Variable doesn't type narrow on reassignment to itself?

[wyattscarpenter]

In the following code, I expect stdout in f to narrow itself to TextIO, but pyright complains about passing it to g. Is this intended behavior, or a bug?

from typing import TextIO, reveal_type
import sys

reveal_type(sys.stdout) # it says TextIO | Any (it's a MaybeNone from typeshed)

def g(stdout: TextIO):
  pass

def f(stdout: TextIO | None = None):
  stdout = stdout if stdout is not None else sys.stdout
  g(stdout) # it says stdout is TextIO | None so it can't be assigned.

def example_function_which_is_fine():
  g(sys.stdout)

Code sample in pyright playground

[erictraut]

The behavior you're seeing here is due to the Any type in the sys.stdout type definition. Since that Any could be a None, it's unsafe to narrow the type of stdout to just TextIO. So this is intended behavior, not a bug.

[wyattscarpenter]

That's surprising to me. I thought it would be narrowed to TextIO | Any, which would then be assignable to TextIO.

Oh, I guess this makes it clearer to me (hovertext from pyright playground included as comments):

def f2(stdout: TextIO | None = None):
  if stdout is not None:
    stdout = stdout # (parameter) stdout: TextIO = (parameter) stdout: TextIO
  else:
    stdout = sys.stdout # (parameter) stdout: TextIO | None = (variable) stdout: TextIO | Any
  g(stdout) # it says stdout is TextIO | None so it can't be assigned.

When it considers assigning sys.stdout to stdout, it says "yeah, that Any could be a None. I will keep the original type", and thus doesn't change it.

Presumably, the ternary is doing this as well.

The same is true also for this even simpler case:

def f3(stdout: TextIO | None = None):
  stdout = sys.stdout
  g(stdout) # it says stdout is TextIO | None so it can't be assigned.

[erictraut]

I thought it would be narrowed to TextIO | Any

TextIO | Any is not narrower than the declared type (TextIO | None). It's wider.