input: 
  class HashTable:
      def __init__(self):
          self.table = [[] for _ in range(256)]
   
      def insert(self, key, value):
          h = hash(key) % 256
          for i, kv in enumerate(self.table[h]):
              if kv[0] == key:
                  self.table[h][i] = (key, value)
                  return
          self.table[h].append((key, value))
   
      def retrieve(self, key):
          h = hash(key) % 256
          for kv in self.table[h]:
              if kv[0] == key:
                  return kv[1]
   
      def delete(self, key):
          h = hash(key) % 256
          for i, kv in enumerate(self.table[h]):
              if kv[0] == key:
                  del self.table[h][i]
  
  # Example usage:
  # >>> ht = HashTable()
  # >>> ht.insert("apple", 5)
  # >>> ht.retrieve("apple")
  # 5
  # >>> ht.delete("apple")
  # >>> ht.retrieve("apple")
  # None
  
output:     def __contains__(self, key):
          h = hash(key) % 256
          for kv in self.table[h]:
              if kv[0] == key:
                  return True
          return False