Support networkx.Graphs as input to Lattice

.pylintdict

@@ -279,6 +279,7 @@ natom
 nd
 ndarray
 nelec
+networkx
 neuropeptide
 nicholas
 nielsen

qiskit_nature/problems/second_quantization/lattice/lattices/lattice.py

@@ -19,11 +19,15 @@
 
 import numpy as np
 
-from retworkx import NodeIndices, PyGraph, WeightedEdgeList, adjacency_matrix
+from retworkx import NodeIndices, PyGraph, WeightedEdgeList, adjacency_matrix, networkx_converter
 from retworkx.visualization import mpl_draw
 
 from qiskit.utils import optionals as _optionals
 
+if _optionals.HAS_NETWORKX:
+    # pylint: disable=unused-import
+    import networkx as nx
+
 if _optionals.HAS_MATPLOTLIB:
     # pylint: disable=unused-import
     from matplotlib.axes import Axes
@@ -109,15 +113,21 @@ class LatticeDrawStyle:
 class Lattice:
     """General Lattice."""
 
-    def __init__(self, graph: PyGraph) -> None:
+    def __init__(self, graph: Union[PyGraph, "nx.Graph"]) -> None:
         """
         Args:
-            graph: Input graph for Lattice. `graph.multigraph` must be False.
+            graph: Input graph for Lattice. Can be provided as ``retworkx.PyGraph``, which is
+                used internally, or, for convenience, as ``networkx.Graph``. The graph
+                cannot be a multigraph.
 
         Raises:
             ValueError: If the input graph is a multigraph.
             ValueError: If the graph edges are non-numeric.
         """
+        if not isinstance(graph, PyGraph):
+            _optionals.HAS_NETWORKX.require_now("Lattice construction from networkx.Graph")
+            graph = networkx_converter(graph)
+
         if graph.multigraph:
             raise ValueError(
                 f"Invalid `graph.multigraph` {graph.multigraph} is given. "

releasenotes/notes/lattice-from-networkx-20c7c8119af77f36.yaml

@@ -0,0 +1,17 @@
+---
+features:
+  - |
+    Add the option to initialize a :class:`~qiskit_nature.problems.second_quantization.lattice.Lattice`
+    from a ``networkx.Graph`` object, which will be internally converted to a ``retworkx.PyGraph``
+    for performance. 
+
+    For example, you can now construct a lattice as
+
+    .. code-block:: python
+
+        import networkx as nx
+        from qiskit_nature.problems.second_quantization.lattice import Lattice
+
+        # 3-regular random graph on 6 nodes
+        graph = nx.generators.random_graphs.random_regular_graph(3, n=6)
+        lattice = Lattice(graph)

test/problems/second_quantization/lattice/lattices/test_lattice.py

@@ -13,12 +13,19 @@
 """Test for Lattice."""
 from test import QiskitNatureTestCase
 
+import unittest
 import numpy as np
 from numpy.testing import assert_array_equal
+
 from retworkx import PyGraph, is_isomorphic
 
+from qiskit.utils import optionals as _optionals
+
 from qiskit_nature.problems.second_quantization.lattice import Lattice
 
+if _optionals.HAS_NETWORKX:
+    import networkx as nx
+
 
 class TestLattice(QiskitNatureTestCase):
     """Test Lattice."""
@@ -134,6 +141,22 @@ def test_to_adjacency_matrix(self):
         target_matrix = np.array([[0, 1, 1], [1, 0, 0], [1, 0, 1]])
         assert_array_equal(lattice.to_adjacency_matrix(), target_matrix)
 
+    @unittest.skipIf(not _optionals.HAS_NETWORKX, "networkx not available.")
+    def test_from_networkx(self):
+        """Test initialization from a networkx graph."""
+        graph = nx.Graph()
+        graph.add_nodes_from(range(5))
+        graph.add_edges_from([(i, i + 1) for i in range(4)])
+        lattice = Lattice(graph)
+
+        target_graph = PyGraph()
+        target_graph.add_nodes_from(range(5))
+        target_graph.add_edges_from([(i, i + 1, 1) for i in range(4)])
+
+        self.assertTrue(
+            is_isomorphic(lattice.graph, target_graph, edge_matcher=lambda x, y: x == y)
+        )
+
     def test_nonnumeric_weight_raises(self):
         """Test the initialization with a graph with non-numeric edge weights raises."""
         graph = PyGraph(multigraph=False)