You can't add extra attributes that weren't part of the original dataclass definition:

  >>> from dataclasses import dataclass
  >>> @dataclass
  ... class C: pass
  ... 
  >>> C().x = 1
  >>> @dataclass(slots=True)
  ... class D: pass
  ... 
  >>> D().x = 1
  Traceback (most recent call last):
    File "<python-input-4>", line 1, in <module>
      D().x = 1
      ^^^^^
  AttributeError: 'D' object has no attribute 'x' and no __dict__ for setting new attributes

Also some of the introspection stops working e.g. vars().

If you're using dataclasses it's less of an issue because dataclasses.asdict.

I rarely need to dynamically add attributes myself on dataclasses like this but unfortunately this also means things like `@cached_property` won't work because it can't internally cache the method result anywhere.