code: def _default_data(self, *args, **kwargs):
          
          data = super(Trading, self)._default_data(*args, **kwargs)
          data['key'] = self.key
          nonce = self.get_nonce()
          msg = str(nonce) + self.username + self.key
  
          signature = hmac.new(
              self.secret.encode('utf-8'), msg=msg.encode('utf-8'),
              digestmod=hashlib.sha256).hexdigest().upper()
          data['signature'] = signature
          data['nonce'] = nonce
          return data
docs: Generate a one-time signature and other data required to send a secure
          POST request to the Bitstamp API.