"Partially unknown" message in for loop.

[sindzicat]

Hello!

When I type:

arr1: list[int] = [1, 2, 3, 4, 5]
arr2: list[float] = [5/2, 3/2, 7/2]
el: int | float

for el in [*arr1, *arr2]:
    print(el)

I get an error message for el variable:

Type of "el" is partially unknown
  Type of "el" is "Unknown | int | float"Pylance[reportUnknownVariableType](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportUnknownVariableType)
(variable) el: Unknown | int | float

Then I changed my code slightly, replaced [*arr1, *arr2] with arr1 + arr2. The error message gone. Modified code, that works:

arr1: list[int] = [1, 2, 3, 4, 5]
arr2: list[float] = [5/2, 3/2, 7/2]
el: int | float

for el in arr1 + arr2:
    print(el)

Version: 1.95.3 (user setup)
Commit: f1a4fb101478ce6ec82fe9627c43efbf9e98c813
Date: 2024-11-13T14:50:04.152Z
Electron: 32.2.1
ElectronBuildId: 10427718
Chromium: 128.0.6613.186
Node.js: 20.18.0
V8: 12.8.374.38-electron.0
OS: Windows_NT x64 10.0.17763

Name: Pylance
Id: ms-python.vscode-pylance
Description: A performant, feature-rich language server for Python in VS Code
Version: 2024.11.101
Publisher: Microsoft
VS Marketplace Link: https://marketplace.visualstudio.com/items?itemName=ms-python.vscode-pylance

It seems to me, I found a bug.

[erictraut]

Could someone from the pylance team please transfer this to the pyright project? Thanks!

@sindzicat, I presume that you're using "strict" type checking mode? The reportUnknownVariableType check is not enabled by default in "standard" or "basic" type checking modes, so I wouldn't expect this error to show up when using default settings. Are there any other non-default settings that you're using?

Based on your description, this sounds like a bug, but I'm unable to repro it with the code sample you've provided. Could you confirm that you're able to repro the problem with a new project and a new source file with only the contents of the code above?

Are members of the pylance team able to repro this?

[sindzicat]

@erictraut, yes, I use "strict" type checking mode.

All settings I modified for Pylance:

Originally I wanted to create this issue in pyright repo first, but then decided it's for Pylance. I may to recreate my issue in pyright repo if needed. Moved already.

[erictraut]

I'm not able to repro the problem. Here it is in pyright playground.

Could you confirm that you're able to repro the problem with a new project and a new source file with only the contents of the code above?

[sindzicat]

@erictraut, I used pre-release version of Pylance. I switched back to production version of Pylance, but I still see this error.

I see that pyright works correctly in playground.

How can I check, what pyright version is used in Pylance? Thanks!

[Azureblade3808]

The key may be strictListInference=false.

There is an option for it in Pylance, which is disabled by default.

[erictraut]

If you enable strict mode but disable strictListInference, you will see this error. This is expected, so it's not a bug. Refer to the pyright documentation for more details.