Determine whether there is a winner in tic-tac-toe game
Your code is WET.


var[0] != 0 and len(set(var)) == 1



Is repeated four times.
field[len(field)-i-1] can be simplified to field[~i].
I'd personally just use a couple of any and a couple of ors.


def is_unique_player(values):
    return values[0] != 0 and len(set(values)) == 1


def is_win(field):
    N = len(field)
    return (
        any(is_unique_player(row) for row in field)
        or any(is_unique_player(column) for column in zip(*field))
        or is_unique_player([field[i][i] for i in range(N)])
        or is_unique_player([field[~i][i] for i in range(N)])
    )