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TypeError/SystemError for dataclasses that uses default_factory and is subclassed #1204
Description
from importlib.metadata import version
from dataclasses import dataclass, field
@dataclass
class _Test:
x: dict = field(default_factory=dict)
class Test(_Test):
pass
print("Mypy version:", version("mypy"))
print(Test())With mypyc test.py && python -c "import test" leads to:
❯ mypyc test.py && python -c "import test"
test.py:7: warning: Unsupported default attribute value
running build_ext
building 'test' extension
clang -fno-strict-overflow -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -O3 -Wall -I/private/tmp/mypyc/.venv/lib/python3.14/site-packages/mypyc/lib-rt -I/private/tmp/mypyc/.venv/include -I/opt/homebrew/opt/[email protected]/Frameworks/Python.framework/Versions/3.14/include/python3.14 -c build/__native.c -o build/temp.macosx-26.0-arm64-cpython-314/build/__native.o -O3 -g1 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable -Wno-ignored-optimization-argument -Wno-array-bounds -Wno-stringop-overread -Wno-stringop-overflow -Wno-cpp -Wno-array-bounds -Wno-stringop-overread -Wno-stringop-overflow
clang -bundle -undefined dynamic_lookup build/temp.macosx-26.0-arm64-cpython-314/build/__native.o -o build/lib.macosx-26.0-arm64-cpython-314/test.cpython-314-darwin.so
copying build/lib.macosx-26.0-arm64-cpython-314/test.cpython-314-darwin.so ->
Mypy version: 2.3.0
TypeError: dict object expected; got dataclasses.Field
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<string>", line 1, in <module>
import test
File "test.py", line 15, in <module>
print(Test())
File "<string>", line 3, in __init__
SystemError: <class 'dict'> returned a result with an exception setRan into this in the wild while trying to compile isort with mypyc (PyCQA/isort#2586).
Code runs fine without mypyc compilation.
Activity
DanielNoord commentedon Jul 28, 2026
Submitted a fix in python/mypy#21785. Using that fix does seem to compile
isortcorrectly, or at least, it avoids the previous crash :)[mypyc] Fix `default_factory` for inherited dataclass (#21785)
[mypyc] Fix `default_factory` for inherited dataclass (#21785)
[mypyc] Fix `default_factory` for inherited dataclass (#21785)