I think the important thing is that the code look pure from a testing perspective.

Say you've got a function that accesses a key-value store. Ideally, you can factor out the i/o so that you do all your reads up front and all your writes at the end, leaving a pure function in the middle. But if the code is too tangled up in its side effects for that, the next best thing is to create a fake KV store and then wrap the function like this:

  def doTest(input, initialState):
    kv = makeFake(initialState)
    result = doThing(kv, input)
    return result, kv.dumpContents()

doThing isn't a pure function, but doTest is. Now you can write tests like this:

  result, outputState = doTest(input, initialState)
  assert (result, outputState) == expected

I guess you could call that "imperative core, functional shell," lol.