Combining Dictionaries Of Lists In Python

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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.

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