Sync up behavior of is_{type} for empty graphs

networkx/algorithms/chordal.py

@@ -339,6 +339,8 @@ def _find_chordality_breaker(G, s=None, treewidth_bound=sys.maxsize):
 
     It ignores any self loops.
     """
+    if len(G) == 0:
+        raise nx.NetworkXPointlessConcept("Graph has no nodes.")
     unnumbered = set(G)
     if s is None:
         s = arbitrary_element(G)

networkx/algorithms/dag.py

@@ -641,7 +641,8 @@ def is_aperiodic(G):
     """
     if not G.is_directed():
         raise nx.NetworkXError("is_aperiodic not defined for undirected graphs")
-
+    if len(G) == 0:
+        raise nx.NetworkXPointlessConcept("Graph has no nodes.")
     s = arbitrary_element(G)
     levels = {s: 0}
     this_level = [s]

networkx/algorithms/distance_regular.py

@@ -147,6 +147,8 @@ def intersection_array(G):
     global_parameters
     """
     # test for regular graph (all degrees must be equal)
+    if len(G) == 0:
+        raise nx.NetworkXPointlessConcept("Graph has no nodes.")
     degree = iter(G.degree())
     (_, k) = next(degree)
     for _, knext in degree:

networkx/algorithms/regular.py

@@ -29,6 +29,8 @@ def is_regular(G):
     True
 
     """
+    if len(G) == 0:
+        raise nx.NetworkXPointlessConcept("Graph has no nodes.")
     n1 = nx.utils.arbitrary_element(G)
     if not G.is_directed():
         d1 = G.degree(n1)