Narwhals: from_native() return type incompatible with attribute assignment

[akmalsoliev]

Describe the Bug

Hey folks,
Based on the discussion on the call this is the MRE:

import narwhals as nw
class Validate:
    def __init__(self, frame: IntoFrame) -> None:
        # ensuring that the Frame is of Narwhals type
        self.frame = nw.from_native(frame)

Error:

ERROR `LazyFrame[NativeDataFrame | NativeLazyFrame | _NativeIbis]` is not assignable to attribute `frame` with type `NativeDataFrame | NativeLazyFrame | _NativeIbis` [bad-assignment]
  --> validoopsie/validate.py:43:26
   |
43 |             self.frame = nw.from_native(frame)
   |                          ^^^^^^^^^^^^^^^^^^^^^
   |
 INFO 1 error (2 ignored)

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[connernilsen]

Thanks for turning this into an issue! I see Sam already looked at it, but going to tag @stroxler, @rchen152 in case they might also know what's going on

[rchen152]

@akmalsoliev Would you be able to share more of the surrounding code? What is the IntoFrame type? Also, does self.frame have a type annotation or a previous definition?

[akmalsoliev]

hey @rchen152, yes, it is IntoFrame is TypeAlias in Narwhals: https://narwhals-dev.github.io/narwhals/api-reference/typing/#narwhals.typing.IntoFrame

[rchen152]

Thanks! Unfortunately, I still can't reproduce this bug. With pyrefly 0.44.1 and narwhals 2.13.0:

$ cat foo.py 
import narwhals as nw
from narwhals.typing import IntoFrame

class Validate:
    def __init__(self, frame: IntoFrame) -> None:
        # ensuring that the Frame is of Narwhals type
        self.frame = nw.from_native(frame)
$ pyrefly check foo.py 
 INFO 0 errors

I suspect some other part of your program is doing something that gives self.frame an expected type.

[akmalsoliev]

It appears that you folks fixed the issues with 0.44.0 release 🎉🎉🎉🎉🎉