work.py

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

def dfs (graph,start,visited=None):
    if visited is None:
        visited=set()
    visited.add(start)
    for next in graph[start]-visited:
        visited.add(next)
        dfs(graph,next,visited)

    return visited

print  dfs(graph,'A')