Representing a tree and doing a pre-order traversal

gistfile1.py

from __future__ import print_function

# The tree:
#        a
#    b       c
#  d   e   f   g
# h i j k l m n o

graph = {
    'a': ['b', 'c'],
    'b': ['d', 'e'],
    'c': ['f', 'g'],
    'd': ['h', 'i'],
    'e': ['j', 'k'],
    'f': ['l', 'm'],
    'g': ['n', 'o'],
}

def trace(start, graph):
    out = [start]
    try:
        for key in graph[start]:
            sub = trace(key, graph)
            out.extend(sub)

    except KeyError:
        pass

    return out


if __name__ == '__main__':
    print(trace('a', graph))