This section seems to agree with me, where it explains why normal classes can’t be subclassed to protocols:
> Now, C is a subtype of Proto, and Proto is a subtype of Base. But C cannot be a subtype of Base (since the latter is not a protocol). This situation would be really weird. In addition, there is an ambiguity about whether attributes of Base should become protocol members of Proto.
https://peps.python.org/pep-0544/#protocols-subclassing-norm...
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I’m not sure why you think it’s “obvious” that you can use a third party library to solve the problem — or why that addresses my complaint that the built-in type system doesn’t work for that.
If anything, the existence of a third party library hints the standard library doesn’t cover the use case.
https://peps.python.org/pep-0544/#:~:text=Structural%20subty....
> substituted-for class to be defined as a protocol.
... which is false
> Similarly, the heading on the dictionary says “for a fixed set of keys” — but what if I want a dynamic heterodox dict? Eg, unpacking JSON.
You can quite obviously decode to recursive types (e.g. using pydantic), not sure what the problem is.