If you want a code example for the above, it would be like this:

defmodule MyTest do
  use ExUnit.Case, async: false

  setup do
    put_application_env_for_test(:some_app, :some_key, :some_value)
  end

  defp put_application_env_for_test(app, key, value) do
    previous_value = Application.get_env(app, key)
    Application.put_env(app, key, value)
    on_exit(fn -> Application.put_env(app, key, previous_value) end)
  end
end

This will break as soon as you use async: true in your tests. Which may or may not be a problem for you.

You probably also want to do the cleanup after the tests, not before. If the tests run in random order, they will jump to some other test file after these tests are done, and the variables may be affecting the behavior in other tests. If you do the cleanups in setup blocks executed before each test here, you want to do it in other test files as well. Otherwise weird things will happen.

Alternative to this is to clean up in on_exit callback: https://hexdocs.pm/ex_unit/ExUnit.Callbacks.html#on_exit/2

So you can go with what you have provided:

1. the tests are async: false
2. the cleanup is done in on_exit block

If you use ProcessTree then you can effectively modify the application environment for most processes while keeping the ability to use async: true tests. It works by looking for env first in the process dictionary of the current process, and then the dictionary of all ancestor and caller processes:

I’ve been using it quite a bit and it’s a really nice way to allow using the application environment while still using async: true.