Combining Dictionaries Of Lists In Python
dictionary, list, python
Solution
If the list of tuples is sorted, `itertools.groupby`, as suggested by @gnibbler, is not a bad alternative to `defaultdict`, but it needs to be used differently than he suggested:
import itertools
import operator
def lot_to_dict(lot):
key = operator.itemgetter(0)
# if lot's not sorted, you also need...:
# lot = sorted(lot, key=key)
# NOT in-place lot.sort to avoid changing it!
grob = itertools.groupby(lot, key)
return dict((k, [v[1] for v in itr]) for k, itr in grob)
For "merging" dicts of lists into a new d.o.l...:
def merge_dols(dol1, dol2):
keys = set(dol1).union(dol2)
no = []
return dict((k, dol1.get(k, no) + dol2.get(k, no)) for k in keys)
I'm giving `[]` a nickname `no` to avoid uselessly constructing a lot of empty lists, given that performance is important. If the sets of the dols' keys overlap only modestly, faster would be:
def merge_dols(dol1, dol2):
result = dict(dol1, **dol2)
result.update((k, dol1[k] + dol2[k])
for k in set(dol1).intersection(dol2))
return result
since this uses list-catenation only for overlapping keys -- so, if those are few, it will be faster.
Problem
I have a very large collection of (p, q) tuples that I would like to convert into a dictionary of lists where the first item in each tuple is a key that indexes a list that contains q. Example: ``` Original List: (1, 2), (1, 3), (2, 3) Resultant Dictionary: {1:[2, 3], 2:[3]} ``` Furthermore, I would like to efficiently combine these dictionaries. Example: ``` Original Dictionaries: {1:[2, 3], 2:[3]}, {1:[4], 3:[1]} Resultant Dictionary: {1:[2, 3, 4], 2:[3], 3:[1]} ``` These operations reside within an inner loop, so I would prefer that they be as fast as possible. Thanks in advance