For a multi-class classification problem, how to transform the target variable to a form that is usable by sklearn algorithms?
Welcome to the site! Do a google search for "one hot encoding" and all will become clear. 

In a nutshell, don't think of it as numbers "assigned" of 0-159. It's more about a "slot" for each category and using one hot encoding to let the algorithm know which slot a particular record belongs to.