def keyOnFirstItem(item_pair,item_sim_data):
    (item1_id,item2_id) = item_pair
    return item1_id,(item2_id,item_sim_data)
For each item-item pair, make the first item's id the key
dataset/ETHPy150Open evancasey/spark-knn-recommender/algorithms/itemBasedRecommender.py/keyOnFirstItem