How to get value from two columns of a pandas DataFrame, based on the values of another column?

Question

I have a DataFrame with 3 columns. The 1st column is Temperature. The other two columns of output variables. Sample DataFrame:

import pandas as pd
import numpy as np

data = [(25, 1, 2),
        (30, 1, 2),
        (25, 1, 2),
        (25, 1, 2),
        (np.nan, 1, 2),
        (40, 1, 2) ]

data_df = pd.DataFrame(data, columns = ['temperature', 'y_1', 'y_2'])

What I want to do is adding a new column to the DataFrame as below:

• If value of temperature is 25, use column 'y_1'.

• For other cases, use value of column 'y_2'.

For the above example, the new column values will be :

[1, 2, 1, 1, 2, 2]

Thank you for your help.