session.create_tmp has slightly surprising behavior

[Julian]

How would this feature be useful?

I'm in the middle of diagnosing some build instability (files appearing in a directory which I don't expect to be there) and in the process noticed something about .create_tmp -- I'm not sure it's related to my specific issue yet, but to me it seems worth addressing independently. Filing to see what others think.

Specifically... session.create_tmp is documented as "create and return a temporary directory" -- but it actually seems to always use a fixed directory path, tmp, and one that may exist "accidentally" if some other thing happened to create a directory named that, or get polluted accidentally if some other process starts writing to that path.

More surprisingly I think is if someone does this:

@session()
def foo(session):
    one = session.create_tmp()
    two = session.create_tmp()

they actually are unlikely to realize that they've gotten the same exact path twice there, one and two are going to be the same tmp/ directory.

Describe the solution you'd like

I'd personally definitely expect behavior more like tempfile.TemporaryDirectory and os.mkdtemp, namely that creating temporary directories gives me a path to a directory I can be sure:

• didn't exist beforehand
• isn't going to be being used by some other process
• is going to differ if I call it twice

Describe alternatives you've considered

Certainly just ignoring session.create_tmp is an option, but filing this as it seems a bit like a small footgun as-is.

Anything else?

No response

[Julian]

It was pointed out to me that create_tmp also sets TMPDIR (which indeed explains my original issue) -- that seems like even more surprising behavior than the first part to me. It now seems worth avoiding this function entirely to me, so going to close this, but obviously if someone agrees with my assessment (on both being quite surprising behavior) and thinks there's some way to nudge it to have less surprising behavior, let me know.

[pquentin]

if someone agrees with my assessment (on both being quite surprising behavior) and thinks there's some way to nudge it to have less surprising behavior, let me know.

Since you asked :) I was expecting create_tmp to be a simple wrapper around https://docs.python.org/3/library/tempfile.html#tempfile.TemporaryDirectory that does not do anything fancy like setting TMPDIR. As it stands, create_tmp is an attractive nuisance. It appears useful, but has multiple important caveats.