graph = {
1: [2, 3],
2: [4]
}

  
>>> def walk_graph(tree, node):
...     yield node
...     if node in tree:
...         for i in tree[node]:
...             for j in walk_graph(tree, i):
...                 yield j
... 
>>> graph = {
...     1: [2, 3],
...     2: [4]
... }
>>> 
>>> tuple(walk_graph(graph, 1))
(1, 2, 4, 3)
>>>

 >>> set(reduce(add,graph.values())+graph.keys())
set([1, 2, 3, 4])

 graph = {
    1: [2, 3],
    2: [3, 4],
    4: [1]
}