Returns variable nodes present in the graph.

        Before calling this method make sure that all the factors are added
        properly.

        Examples
        --------
        >>> from pgmpy.models import FactorGraph
        >>> from pgmpy.factors.discrete import DiscreteFactor
        >>> G = FactorGraph()
        >>> G.add_nodes_from(['a', 'b', 'c'])
        >>> phi1 = DiscreteFactor(['a', 'b'], [2, 2], np.random.rand(4))
        >>> phi2 = DiscreteFactor(['b', 'c'], [2, 2], np.random.rand(4))
        >>> G.add_nodes_from([phi1, phi2])
        >>> G.add_factors(phi1, phi2)
        >>> G.add_edges_from([('a', phi1), ('b', phi1),
        ...                   ('b', phi2), ('c', phi2)])
        >>> G.get_variable_nodes()
        ['a', 'b']

def get_variable_nodes(self):
        """
        Returns variable nodes present in the graph.

        Before calling this method make sure that all the factors are added
        properly.

        Examples
        --------
        >>> from pgmpy.models import FactorGraph
        >>> from pgmpy.factors.discrete import DiscreteFactor
        >>> G = FactorGraph()
        >>> G.add_nodes_from(['a', 'b', 'c'])
        >>> phi1 = DiscreteFactor(['a', 'b'], [2, 2], np.random.rand(4))
        >>> phi2 = DiscreteFactor(['b', 'c'], [2, 2], np.random.rand(4))
        >>> G.add_nodes_from([phi1, phi2])
        >>> G.add_factors(phi1, phi2)
        >>> G.add_edges_from([('a', phi1), ('b', phi1),
        ...                   ('b', phi2), ('c', phi2)])
        >>> G.get_variable_nodes()
        ['a', 'b']
        """
        self.check_model()

        variable_nodes = set([x for factor in self.factors for x in factor.scope()])
        return list(variable_nodes)