Creating a set with dataclass and dict

[hobbitdur]

Hi everyone.
I am trying to play with the new @dataclass and TypedDict from 3.7 and 3.8
But after searching and reading lots of documentation I cannot find a proper answer on this. I think the answer is simple but it's not clear to me.
Here the snippet:

from dataclasses import dataclass


@dataclass(frozen=True, eq=True, repr=False)
class Building:
    name: str
    property: dict


foo = Building(name="Test", property={"key": 10})
var = {foo}

and the error:

  File "<string>", line 3, in __hash__
TypeError: unhashable type: 'dict'

Ok the message is clear, but I don't understand why all nested element of a set need to be hashable. I don't find anything that tell me: a set take only hashable element that have itself hashable element

Obviously if I replace the property with a hashable object it works.

Thank you for your help

Hobbit