Passing NamedTemporaryFile to a subprocess on windows

Question

The python docs for tempfile.NamedTemporaryFile say:

Whether the name can be used to open the file a second time, while the named temporary file is still open, varies across platforms (it can be so used on Unix; it cannot on Windows NT or later).

I have a program that's invoked as prog input.txt, that I can't touch. I want to write a python function to give this a string.

Here's various approaches that don't quite work:

from tempfile import NamedTemporaryFile

Not obvious if legal on windows

with NamedTemporaryFile() as f:
    f.write(contents)
    subprocess.check_call(['prog', f.name])  # legal on windows?

Might delete the file too early

with NamedTemporaryFile() as f:
    f.write(contents)
    f.close()  # does this delete the file?
    subprocess.check_call(['prog', f.name])

Doesn't clean up properly

with NamedTemporaryFile(delete=False) as f:
    f.write(contents)  # if this fails, we never clean up!
try:
    subprocess.check_call(['prog', f.name])
finally:
    os.unlink(f.name)

A little ugly

f = NamedTemporaryFile(delete=False)
try:
    with f:
        f.write(contents)
    subprocess.check_call(['prog', f.name])
finally:
    os.unlink(f.name)

Which of these are correct?

user_1222951 · Answer

As you suspected, the first three variants are all broken - the first one will throw a PermissionError on Windows; the second one does indeed delete the file too early; and the third one doesn't handle exceptions properly.

Your fourth snippet is therefore the correct way to do it.

However, as you said, it is a little ugly. I suggest wrapping it in a function to improve readability and reusability:

import os
from contextlib import contextmanager
from tempfile import NamedTemporaryFile

@contextmanager
def ClosedNamedTempfile(contents, mode='w'):
    f = NamedTemporaryFile(delete=False, mode=mode)
    try:
        with f:
            f.write(contents)
        yield f.name
    finally:
        os.unlink(f.name)

Which allows us to use it like so:

with ClosedNamedTempfile('foobar
') as f:
    subprocess.check_call(['prog', f])

Passing NamedTemporaryFile to a subprocess on windows

1 Answers