>>> parent = ts.Context({
...     'cache_pool': {
...         'total_bytes_limit': 5000000
...     },
...     'file_io_concurrency': {
...         'limit': 10
...     }
... })
>>> child = ts.Context({'cache_pool': {
...     'total_bytes_limit': 10000000
... }},
...                    parent=parent)
>>> assert child.parent is parent
>>> parent['cache_pool'].to_json()
{'total_bytes_limit': 5000000}
>>> child['cache_pool'].to_json()
{'total_bytes_limit': 10000000}
>>> child['file_io_concurrency'].to_json()
{'limit': 10}