Don't know about elegant. The following doesn't have the O(N^2) performance: def merge_counts(mylist): thing_lookup = {} # map thing -> item

Don't know about elegant. The following doesn't have the O(N^2) performance: def merge_counts(mylist): thing_lookup = {} # map thing -> item newlist = [] for item in mylist: thing = item["thing"] if thing in thing_lookup: # seen before; update count. thing_lookup[thing]["count"] += item["count"] else: # first time I've seen this thing newlist.append(item) thing_lookup[thing] = item return newlist It, like the original, mutates the original dictionary. I would prefer: # first time I've seen this thing thing = thing.copy() # don't mutate the original newlist.append(item)