code: def _buildQuery(self, params):
      # Build the signature.
      nonce = self.__nonce.getNext()
      message = "%d%s%s" % (nonce, self.__clientId, self.__key)
      signature = (
          hmac.new(self.__secret, msg=message, digestmod=hashlib.sha256)
          .hexdigest()
          .upper()
      )
  
      # Headers
      headers = {}
      headers["User-Agent"] = HTTPClient.USER_AGENT
  
      # POST data.
      data = {}
      data.update(params)
      data["key"] = self.__key
      data["signature"] = signature
      data["nonce"] = nonce
  
      return (data, headers)
  
imports: import time
  import datetime
  import hmac
  import hashlib
  import requests
  import threading
  from pyalgotrade.utils import dt
  from pyalgotrade.bitstamp import common
  import logging
label_desc: Use image node_id 2 for calling the HTTPClient obj's underlying member method code with example usage: obj._buildQuery(params) and returns: data,  headers