How to update dataframe cells using function return values

问题

I have the following dataframe called df1,

    country ticker price
0   US      MSFT   105.32
1   US      AAPL
2   GERMANY NSU.DE 10.42
3   SG      D05.SI
4   AUS     WOW.AX

I have a function called price_get that looks like this

def price_get(ticker):
   price = somefunction
   return price

The function has to go online to look up the value so it takes a few seconds to run each time.

I want to only use this function on the cells which don't have a price in them, (price cells are empty).

So the function would only be used on rows 1, 3 & 4 in this dataframe and update the price values in them to look something like this

    country ticker price
0   US      MSFT   105.32
1   US      AAPL   150.22
2   GERMANY NSU.DE 10.42
3   SG      D05.SI 23.44
4   AUS     WOW.AX 12.33

How do I apply the price_get function to only the price rows which are empty, use the ticker column string as an input, and update the price column?

回答1:

You should use Boolean Masking for this task, example:

df.loc[df['price'].isna(), 'ticker'] = df.loc[df['price'].isna(), 'ticker'].apply(price_get)

回答2:

Let us do filter then apply your function

df.price.fillna(df.loc[df.price.isnull(),'ticker'].apply(yourfunction),inplace=True)

来源：https://stackoverflow.com/questions/61647693/how-to-update-dataframe-cells-using-function-return-values