I mean you got it but it's something a lot of people want. The semantic reason for it is so you can look up an arbitrary path on a dict and if it's not present get a default, usually None. It can be done by catching KeyError but it has to happen on the caller side which is annoying. I can't make a real nested mapping that returns none if the keys aren't there.

    d = magicdict()
    is42 = d["foo"]["bar"]["baz"]
      # -> You can read any path and get a default if it doesn't exist.

    d["hello"]["world"] = 420 
      # -> You can set any path and d will then contain { "hello": { "world": 420 }

People use things like jmespath to do this but the fundamental issue is that __getitem__ isn't None safe when you want nested dicts. It's a godsend when dealing with JSON.

I feel like we're maybe too in the weeds, I should have just said "now have two expressions in your lambda."