Returns a truncated random walk.

        path_length: Length of the random walk.
        alpha: probability of restarts.
        start: the start node of the random walk.

def random_walk(self, path_length, alpha=0, rand=random.Random(), start=None):
    """ Returns a truncated random walk.

        path_length: Length of the random walk.
        alpha: probability of restarts.
        start: the start node of the random walk.
    """
    G = self
    if start:
      path = [start]
    else:
      # Sampling is uniform w.r.t V, and not w.r.t E
      path = [rand.choice(list(G.keys()))]

    while len(path) < path_length:
      cur = path[-1]
      if len(G[cur]) > 0:
        if rand.random() >= alpha:
          path.append(rand.choice(G[cur]))
        else:
          path.append(path[0])
      else:
        break
    return [str(node) for node in path]