It works now, thank you!
I had to change the order of my calls though, which makes me wonder...
My original attempt was to first add the 'point in time" source then set the claim to 'preferred' - which failed for some reason.
item.addClaim(alexa_ranking_claim, summary=u'Updating Alexa ranking')#1st
retrieved_source = _get_point_in_time_source()
alexa_ranking_claim.addSources([retrieved_source], summary=u'Adding retrieve date.')#2nd
alexa_ranking_claim.changeRank('preferred')#3rd
When changing the the execution order to set the rank to 'preferred' and then adding the sources the call works fine, i.e. no error and both the new ranking as well as the point in time source are taken in consideration
item.addClaim(alexa_ranking_claim, summary=u'Updating Alexa ranking')#1st
alexa_ranking_claim.changeRank('preferred')#3rd
retrieved_source = _get_point_in_time_source()
alexa_ranking_claim.addSources([retrieved_source], summary=u'Adding retrieve date.')2nd
Anyway - thanks again for your help!
--Tozibb (talk) 18:56, 16 October 2017 (UTC)