> During that time, I think the output of the transform was still subject
> to changes, and so this test is written. But now it's already stable,
> and everyone seems to be convinced that those methods should return
> CSR. So, should I remove the type check? Or will it break somehow?

I'd say, remove it, and whoever breaks your test deals with it.

> It doesn't do any harm other than one additional check on hasattr,
> right?

It means that people can introduce API changes without us noticing as the
tests still pass.