pandas math operation with where condition

I want to do math on a condition in a dataframe, but I couldn’t.

my code:

open_position=df.loc[df['sale_price'].isna(), 'purchase_price'*'amount'].sum()

I want to do. Multiply df['purchase_price'] and df['amount'] in rows where df['sales_price'] is NaN and get the sum of all these.

it gives the following error.

TypeError: can't multiply sequence by non-int of type 'str'

Can you help with this?

>Solution :

Try this:

df.loc[df['sale_price'].isna(), 'purchase_price'].mul(df['amount']).sum()

or

df['product'] = df['purchase_price'] * df['amount']
df.loc[df['sale_price'].isna(), 'product'].sum()