This LGTM but I don't think you should close #4694. Or if you do, you should re-open the issue that #4618 seeked to fix.

Fair enough, will leave it open.

@tseaver can you shed some light on the failure here: https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/5141?

nvm, @dhermes helped me figure it out.

And Chuck Norris!

Is that linked in the contributing guide? If not, it should be. P0.

Ignoring conflict error as it's just due to noise from another spanner system tests (I will not release this without a green build on the release PR).

Rather than raising a ValueError if the caller passes a start_open / start_closed but neither end_open / end_closed, the constructor should just assign the empty list to end_open (this was the documented workaround for the caller in #4618: we can just do it on the caller's behalf).

Likewise for the case where the caller passes an end_open / end_closed but neither of start_open / start_closed: the constructor should just assign start_open = [].

For the first case, I think assigning it to end_closed is a better choice, as I believe that end_open excludes everything.

Likewise for the end_closed. Should I make the change?

Go for it, if you're faster than CI I'll rebase it into #4732.