Handle the case is_multigraph and max_edges is None (#15)

hypothesis_networkx/strategy.py

@@ -131,15 +131,15 @@ def graph_builder(draw,
         # Correct for number of edges already made if graph is connected.
         # This may mean we added more edges than originally allowed.
         max_edges -= len(graph.edges)
-    if max_edges < 0:
-        max_edges = 0
+        if max_edges < 0:
+            max_edges = 0
 
     # Likewise for min_edges
     # We already added some edges, so subtract those.
     min_edges -= len(graph.edges)
     if min_edges < 0:
         min_edges = 0
-    elif min_edges > max_edges:
+    elif max_edges is not None and min_edges > max_edges:
         min_edges = max_edges
 
     def edge_filter(edge):

tests/test_graph_builder.py

@@ -103,6 +103,13 @@ def test_graph_builder(data):
             assert not connected or nx.is_connected(graph)
 
 
+def test_multigraph_max_edges_none():
+    try:
+        graph_builder(graph_type=nx.MultiGraph, max_edges=None).example()
+    except Exception:
+        pytest.fail()
+
+
 @settings(deadline=None)
 @given(st.data())
 def test_node_edge_data(data):