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]
  
  # Test cases:
  # Inserting and retrieving an element
  hash_table = HashTable()
  hash_table.insert("key1", "value1")
  assert hash_table.retrieve("key1") == "value1"
  
  # Deleting an element and checking if it's gone
  hash_table.delete("key1")
  assert hash_table.retrieve("key1") is None
  
  # Inserting and updating an element
  hash_table.insert("key1", "updated_value1")
  assert hash_table.retrieve("key1") == "updated_value1"
  
output:     def contains_key(self, key):
          h = hash(key) % 256
          for kv in self.table[h]:
              if kv[0] == key:
                  return True
          return False
  
      def keys(self):
          key_list = []
          for bucket in self.table:
              for kv in bucket:
                  key_list.append(kv[0])
          return key_list
  
      def values(self):
          value_list = []
          for bucket in self.table:
              for kv in bucket:
                  value_list.append(kv[1])
          return value_list
  
      def items(self):
          item_list = []
          for bucket in self.table:
              for kv in bucket:
                  item_list.append(kv)
          return item_list
  
  # Additional test cases:
  # Checking if a key exists
  assert hash_table.contains_key("key1") == True
  assert hash_table.contains_key("nonexistent_key") == False
  
  # Retrieving all keys
  assert set(hash_table.keys()) == {"key1"}
  
  # Retrieving all values
  assert set(hash_table.values()) == {"updated_value1"}
  
  # Retrieving all items
  assert set(hash_table.items()) == {("key1", "updated_value1")}