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Cannot inherit frozen dataclass from a non-frozen one error when using pydantic frozen=True #14857
Description
Bug Report
Specifying frozen=True as model class kwargs for pydantic models causes the following error to be emitted from mypy:
Cannot inherit frozen dataclass from a non-frozen one
To Reproduce
- Install pydantic and mypy:
pip install pydantic mypy - Create
test.pywith the following content:
from pydantic import BaseModel
class MyModel(BaseModel, frozen=True):
my_val: int- Run
mypyon the test file:mypy test.py
Expected Behavior
No errors reported (as was the case with mypy 1.0.1):
> mypy test.py
Success: no issues found in 1 source file
Actual Behavior
Errors reported:
> mypy test.py
test.py:3: error: Cannot inherit frozen dataclass from a non-frozen one [misc]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.1.1
- Python version used: 3.10
Activity
wesleywright commentedon Mar 10, 2023
Similar to in #14850, I believe this is actually an issue with the Pydantic library's use of
dataclass_transform. From looking at the code, it seems thatBaseModelis itself using the transform and should be treated as a dataclass, so per the standard, frozen dataclasses cannot subclass it.You may be able to use the same workaround as suggested in #14850 (using the latest version of the Pydantic plugin).
wesleywright commentedon Mar 10, 2023
After double checking the spec (whoops), there is actually a provision for this:
This seems to be the only way to declare a class that has dataclass semantics but which is treated as neither frozen nor non-frozen. This is definitely a bug in MyPy; I will try to create a PR to fix it.
[dataclass_transform] fix frozen behavior for base classes with direc…