https://github.com/networkx/networkx/commit/1203164760f7be90d5c3a50ab7ab99f04453af8f
Fix UnionFind set extraction (#3224)

Paths are not guaranteed to be fully pruned by the current
implementation if subtrees are unioned, e.g.:
> uf = UnionFind()
> uf.union(1, 2)
> uf.union(3, 4)
> uf.union(4, 5)
> uf.union(1, 5)

In the current implementation, parents[1]=3 and parents[2]=1.
Thus, the mere call of networkx.utils.groups(parents) will yield a
wrong result:
[set([2]), set([1, 3, 4, 5])]

This patch fixes this behavior by simply doing a "find" operation
(which, in turn, does full path pruning) on every key before calling
groups.

--- networkx/utils/union_find.py.orig	2019-04-11 14:52:34.000000000 -0600
+++ networkx/utils/union_find.py	2019-09-11 09:27:32.240303742 -0600
@@ -90,6 +90,10 @@ class UnionFind:
             [['x', 'y'], ['z']]
 
         """
+        # Ensure fully pruned paths
+        for x in self.parents.keys():
+            _ = self[x] # Evaluated for side-effect only
+
         # TODO In Python 3.3+, this should be `yield from ...`.
         for block in groups(self.parents).values():
             yield block