def addNode(self, node_id, x, y):
    if self.node_list[node_id] != None:
        raise exception("This node ID already exists.")
    else:
        if self.spacialMatrix[x][y] != None:
            raise exception("This node position is already populated.")
        else:
            self.spacialMatrix[x][y] = node_id
            self.node_list[node_id] = [x,y]

def addNode(self, node_id, x, y):
    if self.node_list[node_id] != None:

raise exception("This node ID already exists.")

else:

if self.spacialMatrix[x][y] != None:
        raise exception("This node position is already populated.")

else:
        self.spacialMatrix[x][y] = node_id
        self.node_list[node_id] = [x,y]

class NodeError(exception):
    """
    For node related errors.
    """
    pass

def addNode(self, node_id, x, y):
    if self.node_list[node_id] is not None:
        raise NodeError("This node ID already exists.")
    if self.spacialMatrix[x][y] is not None:
        raise NodeError("This node position is already populated.")
    self.spacialMatrix[x][y] = node_id
    self.node_list[node_id] = [x,y]

def addNode(self, node_id, x, y):
    assert self.node_list[node_id] is None
    assert self.spacialMatrix[x][y] is None

    self.spacialMatrix[x][y] = node_id
    self.node_list[node_id] = [x, y]