Is there a way to preserve duplicate keys in python dictionary [duplicate]

I can think of two simple options, assuming you want to keep using a dictionary.

1. You could map keys to lists of items. A defaultdict from the collections module makes this easy.

   >>> import collections
   >>> data = collections.defaultdict(list)
   >>> for k, v in (('a', 'b'), ('a', 'c'), ('b', 'c')):
   ...     data[k].append(v)
   ... 
   >>> data
   defaultdict(<type 'list'>, {'a': ['b', 'c'], 'b': ['c']})
   

2. You could use additional data to disambiguate the keys. This could be a timestamp, a unique id number, or something else. This has the advantage of preserving a one-to-one relationship between keys and values, and the disadvantage of making lookup more complex, since you always have to specify an id. The example below shows how this might work; whether it's good for you depends on the problem domain:

   >>> for k, v in (('a', 'b'), ('a', 'c'), ('b', 'c')):
   ...     i = 0
   ...     while (k, i) in data:
   ...         i += 1
   ...     data[(k, i)] = v
   ... 
   >>> data
   {('a', 1): 'c', ('b', 0): 'c', ('a', 0): 'b'}
   

While I'm not 100% sure, I'm pretty sure the answer is no. That sort of violates the purpose of a dictionary in python. How about you change the value to lists so instead of

{Key:value}

you have

{Key:[Value1,value2]}

An alternative to the defaultdict might be

d = {}
d.setdefault(newkey, []).append(newvalue)

This does the same: append newvalue to a list which is either already in the dictionary at the given newkey, or if not, will be put there.