Stateful closure

Harder to read, but fits function first API design.

def increment_with_report(current, increments):
    added_count = 0
    def missing():
        nonlocal added_count  # Stateful closure
        added_count += 1
    return 0

result = defaultdict(missing, current)
for key, amount in increments:
    result[key] += amount
return result, added_count”