input: 
  def build_blockchain():
      """
      Build a simple blockchain system in Python.
  
      >>> build_blockchain() == [create_genesis_block()]
      True
      >>> blockchain = build_blockchain()
      >>> blockchain[0].hash == blockchain[0].hash_block()
      True
      """
  
output:     
      class Block:
          def __init__(self, index, previous_hash, timestamp, data, hash):
              self.index = index
              self.previous_hash = previous_hash
              self.timestamp = timestamp
              self.data = data
              self.hash = hash
  
          def hash_block(self):
              sha = hashlib.sha256()
              sha.update(f"{self.index}{self.previous_hash}{self.timestamp}{self.data}".encode('utf-8'))
              return sha.hexdigest()
  
      def create_genesis_block():
          return Block(0, "0", time.time(), "Genesis Block", Block.hash_block(Block(0, "0", time.time(), "Genesis Block", "")))
  
      blockchain = [create_genesis_block()]
      return blockchain
  