Reduce list to a dictionary of lists

I’m pulling a bunch of rows out of my database, many of which share a similar ID. I want to group the ones with the same ID into a list and generate a map that maps from the ID to the list of results. I have the following code, but it seems like there must be a simpler way of doing it with reduce or dictionary comprehension:

result = {}
for row in rows:
  type_id = row['type_id']
  values = result.get(type_id, [])
  values.append(row)
  result[type_id] = values

Answer

collections.defaultdict is your friend.

collections.defaultdict(list) will automatically create new lists for each key when you index into the defaultdict, so all you need is:

import collections

result = collections.defaultdict(list)
for row in rows:
  result[row['type_id']].append(row)