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Comment by kotlin2 | Hacker News Reader

    c = defaultdict(lambda: 42)
    b = defaultdict(lambda: c)
    a = defaultdict(lambda: b)
    a["a"]["b"]["c"]  # --> 42

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Okay fair, I deserve that. I assumed it was obvious I meant arbitrary depth.

Also d["a"] and d["a"]["b"] aren't 42.

If d["a"]["b"] is 42, then how could d["a"]["b"]["c"] also be 42? What you want doesn't make sense semantically. Normally, we'd expect these two statements to be equivalent

d["a"]["b"]["c"] == (d["a"]["b"])["c"]

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."

What languages allow such a construct? It seems like it would be super confusing if these two code samples produced different values:

    # One
    a = d["a"]["b"]["c"]
    
    # Two
    a = d["a"]["b"]
    b = a["c"]

The MagicMock class from unittest package does what you want.

I have a hard time understanding any use case outside of such mocking.

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