new_dict = modify_object_dict(object_dict, object)

object_dict = {}   # dictionary is initially empty
RDD = (sc.parallelize(Objects)
   .map(lambda object: modify_object_dict(object_dict, object))

class Foobar(object):
    def __init__(self, name, x=None, y=None, z=None):
        self.name = name
        self.x = x
        self.y = y
        self.z = z

objects = sc.parallelize([
    {"name": "foo", "x": 1}, {"name": "foo", "y": 3},
    {"name": "bar", "z": 4}
]).map(lambda x: Foobar(**x))

pairs = objects.map(lambda obj: (obj.name, obj))

rdd = pairs.groupByKey().mapValues(lambda iter: ...)

def seq_op(obj_dict, obj):
    # equivalent to modify_object_dict
    # Lets assume it is as simple as this
    obj_dict.update((k, getattr(obj, k)) for k in ("x", "y", "z"))
    return obj_dict

def comb_op(obj_dict_1, obj_dict_2):
    # lets it is a simple union
    obj_dict_1.update(obj_dict_2)
    return obj_dict_1

dicts = pairs.aggregateByKey({}, seq_op, comb_op)

dicts.collectAsMap()
## {'bar': {'x': None, 'y': None, 'z': 4},
##     'foo': {'x': None, 'y': 3, 'z': None}}