TicTacToe in Python 3
PyLint is a useful tool for checking your code, ensuring good coding habits.  For example, PyLint doesn’t like a lack of spaces around operators or after commas.  Neither do my old weak eyes.  Instead of this:

pos_matrix=[[' ',' ',' '],[' ',' ',' '],[' ',' ',' ']]


... code like this:

pos_matrix = [[' ', ' ', ' '], [' ', ' ', ' '], [' ', ' ', ' ']]




An if statement that does a return or exit doesn’t need to be followed by an else or elif; if the if statement is true, the execution path doesn’t need to skip around the else clause, because execution has already left the function.

For clarity:

if pos_matrix[pos[0]-1][pos[1]-1]!=' ':
    print('Invalid input')
    exit
else:
    pos_matrix[pos[0]-1][pos[1]-1]=symbol

for i in range(0,3):
    # print rows of game grid


After the if-else statement is the code to print out the game grid.  It appears this code belongs to both paths through the if-else, where as it will only execute if the False path is taken.  As such, it should be indented to belong as part of the else: clause:

if pos_matrix[pos[0] - 1][pos[1] - 1] != ' ':
    print('Invalid input')
    exit
else:
    pos_matrix[pos[0] - 1][pos[1] - 1] = symbol

    for i in range(0,3):
        # print rows of game grid


Alternately, the else: can be omitted entirely, and the statement in the else: clause outdented to the same level as the if statement:

if pos_matrix[pos[0]-1][pos[1]-1]!=' ':
    print('Invalid input')
    exit

pos_matrix[pos[0]-1][pos[1]-1]=symbol

for i in range(0,3):
    # print rows of game grid




Invalid input should not unconditionally terminate the program.

if pos_matrix[pos[0]-1][pos[1]-1]!=' ':
    print('Invalid input')
    exit


This code makes it harder to use in a larger program, since an invalid input will terminate the entire program.

Instead, a ValueError exception should be raised:

if pos_matrix[pos[0] - 1][pos[1] - 1] != ' ':
    raise ValueError('Invalid input')


This allows the caller to use a try ... except: structure.  The user could enter an invalid input, the function detects this and raises the error.  The caller catches the error, and request the user re-enter a valid move.

Without the appropriate try ... except: statement, raising the exception will also terminate the program.



You decode pos using [pos[0]-1][pos[1]-1] multiple times.  This is ugly and hard to follow.  Use variable names to avoid repeating the same calculations:

row, col = pos[0] - 1, pos[1] - 1


Then you can write:

if pos_matrix[row][col] != ' ':




Instead of looping over range(0, 3) or [0, 1, 2], just use range(3):

for i in range(3):
    ....




def my_tictactoe(pos,symbol):
    ...
    pos_matrix[pos[0]-1][pos[1]-1]=symbol
    ...
    print('GAME OVER. \n Player with symbol {x} wins!'.format(x=pos_matrix[pos[0]-1][pos[1]-1]))


Why the complex expression to set xin the format statement?  You’ve just assigned symbolto that pos_matrix entry; just use that!

print('GAME OVER. \n Player with symbol {x} wins!'.format(x=symbol))


Or better, if you are using python 3.6 or later, use an f-string:

print(f'GAME OVER. \n Player with symbol {symbol} wins!')




Your is_win code is more complex than needed because you are checking if anyone has won, which means a row, column or diagonal has 3 of the same symbol, which is not a space.  If you passed the symbol to the function, you can check if that player has won, which means checking for 3 of that symbol only, which is slightly shorter &amp; simpler.

def is_win(symbol):
    for i in range(3):
        if all( pos_matrix[i][col] == symbol for col in range(3)):
            return True
        if all( pos_matrix[row][i] == symbol for row in range(3)):
            return True
    if all( pos_matrix[i][i] == symbol for i in range(3)):
        return True
    if all( pos_matrix[i][2 - i] == symbol for i in range(3)):
        return True
    return False




Who's turn is it, anyway?

You call my_tictactoe(...) and pass in the character to place in the grid.  As Hack Saw pointed out, this could be a 'p'!

Instead, it would be better for the game to keep track of who's turn it is, and automatically use the appropriate symbol.

For example, you could set a global symbol variable initially to 'x', and then have it flip between 'x' and 'o' after processing a valid move.

symbol = 'x'

def my_tictactoe(pos):

    # ... existing validation, printing, and win detection code ...

    # Alternate players: 'x' &lt;--&gt; 'o'
    symbol = 'o' if symbol == 'x' else 'x'


As a bonus, after one player wins, the other player will automatically become the first player.



To test the program, with the automatic player tracking, (expanding on Hack Saw's suggestion a bit) the test code could become:

def main():

    for pos in ((1,1), (2,2), (3,2), (1,3), (2,1), (3,3), (3,1), (1,2)):
        my_tictactoe(pos)

if __name__ == "__main__":
    main()