Improve test coverage for mst.py and bug fix in prim_mst_edges() (#6486)

* Improve test coverage for Kruskal & Prim

* Bug fix in prim_mst_edges()

1 files changed, 10 insertions, 0 deletions

diff --git a/networkx/algorithms/tree/mst.py b/networkx/algorithms/tree/mst.py
index a866aeef..8ec693d6 100644
--- a/networkx/algorithms/tree/mst.py
+++ b/networkx/algorithms/tree/mst.py
@@ -334,12 +334,22 @@ def prim_mst_edges(G, minimum, weight="weight", keys=True, data=True, ignore_nan
                         continue
                     for k2, d2 in keydict.items():
                         new_weight = d2.get(weight, 1) * sign
+                        if isnan(new_weight):
+                            if ignore_nan:
+                                continue
+                            msg = f"NaN found as an edge weight. Edge {(v, w, k2, d2)}"
+                            raise ValueError(msg)
                         push(frontier, (new_weight, next(c), v, w, k2, d2))
             else:
                 for w, d2 in G.adj[v].items():
                     if w in visited:
                         continue
                     new_weight = d2.get(weight, 1) * sign
+                    if isnan(new_weight):
+                        if ignore_nan:
+                            continue
+                        msg = f"NaN found as an edge weight. Edge {(v, w, d2)}"
+                        raise ValueError(msg)
                     push(frontier, (new_weight, next(c), v, w, d2))