Merging dictionary value lists in python

Question

I'm trying to merge three dictionaries, which all have the same keys, and either lists of values, or single values.

one={'a': [1, 2], 'c': [5, 6], 'b': [3, 4]} two={'a': [2.4, 3.4], 'c': [5.6, 7.6], 'b': [3.5, 4.5]} three={'a': 1.2, 'c': 3.4, 'b': 2.3} 

What I need is for all the items in the values to be added to one list.

result={'a': [1, 2, 2.4, 3.4, 1.2], 'c': [5, 6, 5.6, 7.6, 2.3], 'b': [3, 4, 3.5, 4.5, 3.4]} 

I have tried several things, but most put the values into nested lists. E.g.

out=dict((k, [one[k], two.get(k), three.get(k)]) for k in one) {'a': [[1, 2], [2.4, 3.4], 1.2], 'c': [[5, 6], [5.6, 7.6], 3.4], 'b': [[3, 4], [3.5, 4.5], 2.3]} 

I tried updating it by looping through the values:

out.update((k, [x for x in v]) for k,v in out.iteritems()) 

but the results was exactly the same. I have tried to simply add the lists, but because the third dictionary has only a float, I couldn't do it.

check=dict((k, [one[k]+two[k]+three[k]]) for k in one) 

So I tried to first add the lists in values of one and two, and then append the value of three. Adding the lists worked well, but then when I tried to append the float from the third dictionary, suddenly the whole value went to 'None'

check=dict((k, [one[k]+two[k]]) for k in one) {'a': [[1, 2, 2.4, 3.4]], 'c': [[5, 6, 5.6, 7.6]], 'b': [[3, 4, 3.5, 4.5]]} new=dict((k, v.append(three[k])) for k,v in check.items()) {'a': None, 'c': None, 'b': None} 

Answer 1

As a one-liner, with a dictionary comprehension:

new = {key: value + two[key] + [three[key]] for key, value in one.iteritems()} 

This creates new lists, concatenating the list from one with the corresponding list from two, putting the single value in three into a temporary list to make concatenating easier.

Or with a for loop updating one in-place:

for key, value in one.iteritems():     value.extend(two[key])     value.append(three[key]) 

This uses list.extend() to update original list in-place with the list from two, and list.append() to add the single value from three.

Where you went wrong:

• your first attempt creates a new list with the values from one, two and three nested within rather than concatenating the existing lists. Your attempt to clean that up just copied those nested lists across.

• Your second attempt didn't work because the value in three is not a list so could not be concatenated. I created a new list just for that one value.

• Your last attempt should not have used list.append() in a generator expression, because you store the return value of that method, which is always None (its change is stored in v directly and the list doesn't need returning again).

Demo of the first approach:

>>> one={'a': [1, 2], 'c': [5, 6], 'b': [3, 4]} >>> two={'a': [2.4, 3.4], 'c': [5.6, 7.6], 'b': [3.5, 4.5]} >>> three={'a': 1.2, 'c': 3.4, 'b': 2.3} >>> {key: value + two[key] + [three[key]] for key, value in one.iteritems()} {'a': [1, 2, 2.4, 3.4, 1.2], 'c': [5, 6, 5.6, 7.6, 3.4], 'b': [3, 4, 3.5, 4.5, 2.3]}