Wrest a dictionary of lists (e.g., turn a dict of nodes as keys and lists of groups as values into a dict of groups as keys and lists of nodes as values)

wrest_dict.py

def wrest_dict(orig):
  pairs = [(g, n) for n in orig.keys() for g in orig[n]]
  nodes4group = lamdba g: sorted(n[1] for n in pairs if n[0] == g)
  return {g[0]: nodes4group(g[0]) for g in pairs}