code: def locations_within(a, b, tolerance):
      ret = ''
      b = dict(b)
      for (key, value) in a.items():
          if key not in b:
              raise ValueError("b does not have the key: " + key)
          if abs(int(value) - int(b[key])) > tolerance:
              ret += 'key {0} differs: {1} {2}'.format(key, int(value),
                                                       int(b[key]))
          del b[key]
      if b:
          raise ValueError("keys in b not seen in a: " + ", ".join(b.keys()))
      return ret