Python merge records (lists of dicts) by key

merge.py

def merge_records_by_key(dicts1, dicts2, key):
    """Updates each dict1 containing key `key` from `dicts1`
    with the corresponding dict2 from `dicts2` having
    dict1[key] == dict2[key].
    """
    if len(dicts1) > len(dicts2):
        probe, build = dicts1, dicts2

        hashmap = {d[key]: d for d in build if key in d}

        def up(d):
            d.update(hashmap[d[key]])
    else:
        probe, build = dicts2, dicts1

        hashmap = {}
        for d in build:
            if key in d:
                hashmap.setdefault(d[key], []).append(d)

        def up(d):
            for x in hashmap[d[key]]:
                x.update(d)

    for d in probe:
        with suppress(KeyError):
            up(d)