Question

I came across a BFS code which involves collections and deques but I could not understand it much. I hope some of the pythonistas here can help a n00b out.

from collections import deque

def bfs(g, start):
    queue, enqueued = deque([(None, start)]), set([start])
    while queue:
        parent, n = queue.popleft()
        yield parent, n
        new = set(g[n]) - enqueued
        enqueued |= new
        queue.extend([(n, child) for child in new])

Questions:

1) The |= operator seems to be related to bitwise operations – I have no idea how it relates to a BFS, any hints?

2) popleft() should return only one value from what I understand, so how is it returning parent and n here?

3) Is new the series of nodes visited? If I want the nodes, do I just keep appending them to a list?

Thanks in advance.

Craig

Answer 1

1. a |= b for sets is the same
   as a = a.union(b).

2. popleft() does indeed return
   only one element, which happens to
   be a 2-tuple, and therefore can be
   unpacked into two values.

3. new is the set of not yet
   visited nodes.