'SyntaxError invalid syntax' when adding column with if condition to pandas dataframe

Question

I want to add a new column dfout['EXCHANGE_RATIO'] whose values (rows) will be taken from another dataframe ( dfc['EXCHANGE_RATIO'] ) only when dfout['CURRENCY'] != 'EUR' . When dfout['CURRENCY'] != 'EUR' I search for that value in dfc['CURRENCY_SOURCE'] and I take the value of dfc['EXCHANGE_RATIO'] in that same row.

dfout looks like this:

                DATE_PROCESS  BOOKING_ID DEP_AIRPORT ARR_AIRPORT       DEPARTURE_DATE         ARRIVAL_DATE    PRICE CURRENCY
0    2013-04-19 16:04:13 UTC    76969972         AEL         DEL  2013-04-18 00:00:00                  NaN   409.04      EUR
1    2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00                  NaN   280.70      EUR

dfc looks like this:

    CURRENCY_SOURCE CURRENCY_TARGET  EXCHANGE_RATIO
0               TRL             EUR    9.900000e-08
1               VES             EUR    3.220000e-07

I've tried these 2 ways and both throw out Syntax error: invalid syntax . Why?

dfout['EXCHANGE_RATIO'] = dfout['CURRENCY'].apply(lambda x: dfc.query('CURRENCY_SOURCE'==x)['EXCHANGE_RATIO'] if x != 'EUR')

dfout['EXCHANGE_RATIO'] = dfout['CURRENCY'].apply(lambda x: dfc.loc[dfc['CURRENCY_SOURCE'] == x, 'EXCHANGE_RATIO'].iloc[-1] if x != 'EUR')

Answer 1

you can use the map method:

dfout['EXCHANGE_RATIO'] = dfout['CURRENCY'] \
    .map(dict(zip(dfc['CURRENCY_SOURCE'], dfc['EXCHANGE_RATIO'])))

for example with a dfout like this:

              DATE_PROCESS  BOOKING_ID DEP_AIRPORT ARR_AIRPORT       DEPARTURE_DATE  ARRIVAL_DATE   PRICE CURRENCY
0  2013-04-19 16:04:13 UTC    76969972         AEL         DEL  2013-04-18 00:00:00           NaN  409.04      EUR
1  2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00           NaN  280.70      EUR
2  2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00           NaN  280.70      TRL
3  2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00           NaN  280.70      VES

you would get the following output:

              DATE_PROCESS  BOOKING_ID DEP_AIRPORT ARR_AIRPORT       DEPARTURE_DATE  ARRIVAL_DATE   PRICE CURRENCY  EXCHANGE_RATIO
0  2013-04-19 16:04:13 UTC    76969972         AEL         DEL  2013-04-18 00:00:00           NaN  409.04      EUR             NaN
1  2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00           NaN  280.70      EUR             NaN
2  2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00           NaN  280.70      TRL    9.900000e-08
3  2014-04-17 02:26:46 UTC    76888867         ARP         ZAL  2014-04-19 00:00:00           NaN  280.70      VES    3.220000e-07

and if you want to replace those NaN you can use fillna() :

dfout['EXCHANGE_RATIO'] = dfout['CURRENCY'] \
    .map(dict(zip(dfc['CURRENCY_SOURCE'], dfc['EXCHANGE_RATIO']))) \
    .fillna(1)  # or whatever you want there

Answer 2

Corrected the query syntax and added else to make your code work. You have to call the x with the @ .

dfout['EXCHANGE_RATIO'] = dfout['CURRENCY'].apply(lambda x: dfc.query('CURRENCY_SOURCE==@x')['EXCHANGE_RATIO'][0] if x != 'EUR' else np.NaN)

dfout - input

                                DATE_PROCESS    BOOKING_ID  DEP_AIRPORT ARR_AIRPORT DEPARTURE_DATE  ARRIVAL_DATE    PRICE   CURRENCY
0   2013-04-19  16:04:13    UTC 76969972        AEL         DEL         2013-04-18  00:00:00        NaN             409.04  EUR
1   2014-04-17  02:26:46    UTC 76888867        ARP         ZAL         2014-04-19  00:00:00        NaN             280.70  EUR
2   2014-04-17  02:26:46    UTC 76888867        ARP         ZAL         2014-04-19  00:00:00        NaN             280.70  TRL

dfc

  CURRENCY_SOURCE CURRENCY_TARGET  EXCHANGE_RATIO
0             TRL             EUR    9.900000e-08
1             VES             EUR    3.220000e-07

dfout - Output

                                DATE_PROCESS    BOOKING_ID  DEP_AIRPORT ARR_AIRPORT DEPARTURE_DATE  ARRIVAL_DATE    PRICE   CURRENCY    EXCHANGE_RATIO
0   2013-04-19  16:04:13    UTC 76969972        AEL         DEL         2013-04-18  00:00:00        NaN             409.04  EUR         NaN
1   2014-04-17  02:26:46    UTC 76888867        ARP         ZAL         2014-04-19  00:00:00        NaN             280.70  EUR         NaN
2   2014-04-17  02:26:46    UTC 76888867        ARP         ZAL         2014-04-19  00:00:00        NaN             280.70  TRL         9.900000e-08