Comment by abuckenheimer

I feel the same way, although my instinct is generally to build a custom generator. Only costs a couple lines but is plain old python and quite explicit

    target = {'system': {'planets': [{'name': 'earth', 'moons': 1},
                                     {'name': 'jupiter', 'moons': 69}]}}

    glom(target, {'moon_count': ('system.planets', ['moons'], sum)})
    # vs
    def iter_moons(t):
        for planet in target['system']['planets']:
            yield planet['moons']

    sum(iter_moons(target))

would have to combine with `defaultdict`s if your nested data is only sometimes there though