Passing a None object as a filter argument on listing calls seems to ignore the filter rather than returning no results.
E.g. list_evaluations(setup=setuplist) where setuplist happens to be None will happily download ALL OpenML evaluations (when placed in a paging loop).

Indeed, the code says:

def list_evaluations(function, offset=None, size=None, id=None, task=None,
                     setup=None, flow=None, uploader=None, tag=None):
    if setup is not None:
        api_call += "/setup/%s" % ','.join([str(int(i)) for i in setup])

It would be safer to actually check whether the argument was passed? This can be done with kwargs: https://stackoverflow.com/questions/14749328/python-how-to-check-whether-optional-function-parameter-is-set

What do you think? This actually happened to me by mistake (I assumed all studies have a list of setups - not) and I only noticed after downloading 1.3 million evaluations.
Normally you wouldn't worry too much about stupid user mistakes, but in this case it stresses the server. Maybe adding it just for the listing calls isn't the worst idea.