typing error in combination with pylance for factory=list

[matt-the-midnight-hacker]

Hello I'm very excited that attrs now allows to show constructor parameters when used with pylance.
I'm using attrs-21.2.0
However I have a problem when using a class with factory, the factory appears to be not recognized as default value. For example:

@attr.s(auto_attribs=True)
class A:
    my_i: int = 42
    my_list: List[str] = attr.ib(factory=list)

If I try to instantiate this class with a = A() I get error from pylance: 'Argument missing for parameter "my_list"'.
And for the class definition I get the error "Fields without default values cannot appear after fields with default values".
I noticed it works somehow when I use my_list: List[str] = attr.Factory(list) but with this the type is set to Factory and I get problems if I want to use the list a.my_list.append('fu') will then lead to "Instance of 'Factory' has no 'append' member".

[hynek]

Yes, this has been raised in #795 (comment) . I hope the pyright team sees it our way…

[matt-the-midnight-hacker]

Oh ok, if there is already a ticket for this issue then feel free to close this one. Many thanks for all the hard work you guys do. 🙇

[euresti]

Can you work around it with this?

@attr.s(auto_attribs=True)
class A:
    my_i: int = 42
    my_list: List[str] = attr.Factory(list)

Oh ugg:

I noticed it works somehow when I use my_list: List[str] = attr.Factory(list) but with this the type is set to Factory and I get problems if I want to use the list a.my_list.append('fu') will then lead to "Instance of 'Factory' has no 'append' member".

Looks like pylance doesn't do the Factory magic that we do in the stubs.

[matt-the-midnight-hacker]

Sadly not, maybe pylance could do? Seems like that there is some trouble in finding a aggreement which project will handle default_factory/attr.ib(factory=list) (#795 (comment)). Maybe supporting attr.Factory(list) in pylance is an option? It's somehow "half" supported. On hovering the variable I get the correct type hint (List) but still pyright shows errors that its instance of Factory.

[hynek]

Great news, they added factory!