Python inverted dictionaries

Question

I'm currently writing a function that takes a dictionary with immutable values and returns an inverted dictionary. So far, my code is getting extremely simple tests right, but it still has some kinks to work out

def dict_invert(d): inv_map = {v: k for k, v in d.items()} return inv_map list1 = [1,2,3,4,5,6,7,8] list2 = {[1]:3245,[2]:4356,[3]:6578} d = {['a']:[],['b']:[]} d['a'].append(list1) d['b'].append(list2)

How do I fix my code so that it passes the test cases?

My only thoughts are to change list 2 to [1:32, 2:43, 3:54, 4:65]; however, I would still have a problem with having the "[]" in the right spot. I have no idea how to do that.

Your oddly-named list2 and your d shouldn't even run: lists aren't hashable, so you can't use them as dictionary keys. — DSM
– DSM, Commented Mar 15, 2016 at 16:08
@DSM -- That was my first thought too, but If they really are after the return statement, then they don't run (which might explain why some of the tests pass?) — mgilson
– mgilson, Commented Mar 15, 2016 at 16:10

Jimmy Huch · Accepted Answer · 2016-03-19 09:12:06Z

The trick is to realize that multiple keys can have the same values, so when inverting, you must make sure your values map to a list of keys.

from collections import defaultdict def dict_invert(d): inv_map = defaultdict(list) for k, v in d.items(): inv_map[v].append(k) return inv_map

EDIT:

Just adding a bit of more helpful info...

The defaultdict(list) makes the default value of the dict = list() when accessed via [] or get (when normally it would raise KeyError or return None respectively).

With that defaultdict in place, you can use a bit of logic to group keys together... here's an example to illustrate (from my comment above)

Original dict: K0 -> V0, K1 -> V0, K2 -> V0

Should invert to: V0 -> [K0, K1, K2]

EDIT 2:

Your tests seem to be forcing you into using a normal dict, in which case...

def dict_invert(d): inv_map = {} for k, v in d.items(): if v not in inv_map: inv_map[v] = [] inv_map[v].append(k) return inv_map

Now it's outputting defaultdict(<type 'list'>, {}) for every test. Any idea why it's doing that? Do i need to add a dict.get?

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