Map list of tuples into a dictionary

Question

I've got a list of tuples extracted from a table in a DB which looks like (key , foreignkey , value). There is a many to one relationship between the key and foreignkeys and I'd like to convert it into a dict indexed by the foreignkey containing the sum of all values with that foreignkey, i.e. { foreignkey , sumof( value ) }. I wrote something that's rather verbose:

myDict = {}
for item in myTupleList:
    if item[1] in myDict:
        myDict [ item[1] ] += item[2]
    else:
        myDict [ item[1] ] = item[2]

but after seeing this question's answer or these two there's got to be a more concise way of expressing what I'd like to do. And if this is a repeat, I missed it and will remove the question if you can provide the link.

Answer 1

Assuming all your values are ints, you could use a defaultdict to make this easier:

from collections import defaultdict

myDict = defaultdict(int)

for item in myTupleList:
    myDict[item[1]] += item[2]

defaultdict is like a dictionary, except if you try to get a key that isn't there it fills in the value returned by the callable - in this case, int, which returns 0 when called with no arguments.

UPDATE: Thanks to @gnibbler for reminding me, but tuples can be unpacked in a for loop:

from collections import defaultdict

myDict = defaultdict(int)

for _, key, val in myTupleList:
    myDict[key] += val

Here, the 3-item tuple gets unpacked into the variables _, key, and val. _ is a common placeholder name in Python, used to indicate that the value isn't really important. Using this, we can avoid the hairy item[1] and item[2] indexing. We can't rely on this if the tuples in myTupleList aren't all the same size, but I bet they are.

(We also avoid the situation of someone looking at the code and thinking it's broken because the writer thought arrays were 1-indexed, which is what I thought when I first read the code. I wasn't alleviated of this until I read the question. In the above loop, however, it's obvious that myTupleList is a tuple of three elements, and we just don't need the first one.)

Answer 2

from collections import defaultdict

myDict = defaultdict(int)

for _, key, value in myTupleList:
    myDict[key] += value