dataclass does not properly support generics

Please provide more information to help us understand the issue:

Are you reporting a bug, or opening a feature request?

I believe this is a bug.

Please insert below the code you are checking with mypy,
or a mock-up repro if the source is private. We would appreciate
if you try to simplify your case to a minimal repro.

Here is MRE:

from dataclasses import dataclass
from typing import Generic, NewType, TypeVar

T = TypeVar("T")
V = NewType("V", int)

@dataclass
class Base(Generic[T]):
    attr: T

# If this decorator is commented than the generic works as expected
@dataclass
class Sub(Base[V]):
    pass

Sub(attr=V(1))

What is the actual behavior/output?

Argument "attr" to "Sub" has incompatible type "V"; expected "T"

What is the behavior/output you expect?

The code should type check.

What are the versions of mypy and Python you are using?
Do you see the same issue after installing mypy from Git master?

mypy 0.720
yes, tested against: 8782ae7f789ad5b8cab97d3c8419f9f98b6eb285

What are the mypy flags you are using? (For example --strict-optional)

The defaults, executed with: mypy test_script.py

If mypy crashed with a traceback, please paste
the full traceback below.

Yes, this is a bug. The fix is to map/expand variable types in the supertype along the MRO instead of "blindly" copying them to the subtype. Should be not too hard to implement.

It looks like this is still a problem with the latest release. I think I know how to solve this. I can send a pull request.

Also, looks like the same problem affects the attrs plugin (#5744). I think I can fix both with basically the same code.

fix: resolve typevar before generating dataclasses init method

I opened PRs to fix:

For dataclasses - Fix generic inheritance for dataclass init methods #9380
For attrs - Fix generic inheritance for attrs init methods #9383

Fix generic inheritance for dataclass init methods (#9380)

Getting error related to the same issue -- inheritence in generic dataclasses -- If the TypeVar is in another Generic container

from typing import Generic, TypeVar
from dataclasses import dataclass

T = TypeVar('T', int, str)

class C(Generic[T]):
    pass

@dataclass
class A(Generic[T]):
    x: C[T]

@dataclass
class B(A[int]):
    pass


B(x=C[int]())  # error: Argument "x" to "B" has incompatible type "C[int]"; expected "C[T]"

The equivalent code without dataclasses using __init__ works as expected.

Reported here #10986

Anything that we can do to address it?

@rusiaaman

I encountered the same issue as @rusiaaman.

His example reproduces in python 3.9.7

Open a new bug report with the details. This bug report has been closed.

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dataclass does not properly support generics #7520

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dataclass does not properly support generics #7520

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ilevkivskyi commented on Sep 29, 2019

natemcmaster commented on Aug 30, 2020

natemcmaster commented on Aug 30, 2020

rusiaaman commented on Sep 21, 2021

drewm1980 commented on Jan 24, 2022

natemcmaster commented on Jan 25, 2022

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