def get_partition_function(self):
        r"""
        Returns the partition function for a given undirected graph.

        A partition function is defined as

        .. math:: \sum_{X}(\prod_{i=1}^{m} \phi_i)

        where m is the number of factors present in the graph
        and X are all the random variables present.

        Examples
        --------
        >>> from pgmpy.models import ClusterGraph
        >>> from pgmpy.factors.discrete import DiscreteFactor
        >>> G = ClusterGraph()
        >>> G.add_nodes_from([('a', 'b', 'c'), ('a', 'b'), ('a', 'c')])
        >>> G.add_edges_from([(('a', 'b', 'c'), ('a', 'b')),
        ...                   (('a', 'b', 'c'), ('a', 'c'))])
        >>> phi1 = DiscreteFactor(['a', 'b', 'c'], [2, 2, 2], np.random.rand(8))
        >>> phi2 = DiscreteFactor(['a', 'b'], [2, 2], np.random.rand(4))
        >>> phi3 = DiscreteFactor(['a', 'c'], [2, 2], np.random.rand(4))
        >>> G.add_factors(phi1, phi2, phi3)
        >>> G.get_partition_function()
        """
        if self.check_model():
            factor = self.factors[0]
            factor = factor_product(factor, *[self.factors[i] for i in range(1, len(self.factors))])
            return np.sum(factor.values)