Interview question: implement a class that will take a list of lists and return the next item in a randomly selected list

RandomItemRetriever.py

from random import Random

class RandomItemRetriever:
  '''
  This class will take a list of lists and return the next item in a randomly selected list.
  Time: O(1)
  Space: O(n) where n = number of lists
  '''
  def __init__(self, lists):
    # store original data (never mutated)
    self.lists = lists
    # next index for each list
    self.indexes = [0] * len(lists)
    # array_idxs
    self.array_idxs = [x for x in range(len(lists))]
    # pointer to first available array
    self.start = 0
    
  def get_next(self):
    if self.start >= len(self.array_idxs):
      raise StopIteration()
    
    r = Random()
    idx = r.randint(self.start, len(self.array_idxs)-1)
    arr_idx = self.array_idxs[idx]
    
    item = self.lists[arr_idx][self.indexes[arr_idx]]
    self.indexes[arr_idx] += 1
    
    # if we're done with a list then put it at start of self.arrays and increment self.start
    if self.indexes[arr_idx] == len(self.lists[arr_idx]):
      self.array_idxs[self.start], self.array_idxs[arr_idx] = self.array_idxs[arr_idx], self.array_idxs[self.start]
      self.start += 1
    
    return item