Bug of defaultdict?

buggy_defaultdict.py

import collections

def dfs(G, s, path=[]):
    global explored
    explored.add(s)
    path.append(s)
    for v in G[s]:
        if v not in path:
            path = dfs(G, v, path)
    return path

def dfsloop(G):
    for v in G:  # No error if "for v in G.keys():"
        if v not in explored:
            dfs(G, v)


G = collections.defaultdict(list)

G[1] = [9, 8, 2, 0, 6]
G[2] = [9, 0, 8]
G[3] = [1, 6, 8]
G[6] = [8, 9, 2, 0]
G[7] = [6, 1, 2, 3, 9]
G[8] = [0, 9]
G[9] = [0]


# "for v in G" and "for v in G.keys()" should be the same
print 'for v in G:'
for v in G:
    print v

print 'for v in G.keys():'
for v in G.keys():
    print v

explored = set()
dfsloop(G)