I have a tree:

   A
  / \
 B   C
/ \
D E

which is implemented as a dict

tree = {
  'A': set(['B', 'C']),
  'B': set(['D', 'E']), 
  'C': set(),
  'D': set(),
  'E': set(),
}

I want to sort the nodes.

and I don't know how to write a key function for sort() in this situation

so I write a cmp function:

sorted(tree, cmp=lambda x, y: 1 if x in tree[y] else -1 if y in tree[x] else 0)

and it gets ['A', 'C', 'B', 'E', 'D'].

how to convert cmp to key really confused me and it surely need more typing time.

so I disagree the removal