>>> from collections import defaultdict
>>>
>>> def union_merge(*observations, key="rev_id"):
...   id_map = defaultdict(dict)
...   for obs in observations:
...     for ob in obs:
...       id_map[ob[key]].update(ob)
...
...   for ob in id_map.values():
...     yield ob
...
>>> list(union_merge([{'foo': 1, 'rev_id': 1}], [{'foo': 2, 'rev_id': 1}]))
[{'foo': 2, 'rev_id': 1}]
>>> list(union_merge([{'foo': 1, 'rev_id': 1}], [{'foo': 2, 'rev_id': 1}], [{'foo': 3, 'rev_id': 1}]))
[{'foo': 3, 'rev_id': 1}]