I doubt there is significant literature that considers negative sample weights. The one brief reference I am familiar with is section 3.1.2 in http://tmva.sourceforge.net/docu/TMVAUsersGuide.pdf. I can't find anything else on the great Internet. That shouldn't stop us from protecting against strangeness that may occur if `fps[-1]` goes negative here. Maybe this warrants a separate warning with a clearer message stating that the weighted number of negative samples in `y_true` is negative (similarly for positive samples).