>>> d1={'a':1,'b':2}
>>> {value:key for key,value in d1.iteritems()}
{1: 'a', 2: 'b'}
>>> {value:key for key,value in d1.iteritems()}[2]
'b'

>>> d1={'a':1,'b':2}
>>> dict((value,key) for key,value in d1.iteritems())
{1: 'a', 2: 'b'}

>>> d1={'a':1,'b':2,'c':1}
>>> d=defaultdict(list)
>>> for k,v in d1.iteritems():
... d[v].append(k)
... 
>>> d
defaultdict(, {1: ['a', 'c'], 2: ['b']})