Repository navigation
[Attrs] Invalid typing of init arguments in subclasses of generics #5744
Description
from typing import Generic, List, TypeVar
import attr
T_BASE_ITEM = TypeVar("T_BASE_ITEM", bound="BaseItem")
class BaseItem(object):
pass
class ConcreteItem(BaseItem):
pass
@attr.s
class Container(Generic[T_BASE_ITEM]):
items = attr.ib(type=List[T_BASE_ITEM], factory=list)
@attr.s
class ConcreteContainer(Container[ConcreteItem]):
pass
container1 = ConcreteContainer()
container1.items.append(ConcreteItem())
container1.items.extend([ConcreteItem()])
container1.items = [ConcreteItem()]
container2 = ConcreteContainer(items=[ConcreteItem()])
fails ONLY on the last line:
List item 0 has incompatible type "ConcreteItem"; expected "T_BASE_ITEM"
It seems that typing for the attribute items works as expected. However, if items is used as a keyword argument of the __init__ generated by attrs, typing fails.
Note also that if the following class is added:
@attr.s
class ItemWrapper(Generic[T_BASE_ITEM]):
item = attr.ib(type=T_BASE_ITEM)
mypy fails with error:
'T_BASE_ITEM' is a type variable and only valid in type context
Activity
ilevkivskyi commentedon Oct 8, 2018
Yes, this is clearly a bug. The cause is that
__init__is generated for every subclass that is an attrs class. When__init__(or any other attribute) is accessed on a superclass of a normal class, mypy takes care tomap_instance_to_supertype/expand_type_by_instance, while the plugin doesn't do this.@euresti will you have time to work on this?
ilevkivskyi commentedon Oct 8, 2018
Btw, most likely dataclasses have the same problem.
natemcmaster commentedon Aug 30, 2020
Thanks for the tip about
map_instance_to_supertype. I have proposed a fix in #9383 which uses that API to resolve typevars.Fix generic inheritance for attrs init methods (#9383)