Minimax algorithm not working in tic tac toe game

Question

I have recently learned about minimax and I am trying to make an unbeatable AI tic tac toe game. I don't know why my minimax function does not work and my computer sometime plays un optimal moves.

This is my full code:

from random import randint
import os

def play_again():
    print()
    t=True
    while t:
        p=input("Do you want to play again? Y or N  ").upper()
        if p=='Y' or p=='YES':
            t=False
            return True
        elif p=='N' or p=='NO':
            t=False
            print("Thank You!!")
            return False
        else:
            print('Enter a valid input!')
            
    
        
while True:

    board1=['1','2','3','4','5','6','7','8','9']
    print('    |   |')
    print(' ',board1[0],'|',board1[1],'|',board1[2])
    print('____|___|____')
    print('    |   |')
    print(' ',board1[3],'|',board1[4],'|',board1[5])
    print('____|___|____')
    print('    |   |')
    print(' ',board1[6],'|',board1[7],'|',board1[8])
    print('    |   |')

    
    board=[' ',' ',' ',' ',' ',' ',' ',' ',' ']
    def print_board():
        
        print('    |   |')
        print(' ',board[0],'|',board[1],'|',board[2])
        print('____|___|____')
        print('    |   |')
        print(' ',board[3],'|',board[4],'|',board[5])
        print('____|___|____')
        print('    |   |')
        print(' ',board[6],'|',board[7],'|',board[8])
        print('    |   |')


    if randint(0,1)==0:
        player='Player 1'
    else:
        player='Player 2'


    a='a'
    while a not in ['X','O']:
        a=input("Player 1: Do you want to be X or O: ").upper()
            
        if a not in ['X','O']:
            print("Enter valid output X or O")

    if a=='X':
        c='O'
    else:
        c='X'

    def first():
        if player=='Player 1':
            print()
            print('Player 1 will go first')
            
        else:
            print()
            print('Player 2 will go first')
            
    first()

    def position_check():
        return board[b-1] not in ['X','O']

    def board_full():
        for i in board:
            if i not in ['X','O']:
                return True
        return False


    def win_check(m):
        return ((board[0]==m and board[1]==m and board[2]==m) or
                (board[3]==m and board[4]==m and board[5]==m) or
                (board[6]==m and board[7]==m and board[8]==m) or
                (board[0]==m and board[3]==m and board[6]==m) or
                (board[1]==m and board[4]==m and board[7]==m) or
                (board[2]==m and board[5]==m and board[8]==m) or
                (board[0]==m and board[4]==m and board[8]==m) or
                (board[2]==m and board[4]==m and board[6]==m))

    def minimax(board,depth,ismax):
        
        if win_check(c):
            return 1

        elif win_check(a):
            return -1

        elif not board_full:
            return 0

        if ismax:
            bestscore = -1000

            for i in range(len(board)):
                
                if board[i]==' ':
                    board[i]=c
                    score= minimax(board,depth+1,False)
                    
                    board[i]=' '

                    if score > bestscore:
                        bestscore = score

            return bestscore

        else:
            bestscore = 1000

            for i in range(len(board)):
                if board[i]==' ':
                    
                    board[i]=a

                    score= minimax(board,depth+1,True)
                    
                    board[i]=' '

                    if score < bestscore:
                        bestscore = score

            return bestscore
            
        
    while board_full():
        
        if player=='Player 1':
            b='a'
            within_range=False
            while b.isdigit()==False or within_range==False:
                b=input("Enter your next move (1-9) :")

                if b.isdigit()==False:
                    print('\n'*100)
                    print('Enter a choice between 1-9')

                if b.isdigit()==True:
                    if int(b) in range(1,10):
                        within_range=True
                    else:
                        print('Enter a choice between 1-9')

            b=int(b)

            if position_check():
                board[b-1]=a
                print_board()
                if win_check(a):
                    print('Congratulations Player 1 wins!!')
                    break
                else:
                    player='Player 2'

            else:
                print('Position already occupied')

        if player=='Player 2':

            bestscore = -1000

            for i in range(len(board)):
                if board[i]==' ':
                    board[i]=c

                    score= minimax(board,0,False)
                    
                    board[i]=' '

                    if score > bestscore:
                        bestscore = score
                        bestmove=i
                        b=bestmove

            if position_check:
                
                board[b]=c
                
                print_board()
                
                if win_check(c):
                    print('Congratulations Player 2 wins!!')
                    break
                else:
                    player='Player 1'
                    
