pf, descriptors: Add degree function for compatibility with NX2.0

pypsa/descriptors.py

@@ -89,6 +89,13 @@ def __init__(self, data=None, **attr):
 else:
     raise ImportError("NetworkX version {} is too old. At least 1.10 is needed.".format(nx.__version__))
 
+if _nx_version >= '2.0':
+    def degree(G):
+        return G.degree()
+else:
+    def degree(G):
+        return G.degree_iter()
+
 class Dict(dict):
     """
     Dict is a subclass of dict, which allows you to get AND SET

pypsa/pf.py

@@ -43,7 +43,7 @@
 from itertools import chain
 import time
 
-from .descriptors import get_switchable_as_dense, allocate_series_dataframes, Dict, zsum
+from .descriptors import get_switchable_as_dense, allocate_series_dataframes, Dict, zsum, degree
 
 pd.Series.zsum = zsum
 
@@ -836,7 +836,7 @@ def find_tree(sub_network, weight='x_pu'):
     sub_network.tree = nx.minimum_spanning_tree(graph)
 
     #find bus with highest degree to use as slack
-    tree_slack_bus, slack_degree = max(sub_network.tree.degree_iter(), key=itemgetter(1))
+    tree_slack_bus, slack_degree = max(degree(sub_network.tree), key=itemgetter(1))
     logger.info("Tree slack bus is %s with degree %d.", tree_slack_bus, slack_degree)
 
     #determine which buses are supplied in tree through branch from slack