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up vote 7 down vote favorite 
def bfs(graph, root):
    visited, queue = [], [root]
    while queue:
        vertex = queue.pop(0)
        for w in graph[vertex]:
            if w not in visited:
                visited.append(w)
                queue.append(w)

graph = {0: [1, 2], 1: [2], 2: []}
bfs(graph, 0)

Does this look like a correct implementation of BFS in Python 3?

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up vote 10 down vote accepted
  • sets perform containing checks (w in visited) \$O(1)\$ rather than \$O(n)\$ for lists.
  • collections.deque are better than lists for poping elements at the front (popleft).
  • you should put your example code under an if __name__ == '__main__' clause.
  • w as a variable name does not convey meaning, you should try to come up with something more explicit.
import collections


def breadth_first_search(graph, root): 
    visited, queue = set(), collections.deque([root])
    while queue: 
        vertex = queue.popleft()
        for neighbour in graph[vertex]: 
            if neighbour not in visited: 
                visited.add(neighbour) 
                queue.append(neighbour) 


if __name__ == '__main__':
    graph = {0: [1, 2], 1: [2], 2: []} 
    breadth_first_search(graph, 0)
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