We wrote this code on the document viewer
in class on April 10, 2012

def groups(filename):
    
    d = {}

    f = open(filename)

    for line in f:
        
        line = line.strip()   # ok to leave out
        
        s = line.split(":")

        stg = s[0]                # student groups
        nets = s[1].split(",")    # netids

        for n in nets:

            if n in d:
                d[n].append(stg)  # append/add to existing list
            else:
                d[n] = [stg]      # list with one element


    f.close()   # can leave out

    return {}