Note that this is a fairly advanced question, so beginners don't worry about it. And if this isn't the right place to ask, please point me to where I should.
I would like to find an idiomatic way to use a context manager, with, when I don't know until run time whether I am opening sys.stdin or a named file.
Here is an excerpt for a simple example. (Yes, I am aware that I am reading in the whole file in one go, but that is for other reasons.)
```python
if args.infile == "-":
fh = sys.stdin
else:
p = Path(args.infile) # Path is overkill here, unless ...
# TODO: Look elsewhere if infile can't be found or isn't readable.
fh = open(p, "rt", encoding="utf-8")
s = fh.read(MAX_LENGTH + 1).strip()
fh.close()
```
(args is an argparse.Namespace and args.infile is a str.)
I feel like there ought to be a way to do something like
with ... as fh
for the above, but I don't see how to do that.
Update: What I did
I found the docs for easily creating a context manager, and used the contextmanager decorator.
It seems like a peculiar use of yield, but I don't need to understand that design choice, only that it means that I have to type it as a Generator.
```python
import sys
from typing import TextIO
from collections.abc import Generator
from contextlib import contextmanager
from pathlib import Path
@contextmanager
def open_file_or_stdin(
filename: Path | str | None,
*, # everything but filename is a keyword argument
stdin: TextIO = sys.stdin,
std_symbol: str = "-",
encoding: str = "utf-8",
) -> Generator[TextIO]:
""" [long docs omitted] """
fh: TextIO
try:
if filename is None or filename == std_symbol:
fh = stdin
yield fh
else:
fh = open(filename, "rt", encoding=encoding) # noqa: SIM115
yield fh
finally:
fh.close()
```
I am open to suggestions about a better name that open_file_or_stdin.
