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Overriding __post_init__ in child dataclass leads to type error #9254
Description
Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues
Please provide more information to help us understand the issue:
Consider the following test file:
from dataclasses import dataclass, InitVar
@dataclass
class Parent:
x: int
def __post_init__(self) -> None:
print("hi!")
@dataclass
class Child(Parent):
y: int
z: InitVar[int]
def __post_init__(self, z: int) -> None:
print("hi from child; z is", z)
class OtherParent:
def __init__(self):
print("hi from other parent")
class OtherChild(OtherParent):
def __init__(self, z: int):
print("hi from other child; z is", z)
if __name__ == "__main__":
Child(1, 2, 3)
OtherChild(3)
Running mypy results in the following error:
% mypy dataclass_postinit_test.py
dataclass_postinit_test.py:17: error: Signature of "__post_init__" incompatible with supertype "Parent"
Found 1 error in 1 file (checked 1 source file)
I understand from #1237 that generally it is an error to have subclass methods that differ in this way. But it seems like __post_init__ should not fall into this category, and instead should be treated like __init__ which does not have any type checking error (as the example file also demonstrates with OtherParent/OtherChild).
Activity
gvanrossum commentedon Aug 3, 2020
vnmabus commentedon Oct 19, 2021
Can a general solution be added, like a decorator that you can use in a base class method to not require Liskov in the subclass overrides? Because sometimes it is useful to have a special "startup" or "run" method that initializes the object and should be different for each subclass.
sobolevn commentedon Jun 23, 2023
@vnmabus I think that
typing.no_type_checkcan help. Or you can create your own based ontyping.no_type_check_decorator.sobolevn commentedon Jun 23, 2023
Since I am working on #15498 I will also have a look at this one.
Support better `__post_init__` method signature for `dataclasses` (#1…