>>> from collections import ChainMap
>>> default_struct = {'key': {'key1': 0, 'key2': 0}}
>>> struct = {'key': {'key1': 666}}
>>> context = dict(ChainMap(struct, default_struct))
>>> context
{'key': {'key1': 666}}
>>> context = dict(ChainMap(default_struct, struct))
>>> context
{'key': {'key1': 0, 'key2': 0}}