pythonic dictionary inversion

dict-construction-I.py

# example: dictionary that has students as keys and within each entry has a list of teachers that teach that student
# this snippet will use that dictionary to create a dictionary with teachers as keys and within those entires have a list 
# of students taught by that teacher
for e in dic.keys():
  for d in dic[e]:
    if not data.get(d):
      data[d] = set()
    data[d].add(e)
    
#simplified
for e,sub_dic in dic.iteritems():
  for d in sub_dic:
    data.setdefault(d, set())
    data[d].add(e)