dataclass inheritance with keyword-only args [3.10]

[cdce8p]

Python 3.10 adds the ability to specify keyword-only args in dataclasses. Especially with inheritance, this allows some combinations that would previously have been an error, and are currently still detected as such.

For example, it's now possible to specify fields without a default in a derived class if only keyword-only args in the base class have a default. This will work since keyword-only args need to come after positional/keyword args. https://docs.python.org/3.10/library/dataclasses.html#re-ordering-of-keyword-only-parameters-in-init

from dataclasses import dataclass, KW_ONLY

@dataclass
class Base:
    x: str
    _: KW_ONLY
    y: int = 0
    w: int = 1

@dataclass
class D(Base):
    z: str  # (1)
    a: str = "a"

>>> D("Hello", "World")
D(x='Hello', y=0, w=1, z='World', a='a')

At the moment, pylance indicates an error for z at position (1):

Fields without default values cannot appear after fields with default values

--
On a related note, the tip for D is also not correct.

Due to the reordering it should be

D(x: str, z: str, *, y: int = 0, w: int = 1, a: str = "a") -> None

[erictraut]

This functionality will be included in the next release.

[heejaechang]

fixed in 2021.8.1