async def create_order(self, symbol, action, price, quantity, order_type=ORDER_TYPE_LIMIT, *args, **kwargs):
    " Create an order.\n\n        Args:\n            symbol: Trade target\n            action: Trade direction, `BUY` or `SELL`.\n            price: Price of each contract.\n            quantity: The buying or selling quantity.\n            order_type: Order type, `MARKET` or `LIMIT`.\n\n        Returns:\n            order_no: Order ID if created successfully, otherwise it's None.\n            error: Error information, otherwise it's None.\n        "
    if (action == ORDER_ACTION_BUY):
        side = 'buy'
    else:
        side = 'sell'
    size = abs(float(quantity))
    price = float(price)
    if (order_type == ORDER_TYPE_LIMIT):
        ot = 'limit'
    elif (order_type == ORDER_TYPE_MARKET):
        ot = 'market'
        price = None
    else:
        raise NotImplementedError
    (success, error) = (await self._rest_api.place_order(symbol, side, price, size, ot))
    if error:
        return (None, error)
    if (not success['success']):
        return (None, 'place_order error')
    result = success['result']
    return (str(result['id']), None)
Create an order.

Args:
    symbol: Trade target
    action: Trade direction, `BUY` or `SELL`.
    price: Price of each contract.
    quantity: The buying or selling quantity.
    order_type: Order type, `MARKET` or `LIMIT`.

Returns:
    order_no: Order ID if created successfully, otherwise it's None.
    error: Error information, otherwise it's None.
async def create_order(self, symbol, action, price, quantity, order_type=ORDER_TYPE_LIMIT, *args, **kwargs):
    " Create an order.\n\n        Args:\n            symbol: Trade target\n            action: Trade direction, `BUY` or `SELL`.\n            price: Price of each contract.\n            quantity: The buying or selling quantity.\n            order_type: Order type, `MARKET` or `LIMIT`.\n\n        Returns:\n            order_no: Order ID if created successfully, otherwise it's None.\n            error: Error information, otherwise it's None.\n        "
    if (action == ORDER_ACTION_BUY):
        side = 'buy'
    else:
        side = 'sell'
    size = abs(float(quantity))
    price = float(price)
    if (order_type == ORDER_TYPE_LIMIT):
        ot = 'limit'
    elif (order_type == ORDER_TYPE_MARKET):
        ot = 'market'
        price = None
    else:
        raise NotImplementedError
    (success, error) = (await self._rest_api.place_order(symbol, side, price, size, ot))
    if error:
        return (None, error)
    if (not success['success']):
        return (None, 'place_order error')
    result = success['result']
    return (str(result['id']), None)