Python 3.13 (dev) bug: cannot inherit frozen dataclass from a non-frozen one

[simonw]

Running Pint against the current Python 3.13 developer preview throws this error:

pint/__init__.py:18: in <module>
    from .delegates.formatter._format_helpers import formatter
pint/delegates/__init__.py:12: in <module>
    from . import txt_defparser
pint/delegates/txt_defparser/__init__.py:12: in <module>
    from .defparser import DefParser
pint/delegates/txt_defparser/defparser.py:10: in <module>
    from . import block, common, context, defaults, group, plain, system
pint/delegates/txt_defparser/common.py:23: in <module>
    @dataclass(frozen=True)
../../../.pyenv/versions/3.13-dev/lib/python3.13/dataclasses.py:1247: in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
../../../.pyenv/versions/3.13-dev/lib/python3.13/dataclasses.py:1015: in _process_class
    raise TypeError('cannot inherit frozen dataclass from a '
E   TypeError: cannot inherit frozen dataclass from a non-frozen one

[simonw]

Steps to reproduce:

cd /tmp
git clone https://github.com/hgrecco/pint
cd pint
python3.13 -m venv venv
# Or for me that was:
# ~/.pyenv/versions/3.13-dev/bin/python -m venv venv
source venv/bin/activate
python -m pip install pytest
python -m pip install -e .
pytest

[simonw]

It's possible this issue is related:

• gh-109409: Fix inheritance of frozen dataclass from non-frozen dataclass mixins python/cpython#109437

See cPython commit: python/cpython@b6000d2

[simonw]

I think this is the code that the trackback complains about:

pint/pint/delegates/txt_defparser/common.py

Lines 15 to 25 in f2e4081

	from dataclasses import dataclass, field
	import flexparser as fp
	from ... import errors
	from ..base_defparser import ParserConfig
	@dataclass(frozen=True)
	class DefinitionSyntaxError(errors.DefinitionSyntaxError, fp.ParsingError):
	"""A syntax error was found in a definition. Combines:

And yes, DefinitionSyntaxError() there is marked as frozen=True - but the errors.DefinitionSyntaxError class it extends is marked frozen=False here:

pint/pint/errors.py

Lines 105 to 115 in f2e4081

	@dataclass(frozen=False)
	class DefinitionSyntaxError(ValueError, PintError):
	"""Raised when a textual definition has a syntax error."""
	msg: str
	def __str__(self):
	return self.msg
	def __reduce__(self):
	return self.__class__, tuple(getattr(self, f.name) for f in fields(self))

[simonw]

Modifying pint/errors.py and replacing every instance of @dataclass(frozen=False) with @dataclass(frozen=True) addresses this problem, but I don't know if it's the right solution.

diff --git a/pint/errors.py b/pint/errors.py
index 59d3b45..f080f52 100644
--- a/pint/errors.py
+++ b/pint/errors.py
@@ -81,12 +81,12 @@ class WithDefErr:
         return DefinitionError(self.name, self.__class__, msg)
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class PintError(Exception):
     """Base exception for all Pint errors."""
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class DefinitionError(ValueError, PintError):
     """Raised when a definition is not properly constructed."""
 
@@ -102,7 +102,7 @@ class DefinitionError(ValueError, PintError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class DefinitionSyntaxError(ValueError, PintError):
     """Raised when a textual definition has a syntax error."""
 
@@ -115,7 +115,7 @@ class DefinitionSyntaxError(ValueError, PintError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class RedefinitionError(ValueError, PintError):
     """Raised when a unit or prefix is redefined."""
 
@@ -130,7 +130,7 @@ class RedefinitionError(ValueError, PintError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class UndefinedUnitError(AttributeError, PintError):
     """Raised when the units are not defined in the unit registry."""
 
@@ -150,13 +150,13 @@ class UndefinedUnitError(AttributeError, PintError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class PintTypeError(TypeError, PintError):
     def __reduce__(self):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class DimensionalityError(PintTypeError):
     """Raised when trying to convert between incompatible units."""
 
@@ -183,7 +183,7 @@ class DimensionalityError(PintTypeError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class OffsetUnitCalculusError(PintTypeError):
     """Raised on ambiguous operations with offset units."""
 
@@ -208,7 +208,7 @@ class OffsetUnitCalculusError(PintTypeError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class LogarithmicUnitCalculusError(PintTypeError):
     """Raised on inappropriate operations with logarithmic units."""
 
@@ -233,7 +233,7 @@ class LogarithmicUnitCalculusError(PintTypeError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class UnitStrippedWarning(UserWarning, PintError):
     msg: str
 
@@ -241,13 +241,13 @@ class UnitStrippedWarning(UserWarning, PintError):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class UnexpectedScaleInContainer(Exception):
     def __reduce__(self):
         return self.__class__, tuple(getattr(self, f.name) for f in fields(self))
 
 
-@dataclass(frozen=False)
+@dataclass(frozen=True)
 class UndefinedBehavior(UserWarning, PintError):
     msg: str