in range(len(potential_moves)):
            scores.append(mini_max(potential_moves[i], internal_board, next_player))
            internal_board = board
        if isMaximizing:
            score = min(scores)
        else:
            score = max(scores)

        board[probed_position_x][probed_position_y] = " "
        potential_moves.insert(position_index, position)

        return score






def update_game(player_number):
    global gameboard
    global input_var1
    global current_player

    if player_number == 1:
        pos = get_player_input()
    else:
        pos = get_ai_input()

    if pos in available_moves:
        available_moves.remove(pos)
        potential_moves.remove(pos)

        for x in range(len(input_var1)):
            for y in range(len(input_var1[x])):
                if input_var1[x][y] == pos:
                    gameboard[x][y] = player_symbols[player_number]
                    print(" ")
                    print("Player", player_number, "move: ")
    else:
        print("this position is invalid or already taken")
        update_game(player_number)


def check_for_winners(gameboard_state, moves):
    column_check = []
#horizontal
    for x in range(len(gameboard_state[0])):
        if gameboard_state[x].count(gameboard_state[x][0]) == len(gameboard_state[x]) and gameboard_state[x][0] != " ":
            #there is horizontal winner
            return 1
#vertical
    for i in range(len(gameboard_state)):
        for row in gameboard_state:
            column_check.append(row[i])
        if column_check.count(column_check[0]) == len(column_check) and column_check[0] != " ":
            #there is vertical winner
            return 1
        else:
            column_check.clear()
#diagonal right
    for counter, row in enumerate(gameboard_state):
        column_check.append(row[counter])
    if column_check.count(column_check[0]) == len(column_check) and column_check[0] != " ":
            #there is diagonal winner
            return 1
    else:
        column_check.clear()

#diagonal left
    for counter, row in enumerate(gameboard_state):
        column_check.append(row[-counter-1])
    if column_check.count(column_check[0]) == len(column_check) and column_check[0] != " ":
            #there is diagonal winner
            return 1
    else:
        column_check.clear()

#check for TIE
    if len(moves) == 0:
        return 0

    return -3

def change_player():
    global current_player

    if current_player == 1:
        current_player = 2
    else:
        current_player = 1

def main_game_loop(game_ended):
    global aigameboard
    while game_ended == False:

        draw_gameboard()
        update_game(current_player)
        if check_for_winners(gameboard, available_moves) != -3:
            game_ended = True
        else:
            aigameboard = gameboard
            change_player()

    else:
        end_game()

main_game_loop(False)