def add_node(self, node, weight=None):
        """
        Adds a single node to the Graph.

        Parameters
        ----------
        node: str, int, or any hashable python object.
            The node to add to the graph.

        weight: int, float
            The weight of the node.

        Examples
        --------
        >>> from pgmpy.base import DAG
        >>> G = DAG()
        >>> G.add_node(node='A')
        >>> sorted(G.nodes())
        ['A']

        Adding a node with some weight.
        >>> G.add_node(node='B', weight=0.3)

        The weight of these nodes can be accessed as:
        >>> G.node['B']
        {'weight': 0.3}
        >>> G.node['A']
        {'weight': None}
        """

        # Check for networkx 2.0 syntax
        if isinstance(node, tuple) and len(node) == 2 and isinstance(node[1], dict):
            node, attrs = node
            if attrs.get('weight', None) is not None:
                attrs['weight'] = weight
        else:
            attrs = {'weight': weight}

        super(DAG, self).add_node(node, weight=weight)
Adds a single node to the Graph.

        Parameters
        ----------
        node: str, int, or any hashable python object.
            The node to add to the graph.

        weight: int, float
            The weight of the node.

        Examples
        --------
        >>> from pgmpy.base import DAG
        >>> G = DAG()
        >>> G.add_node(node='A')
        >>> sorted(G.nodes())
        ['A']

        Adding a node with some weight.
        >>> G.add_node(node='B', weight=0.3)

        The weight of these nodes can be accessed as:
        >>> G.node['B']
        {'weight': 0.3}
        >>> G.node['A']
        {'weight': None}