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Comment by ssl232 | Hacker News Reader

TIL about dict reverse mapping in 3.10. This will be rather useful:

    people = {
        "Diane": 70,
        "Bob": 78,
        "Emma": 84
    }

    keys = people.keys()
    # dict_keys(['Diane', 'Bob', 'Emma'])

    keys.mapping["Bob"]
    # 78

EDIT: I actually misunderstood this; see comments. The keys.mapping field does not point to the original dictionary. I guess this is actually just about consistency with the way other mappings work? Not so useful (to me) after all.

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Why would you want a collection of keys to behave like the mapping they came from? Why not pass around the dict itself, when you need access to the values? This seems like it extends the responsibilities of "keys" beyond what they should care about, and the mental overhead grows with it.

Or is this about read-only access? There are better, more explicit ways to accomplish that.

I do not see the use for this, over the standard access methods.

    people = {
        "Diane": 70,
        "Bob": 78,
        "Emma": 84
    }

    people.get("Bob")
    # 78

The common usecase is:

    for key in people.keys():
        people.get(key)

    keys = people.keys()
    # dict_keys(['Diane', 'Bob', 'Emma'])

    for key in keys:
        keys.mapping[key]

What is the advantage?

It gives you access to the original dictionary from a view. I've not played with it yet but I guess that means you can modify it.

EDIT: apparently not:

    >>> keys.mapping
    mappingproxy({'Diane': 70, 'Bob': 78, 'Emma': 84})

In this case I actually don't know what the use case is :shrug:

The rationale was here.

https://github.com/python/cpython/issues/85067#issuecomment-...

It seems a bit vague: « Exposing this attribute would help with introspection, making it possible to write efficient functions that operate on dict views. »

I don’t quite get the usecase for it. You’ve got the original dictionary that gives you exactly the same thing. There’s obviously something I’m not thinking of here but I’m not sure why I would want to trade the dictionary for a set of keys, that’s also the dictionary.

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