Ways by node

graph.clj

(defn collect-ways-by-node [ways]
  (reduce (fn [m m2] (merge-with #(concat %1 %2) m1 m2))
           (for [way ways]
                (for [id (:nodes way)]
                     (hash-map (:id way) [id])))))

graph.py

import collections

def ways_by_node(ways):
    ret = collections.defaultdict(list)
    for way in ways:
        way_id = way['id']
        for node_id in way['nodes']:
            ret[node_id].append(way_id)
    return ret