Problem with sys.stdout.reconfigure and TextIOWrapper partially unknown type

[DervishD]

Hi there!

I have the following code in a program, which I think it's pretty standard:

import sys
from io import TextIOWrapper

if sys.stdout and isinstance(sys.stdout, TextIOWrapper):
    sys.stdout.reconfigure(encoding='utf-8')

I'm getting an error from PyRight (well, Pylance, used under VSCode), to wit: Type of "stdout" is partially unknown.  Type of "stdout" is "TextIOWrapper[Unknown]"). This was not happening the last time I worked in this code, a few months ago, so I don't know if it is a bug or not.

Thing is, I don't know how to get rid of this error except using cast(TextIOWrapper, sys.stdout) and I cannot do this because in fact sys.stdout can be redirected and not be of class TextIOWrapper. Also, I don't entirely understand why the cast works here suppressing the error, and I prefer not to use a solution I don't understand.

Am I doing anything wrong here? How can I suppress the error without resorting to dubious code constructs? Is it a bug in Pyright or typeshed or anything like it?

I've googled about this problem for days without finding anything and help from Copilot and ChatGPT has been quite useless.

Thanks in advance for any information about this. I can't believe I'm the only one wanting to force UTF-8 on output AND using Pyright 😆

[erictraut]

Pyright is working correctly here. I've confirmed that the behavior hasn't changed in pyright for a long time. I suspect that the type definitions for stdio have changed over time, which would explain why you observed a difference in behavior. These type definitions are provided in typeshed and bundled with pyright and pylance.

The type of stdout is defined as TextIO | MaybeNone. The TextIOWrapper type is a subclass of TextIO, but it is defined as generic. It takes a single type argument. When you use the isinstance check to narrow the type of TextIO, pyright has no way of inferring the type argument for TextIOWrapper, so it infers Unknown. If you know the intended type of the type argument, you could use a cast within the conditional block. Unfortunately, isinstance doesn't allow you to pass a specialized type directly as its second argument.

It appears that you're using strict-mode type checking. By default pyright does not report unknown or partially-unknown types. If you don't want to see these errors reported, you can switch from "strict" to "standard" type checking mode.

[DervishD]

@erictraut thanks A LOT for the explanation. I was focused unto TextIOWrapper and forgot about TextIO.
Yes, I'm using "strict" mode, but I cannot switch to "standard", for now it's mandatory to use "strict" mode. Looks like I'm not doing anything wrong so I may just silent the warning, since the code is OK. I may replace the isinstance() by a try block and ignore AttributeError if reconfigure() is not defined for the actual class sys.stdio belongs to at runtime, or any other solution.

And yes, probably the problem is in typeshed, apparently it has been updated since I last worked on the code.

Thanks a lot, really, I was worried about some unintended side-effect in the code I was using, even though as I wrote it seems pretty standard.