Combining Dictionaries Of Lists In Python

Question

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

Answer 1

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.