Does the cost is updating with new value of shares or old ones ? about finrl-trading HOT 1 OPEN

self.cost is a is a variable to track the cost of the agents strategy and updated after each sell/buy action
+=
it will change the account balance because the cost for trading a stock is directly substracted from the money the agent has.
It will not update the new amount of shares:
What this function does:

1. check if the stock is currently held if yes the sell the amount the agent decided as an action
2. update the balance by incresasing the balance money you get from the selling of that stock (minus transaction costs so (1-TRANSACTION_FFE_PERCENT)) (not min(abs(action,self.state[index+STOCK_DIM+1]) ensures that you can just sell maximum the amount of shares currently held of that stock because if the rescaled action says "sell 10 stocks but you just hold 5 stocks" you can just sell 5 stocks because no short selling is allowed.
3. update the number of that particular stock currently held if you hold 10 stocks and sell 5 then you just hold 5 stocks
4. update the cost variable to track the money spend by follwing the agents strategy
5. update te count of trades so after each sell/buy action it is increased by 1 to see in the end how much trades the agent made

so yes you are right every sell action deductes the transaction cost directly from the balance

i created a similar environment but seperated the balance, stock_prices, num_stocks in the portfolio to increase readability
just think about it as this

def _sell_stock(self, index, action):

self.holdings[index] > 0: (step 1)
#update balance, holding, cost, trade_count
amount = round(min(abs(action),self.holdings[index])) #amount to trade (step 2)
stock_sell_activity = [self.day,index,amount] (aditionally Track sell activity)
self.balance += self.stock_price[index]amount(1-TRANSACTION_FEE_PERCENT-BPS) (step 2) update balance note BPS is a spread considere because bid and aski prices differ
self.holdings[index] -= amount (step 3) update the hodling of the particular stock by substracting the amount sold
self.cost += self.stock_price[index]amount(TRANSACTION_FEE_PERCENT+BPS) (step 4)
self.daily_cost += self.stock_price[index]amount(TRANSACTION_FEE_PERCENT+BPS) (aditionally track daily cost of the strategy)
self.daily_sell_amount += round(self.stock_price[index]amount(1-TRANSACTION_FEE_PERCENT-BPS)) (additonally to tracl daily amount sold )
self.trades+=1 (step 5)
else:
pass

from finrl-trading.