Depth First Search in Python

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algorithm

Python2.7

graph

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, stack = set(), [start]
        while stack:
            vertex = stack.pop(-1)
            if vertex not in visited:
                visited.add(vertex)
                print visited
                stack.extend(graph[vertex] - visited)
    return visited

dfs(graph,'A')
set(['A'])
set(['A', 'B'])
set(['A', 'B', 'D'])
set(['A', 'B', 'E', 'D'])
set(['A', 'B', 'E', 'D', 'F'])
set(['A', 'C', 'B', 'E', 'D', 'F'])
set(['A', 'C', 'B', 'E', 'D', 'F'])

I am trying to do Depth first search and I have printed the visited every time to see what is happening in the visited node. In beginning visited node seems to give correct output but after few iteration visited.add() adds the element in the middle of the set. I am not understanding why this is happening.