[33] > ~/micromamba/envs/pytorch313/lib/python3.13/dataclasses.py(1319)
....
1308     def fields(class_or_instance):
1309         """Return a tuple describing the fields of this dataclass.
1310
1311         Accepts a dataclass or an instance of one. Tuple elements are of
1312         type Field.
1313         """
1314
1315         # Might it be worth caching this, per class?
1316         try:
1317             fields = getattr(class_or_instance, _FIELDS)
1318         except AttributeError:
1319  ->         breakpoint()
1320             raise TypeError('must be called with a dataclass type or instance') from None
1321
1322         # Exclude pseudo-fields.  Note that fields is sorted by insertion
1323         # order, so the order of the tuple is as the fields were defined.
1324         return tuple(f for f in fields.values() if f._field_type is _FIELD)

(Pdb+) class_or_instance
MyTuple(a=tensor([1.]), b=tensor([2.]), ab=tensor([3.]))
(Pdb+) _FIELDS
'__dataclass_fields__'
(Pdb+) getattr(class_or_instance, _FIELDS)
*** AttributeError: 'MyTuple' object has no attribute '__dataclass_fields__'