Replace NetworkX by RetworkX

doc/releases/changelog-dev.md

@@ -122,7 +122,12 @@
 * Interferometer is now a class with `shape` method.
   [(#1946)](https://github.com/PennyLaneAI/pennylane/pull/1946)
 
-* Replace NetworkX by RetworkX in CircuitGraph and add RetworkX support to QAOA.
+* The `CircuitGraph`, used to represent circuits via directed acyclic graphs, now
+  uses RetworkX for its internal representation. This results in significant speedup
+  for algorithms that rely on a directed acyclic graph representation.
+  [(#1791)](https://github.com/PennyLaneAI/pennylane/pull/1791)
+
+* The QAOA module now accepts both NetworkX and RetworkX graphs as function inputs.
   [(#1791)](https://github.com/PennyLaneAI/pennylane/pull/1791)
 
 <h3>Breaking changes</h3>

pennylane/circuit_graph.py

@@ -177,7 +177,9 @@ def __init__(self, ops, obs, wires, par_info=None, trainable_params=None):
             # This operator does not depend on any others
 
             # Check if wire[0] in self._grid.values()
-            # is already added to the graph
+            # is already added to the graph; this
+            # condition avoids adding new nodes with
+            # the same value but different indexes
             if wire[0] not in self._graph.nodes():
                 self._graph.add_node(wire[0])
 
@@ -189,6 +191,8 @@ def __init__(self, ops, obs, wires, par_info=None, trainable_params=None):
                     self._graph.add_node(wire[i])
 
                 # Create an edge between this and the previous operator
+                # There isn't any default value for the edge-data in
+                # rx.PyDiGraph.add_edge(); this is set to an empty string
                 self._graph.add_edge(
                     self._graph.nodes().index(wire[i - 1]), self._graph.nodes().index(wire[i]), ""
                 )
@@ -336,6 +340,7 @@ def ancestors(self, ops):
         anc = set(
             self._graph.get_node_data(n)
             for n in set().union(
+                # rx.ancestors() returns node indexes instead of node-values
                 *(rx.ancestors(self._graph, self._graph.nodes().index(o)) for o in ops)
             )
         )
@@ -353,6 +358,7 @@ def descendants(self, ops):
         des = set(
             self._graph.get_node_data(n)
             for n in set().union(
+                # rx.descendants() returns node indexes instead of node-values
                 *(rx.descendants(self._graph, self._graph.nodes().index(o)) for o in ops)
             )
         )
@@ -676,8 +682,13 @@ def has_path(self, a, b):
 
         return (
             len(
-                rx._digraph_dijkstra_shortest_path(
-                    self._graph, self._graph.nodes().index(a), self._graph.nodes().index(b)
+                rx.digraph_dijkstra_shortest_paths(
+                    self._graph,
+                    self._graph.nodes().index(a),
+                    self._graph.nodes().index(b),
+                    weight_fn=None,
+                    default_weight=1.0,
+                    as_undirected=False,
                 )
             )
             != 0