Passing StringIO object to TextIOWrapper object is giving mypy error

Question

I wasn't getting any Mypy error with Mypy 0.812. But after upgrading Mypy to latest (1.14.1) I am getting below error. error: Argument 1 to "_redirect_stdout" has incompatible type "StringIO"; expected "TextIOWrapper[_WrappedBuffer]" [arg-type]

Please help me to understand

• the error
• whats changed in mypy that is raising the error and
• the possible ways to resolve it. Thanks!

@contextlib.contextmanager
def _redirect_stdout(
    new_target: io.TextIOWrapper
) -> Generator[io.TextIOWrapper, None, None]:
    old_target = sys.stdout
    sys.stdout = new_target

    try:
        yield new_target
    finally:
        sys.stdout = old_target


target_stdout = io.StringIO()

with _redirect_stdout(target_stdout):
    print(['These', 'are', 'sample', 'strings.'])

Answer 1

Mypy 0.812 is ancient: It was released in February 2021. There have been many, many changes since then. This answer will thus not cover the "exact" change, if there is even such a thing.

The type StringIO is not assignable to TextIOWrapper, as they are defined as different classes, and with different bases, to boot:

# https://github.com/python/typeshed/blob/101287091c/stdlib/_io.pyi#L157
class TextIOWrapper(TextIOBase, _TextIOBase, TextIO, Generic[_BufferT_co]):
    ...

# https://github.com/python/typeshed/blob/101287091c/stdlib/_io.pyi#L191
class StringIO(TextIOBase, _TextIOBase, TextIO):
    ...

They do, however, share a few base classes. _TextIOBase is private, as signalled by its name, so it's out of the question. That leaves TextIOBase and TextIO:

_{(playgrounds: Mypy, Pyright)}

from io import StringIO, TextIOBase
from typing import TextIO

def a(v: TextIOBase) -> None: ...
def b(v: TextIO) -> None: ...

a(StringIO())  # fine
b(StringIO())  # fine

sys.stdout has the type TextIO, so the best choice overall is TextIO:

# https://github.com/python/typeshed/blob/101287091c/stdlib/sys/__init__.pyi#L67
stdout: TextIO | MaybeNone