How to test session-scoped fixture clean-up when writing plugins?

[butla]

I'm trying to modify tests for pytest-docker. One thing that I'd like to add is testing that the session-scoped docker_services fixture (which spins up and later destroys a Docker Compose environment) gets rid of the Docker volume it creates.

All of the approaches I have in mind have problems:

1. use the recommended black box testing and create a small test that will use the fixture.
   The problem: All of the names are generated randomly. I don't know ahead of time how the volume will be called, so I can't check if it's gone. I could count the volumes before the run and after the run, but that would be flaky if some other process on the machine is working with Docker in the background.
2. write a test using the fixture, do some assertions in it.
   The problem: Any fixture or test I can create that references docker_services will have a smaller scope, so I can't run any test code after the fixture is cleaned.

Any thoughts on the subject? Is there something obvious that I'm missing?

[nicoddemus]

Hi @butla,

I think you got the gist of it.

One solution I can think of is to write the volume id somehow to stdout or a known file location (inside tmpdir) during option 1. After testdir.runpytest_subprocess(), you can read the id and check that the volume does not exist anymore.

Others might suggest more elegant solutions.

[butla]

I'm gonna submit a PR to the docs about testing plugins if we get an elegant solution or if this thread :-) I'll go with your suggestion - seems to be robust enough.

[nicoddemus]

I'm gonna submit a PR to the docs about testing plugins

Great, appreciate it! 😁

[butla]

So the approach I took [here] (https://github.com/AndreLouisCaron/pytest-docker/blob/master/tests/test_integration.py#L42) can be described like this:

• the outer test (the test testing the test fixture :) ) prepares a name for the file that will be used to pass data from the inner test
• the filename gets inserted into the string with inner test's Python code by string formatting
• inner tests happen, they write something to the file
• inner tests end, the outer test can retrieve whatever it needs from the file.

Any ideas for a more elegant aproach? If there aren't any, I'll submit that for the docs.

[nicoddemus]

I think that approach is fine @butla, I've used it myself on the past.