There are some DocTest failures in python 3 in lines like this. The order of the keys in dictionaries changes between python versions (and also between platforms). Maybe we could just skip the last two lines of the example and end with  `>>> flow, F = nx.preflow_push(G, 'x', 'y')` or do something more elaborated such as:

```
print(flow)
for node, nbrs in sorted(F.items()):
    print(node)
    for n, value in sorted(nbrs.items()):
         print('  {0} : {1}'.format(n, value))
```