                


    else:
        print('Its a DRAW!!')


    if not play_again():
        break

When I play position 1 comp should play position 5 but it doesn't but it always plays position 2 and eventually looses Where is the mistake in my code I just learned about minimax and I don't want to use depth concept :

    |   |
  1 | 2 | 3
____|___|____
    |   |
  4 | 5 | 6
____|___|____
    |   |
  7 | 8 | 9
    |   |
Player 1: Do you want to be X or O: x

Player 1 will go first
Enter your next move (1-9) :1
    |   |
  X |   |  
____|___|____
    |   |
    |   |  
____|___|____
    |   |
    |   |  
    |   |
    |   |
  X | O |  
____|___|____
    |   |
    |   |  
____|___|____
    |   |
    |   |  
    |   |

Answer 1

There are a few issues:

• In minimax at this statement:

  elif not board_full:
  

  This condition is never fulfilled, because you are evaluating the function object, not the result from calling the function.

  This should be:

  elif not board_full():
  

  I should say that the name of this function is misleading. board_full returns True when the board is not full, despite its name. You would better call it differently, like: board_has_moves.

• There's a similar problem in the second half of the main loop, where you have:

        if position_check:
  

  This condition is always true, even when the game is over, and so the computer gets to play their last iterated move (i.e. 9) -- overwriting that cell with an invalid move.

  It should have the parentheses.

• The function position_check apparently expects a 1-based value of b, as it works with b-1, but the computer move is stored in b as a 0-based value. So position_check will not work correctly with that. I suggest that you work with a 0-based b value all the way, and adapt position_check to work with b instead of b-1. The human player's move can be set correctly by doing b=int(b) - 1

• If the human player had played the last possible move, then still the second if block is entered without first checking whether the game is over!. If all of the above fixes have been implemented, this will still work out ok, but it is better to make that if player=='Player 2': an else: statement. That way the main loop will only execute one of the two blocks -- never both in the same iteration.

Not a problem, but:

• Choose good names. Names like a, b and c are meaningless and make the code harder to read. It is much clearer if you name those player_1_symbol, move, player_2_symbol
• In play_again you don't need the name t. Just do a while True:.
• It is disturbing that you have defined functions in a loop's body! Don't do that. Move them outside the loop. This means that for position_check and other functions, you should to pass some values as a function argument -- which anyway is good practice.
• I would put the move-loop in a separate function, and that function could have local function(s): I would actually only define minimax as a local function, so the number of arguments can be kept small for this recursive function.
• Avoid the use of global variables, and also put the main logic in a function, like main(). This makes the names that were global into local names and forces you to pass arguments -- which is good practice.

There is certainly much more to improve, but the above changes (and a few more), led to this version of your code:

from random import randint

def play_again():
    print()
    while True:
        p=input("Do you want to play again? Y or N  ").upper()
        if p=='Y' or p=='YES':
            return True
        elif p=='N' or p=='NO':
            print("Thank You!!")
            return False
        else:
            print('Enter a valid input!')

def print_board(board):  # parameter list
    print('    |   |')
    print(' ',board[0],'|',board[1],'|',board[2])
    print('____|___|____')
    print('    |   |')
    print(' ',board[3],'|',board[4],'|',board[5])
    print('____|___|____')
    print('    |   |')
    print(' ',board[6],'|',board[7],'|',board[8])
    print('    |   |')

def first(player):  # argument list
    print()
    if player=='Player 1':
        print('Player 1 will go first')
    else:
        print('Player 2 will go first')
        
def position_check(board, move):
    return board[move] not in ['X','O']

def board_has_moves(board):
    for i in board:
        if i not in ['X','O']:
            return True
    return False

def win_check(board, m):
    return ((board[0]==m and board[1]==m and board[2]==m) or
            (board[3]==m and board[4]==m and board[5]==m) or
            (board[6]==m and board[7]==m and board[8]==m) or
            (board[0]==m and board[3]==m and board[6]==m) or
            (board[1]==m and board[4]==m and board[7]==m) or
            (board[2]==m and board[5]==m and board[8]==m) or
            (board[0]==m and board[4]==m and board[8]==m) or
            (board[2]==m and board[4]==m and board[6]==m))

