input: 
  def create_blockchain_class() -> str:
      """Create a Python class definition for a basic blockchain using only core modules.
      The class should define a method to initialize the blockchain, a method to create a new block,
      a private method to get the current timestamp, and a method to hash a block.
      >>> create_blockchain_class()
      "class Blockchain:\n    def __init__(self):\n        self.chain = []\n        self.transactions = []\n    \n    def create_block(self, nonce, previous_hash):\n        block = {\n            'block_number': len(self.chain) + 1,\n            'timestamp': self._current_timestamp(),\n            'nonce': nonce,\n            'previous_hash': previous_hash\n        }\n        self.chain.append(block)\n        return block\n    \n    def _current_timestamp(self):\n        return round(time.time() * 1000)\n    \n    def hash_block(self, block):\n        string_object = json.dumps(block, sort_keys=True)\n        block_string = string_object.encode()\n        raw_hash = hashlib.sha256(block_string)\n        hex_hash = raw_hash.hexdigest()\n        return hex_hash"
      """
      return "class Blockchain:\n    def __init__(self):\n        self.chain = []\n        self.transactions = []\n    \n    def create_block(self, nonce, previous_hash):\n        block = {\n            'block_number': len(self.chain) + 1,\n            'timestamp': self._current_timestamp(),\n            'nonce': nonce,\n            'previous_hash': previous_hash\n        }\n        self.chain.append(block)\n        return block\n    \n    def _current_timestamp(self):\n        return round(time.time() * 1000)\n    \n    def hash_block(self, block):\n        string_object = json.dumps(block, sort_keys=True)\n        block_string = string_object.encode()\n        raw_hash = hashlib.sha256(block_string)\n        hex_hash = raw_hash.hexdigest()\n        return hex_hash"
  
output:     
      return "class Blockchain:\n    def __init__(self):\n        self.chain = []\n        self.transactions = []\n    \n    def create_block(self, nonce, previous_hash):\n        block = {\n            'block_number': len(self.chain) + 1,\n            'timestamp': self._current_timestamp(),\n            'nonce': nonce,\n            'previous_hash': previous_hash\n        }\n        self.chain.append(block)\n        return block\n    \n    def _current_timestamp(self):\n        return round(time.time() * 1000)\n    \n    def hash_block(self, block):\n        string_object = json.dumps(block, sort_keys=True)\n        block_string = string_object.encode()\n        raw_hash = hashlib.sha256(block_string)\n        hex_hash = raw_hash.hexdigest()\n        return hex_hash"
  