class Board(object):
    """
    A tictactoe board using coordinates.

    Coordinates start at (0,0) in the top left.

    >>> myboard = Board()

    >>> myboard.set((0,1), 'x')
    >>> myboard.set((0,1), 'foo')
    Traceback (most recent call last):
        ...
    ValueError: state must be one of 'x', 'o', or ' '
    >>> myboard.set((7,1), 'o')
    Traceback (most recent call last):
        ...
    ValueError: coord does not exist

    >>> myboard.get((0,1))
    'x'
    >>> myboard.get('banana')
    Traceback (most recent call last):
        ...
    ValueError: coord does not exist

    >>> myboard.set((0,0), 'x')
    >>> myboard.has_row()
    []
    >>> myboard.set((0,2), 'x')
    >>> myboard.has_row()
    ['x']
    """

    def __init__(self):
        self.board = {i: " " for i in itertools.product((0,1,2), repeat=2)}

    def set(self, coord, state):
        """
        Set a space on the board to `state`.

        For e.g.: set((0,1), 'x')
        """
        if state not in ('x', 'o', ' '):
            raise ValueError("state must be one of 'x', 'o', or ' '")
        elif coord not in self.board.keys():
            raise ValueError("coord does not exist")
        else:
            self.board[coord] = state

    def get(self, coord):
        """
        Get the state of the space at `coord`.
        """
        try:
            return self.board[coord]
        except KeyError:
            raise ValueError("coord does not exist")

    def filled(self):
        """
        Return amount of spaces filled.
        """
        f = 0

        for v in self.board.itervalues():
            if v != ' ':
                f += 1

        return f

    def has_row(self):
        """
        Return a list of players who have a row of three.
        """
        across = [[i for i in itertools.product([0,1,2], [n])] for n in xrange(3)]
        down = [[(b, a) for a, b in i] for i in across]
        diag = [
            [(0,0), (1,1), (2,2)],
            [(0,2), (1,1), (2,0)],
        ]

        results = []

        for row in itertools.chain(across, down, diag):
            values = [self.board[c] for c in row]

            if values == ['x', 'x', 'x']:
                results.append('x')
            if values == ['o', 'o', 'o']:
                results.append('o')

        return results