Insert a column with a conditional function into a dataframe

I want to calculate the difference between open and close price but taking to account if a trade is a Buy/Sell.

I have the conditional function below; but l do not know how to insert a new column with the results into the original dataframe which has a list of trades and l am getting an error with this function.

def pl_gap(type):
if type in mt_trades[‘type’] == ‘BUY’:
mt_trades[‘close_price’] – mt_trades[‘open_price’]
else:
mt_trades[‘open_price’] – mt_trades[‘close_price’]

mt_trades[‘pl_gap’] = mt_trades.apply(pl_gap)

>Solution :

Let me know if this works for you or If you can show me the head of your data I can help better without guessing.

Define a function to calculate the profit/loss gap based on trade type

def pl_gap(row):
    if row['type'] == 'BUY':
        return row['close_price'] - row['open_price']
    else:
        return row['open_price'] - row['close_price']

Apply the function to create a new ‘pl_gap’ column

mt_trades['pl_gap'] = mt_trades.apply(lambda row: pl_gap(row), axis=1)