# A function to deal with one game
def playgame(board, player, player_1_symbol, player_2_symbol):
    def minimax(depth, ismax):  # removed board parameter, as already in scope
        if win_check(board, player_2_symbol):  # pass board also
            return 1
        elif win_check(board, player_1_symbol):
            return -1
        elif not board_has_moves(board):  # call it, rename it
            return 0
        if ismax:
            bestscore = -1000
            for i in range(len(board)):
                if board[i]==' ':
                    board[i]=player_2_symbol
                    score= minimax(depth+1,False)
                    board[i]=' '
                    if score > bestscore:
                        bestscore = score
            return bestscore
        else:
            bestscore = 1000
            for i in range(len(board)):
                if board[i]==' ':
                    board[i]=player_1_symbol
                    score= minimax(depth+1,True)
                    board[i]=' '
                    if score < bestscore:
                        bestscore = score
            return bestscore
    
    while board_has_moves(board):  # better name; pass argument
        if player=='Player 1':
            move='a'  # better name
            within_range=False
            while not move.isdigit() or not within_range:
                move=input("Enter your next move (1-9) :")
                if not move.isdigit():
                    print('\n'*100)
                    print('Enter a choice between 1-9')
                if move.isdigit():
                    if int(move) in range(1,10):
                        within_range=True
                    else:
                        print('Enter a choice between 1-9')
            move=int(move)-1  # reduce to make it 0-based
            if position_check(board, move):  # pass the arguments
                board[move]=player_1_symbol   # b is zero based now
                print_board(board)  # pass argument
                if win_check(board, player_1_symbol):  # pass arguments
                    print('Congratulations Player 1 wins!!')
                    break
                else:
                    player='Player 2'
            else:
                print('Position already occupied')
        else:  # changed to avoid code falling thru from the previous block
            bestscore = -1000
            bestmove = 10  # better initialise before loop
            for i in range(len(board)):
                if board[i]==' ':
                    board[i]=player_2_symbol
                    score= minimax(0,False)
                    board[i]=' '
                    if score > bestscore:
                        bestscore = score
                        bestmove=i
            if position_check(board, bestmove):  # add the parentheses to call it
                board[bestmove]=player_2_symbol
                print_board(board)  # pass argument
                if win_check(board, player_2_symbol):
                    print('Congratulations Player 2 wins!!')
                    break
                else:
                    player='Player 1'
    else:
        print('Its a DRAW!!')


# Main program
def main():  # avoid global variables, and put main logic in a function
    while True:
        board1=['1','2','3','4','5','6','7','8','9']
        print('    |   |')
        print(' ',board1[0],'|',board1[1],'|',board1[2])
        print('____|___|____')
        print('    |   |')
        print(' ',board1[3],'|',board1[4],'|',board1[5])
        print('____|___|____')
        print('    |   |')
        print(' ',board1[6],'|',board1[7],'|',board1[8])
        print('    |   |')
        board=[' ',' ',' ',' ',' ',' ',' ',' ',' ']
    
        # simplified:
        player = ('Player 1', 'Player 2')[randint(0,1)]
        player_1_symbol ='a'  # better name
        while player_1_symbol not in ['X','O']:
            player_1_symbol=input("Player 1: Do you want to be X or O: ").upper()
            if player_1_symbol not in ['X','O']:
                print("Enter valid output X or O")
        # use ternary operator, and use better name
        player_2_symbol = 'O' if player_1_symbol=='X' else 'X'
        first(player)  # pass argument
        # move the move-loop logic into another function:
        playgame(board, player, player_1_symbol, player_2_symbol)
        if not play_again():
            break
    
main()