from collections import ChainMap 

prio_1 = {"param_1": "foo"} 
prio_2 = {"param_1": "foobar", "param_2": "bar"} 

combined = ChainMap(prio_1, prio_2) 

print(combined["param_1"]) # outputs 'foo' 
print(combined["param_2"]) # outputs 'bar'

The param_1 from the prio_1 dictionary is dominant, so it isn’t overwritten by the key-value pair from prio_2 dict. prio_2 dict just adds the param_2 because it is not yet in the ChainMap.

The nice thing is that no information is lost, the ChainMap object contains both dictionaries.

You can use it for example in a situation where you want to chain arguments from argparse, os.environ and default parameters. See here

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