Interesting test failure from docbuild here:

Document: getting/tutorial
--------------------------
**********************************************************************
File "getting/tutorial.rst", line 78, in default
Failed example:
    speed = 23 * ureg.snail_speed
Expected:
    Traceback (most recent call last):
    ...
    UndefinedUnitError: 'snail_speed' is not defined in the unit registry
Got:
    Traceback (most recent call last):
      File "/opt/hostedtoolcache/Python/3.10.14/x64/lib/python3.10/doctest.py", line 1350, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest default[0]>", line 1, in <module>
        speed = 23 * ureg.snail_speed
      File "<string>", line 4, in __setattr__
    dataclasses.FrozenInstanceError: cannot assign to field 'name'
**********************************************************************

I'm a bit stuck on what to do next with this one.

CodSpeed Performance Report

Merging #2037 will not alter performance

_{Comparing simonw:python-3.13 (01ea994) with master (1e46b2e)}

Summary

✅ 437 untouched benchmarks

While I have been a long proponent of making exception immutable (i.e. frozen when using a dataclasses) this has bit me many times. Do you know what is the change in Python 3.13 that makes this required?

See: https://github.com/python/cpython/blame/3.13/Lib/dataclasses.py#L1035-L1044
It seems that was always meant to be like that, but it was not correctly checked before for multiple inheritance

I've tried to play with it, but haven't found any good solution with keeping the dataclasses frozen in errors.py. Even if you use functools.cached_property, or other way to avoid mutation within, the errors will eventually get mutated, setting __cause__, etc. How does freezing a class that inherits non-frozen one supposed to work?

I'm not experienced enough to be able to help here, sorry.
Wouldn't using __slots__ make sense?

@hgrecco would it be ok to move them all to frozen=False in order to unblock this issue? The doctest failure is fairly weird, because if you simply raise pint.errors.UndefinedUnitError("anything") you do not get the same traceback issue. I couldn't find the cause of this issue either. What I was able to find is that ureg.Quantity("10 * snail_speed") does output the correct exception 🤷