Create many constrained, random permutation of a list -

i need make random list of permutations. elements can assume integers 0 through x-1. want make y lists, each containing z elements. rules no list may contain same element twice , on lists, number of times each elements used same (or close possible). instance, if elements 0,1,2,3, y 6, , z 2, 1 possible solution is:

 0,3 1,2 3,0 2,1 0,1 2,3 

each row has unique elements , no element has been used more 3 times. if y 7, 2 elements used 4 times, rest 3.

this improved, seems job (python):

import math, random   def get_pool(items, y, z):     slots = y*z      use_each_times = slots/len(items)     exceptions = slots - use_each_times*len(items)       if (use_each_times > y or         exceptions > 0 , use_each_times+1 > y):         raise exception("impossible.")       pool = {}     n in items:         pool[n] = use_each_times      n in random.sample(items, exceptions):         pool[n] += 1      return pool  def rebalance(ret, pool, z):     max_item = none     max_times = none      item, times in pool.items():         if times > max_times:             max_item = item             max_times = times       next, times = max_item, max_times      candidates = []     in range(len(ret)):         item = ret[i]          if next not in item:             candidates.append( (item, i) )       swap, swap_index = random.choice(candidates)      swapi = []     in range(len(swap)):         if swap[i] not in pool:             swapi.append( (swap[i], i) )       which, = random.choice(swapi)      pool[next] -= 1     pool[swap[i]] = 1     swap[i] = next      ret[swap_index] = swap  def plist(items, y, z):     pool = get_pool(items, y, z)      ret = []     while len(pool.keys()) > 0:         while len(pool.keys()) < z:             rebalance(ret, pool, z)          selections = random.sample(pool.keys(), z)          in selections:             pool[i] -= 1             if pool[i] == 0:                 del pool[i]          ret.append( selections )      return ret   print plist([0,1,2,3], 6, 2)