我有一棵树作为广度优先搜索的输入，我想知道算法在哪一级进行？

# Breadth First Search Implementation
graph = { 
    'A':['B','C','D'],
    'B':['A'],
    'C':['A','E','F'],
    'D':['A','G','H'],
    'E':['C'],
    'F':['C'],
    'G':['D'],
    'H':['D']
    }


def breadth_first_search(graph,source):
    """
    This function is the Implementation of the breadth_first_search program
    """
    # Mark each node as not visited
    mark = {}
    for item in graph.keys():
        mark[item] = 0

    queue, output = [],[]

    # Initialize an empty queue with the source node and mark it as explored
    queue.append(source)
    mark[source] = 1
    output.append(source)

    # while queue is not empty
    while queue:
        # remove the first element of the queue and call it vertex
        vertex = queue[0]
        queue.pop(0)
        # for each edge from the vertex do the following
        for vrtx in graph[vertex]:
            # If the vertex is unexplored
            if mark[vrtx] == 0:
                queue.append(vrtx)  # mark it as explored
                mark[vrtx] = 1      # and append it to the queue
                output.append(vrtx) # fill the output vector
    return output

print breadth_first_search(graph, 'A')

它需要树作为输入图，我想要的是，在每次迭代时，它都应打印出正在处理的当前级别。