How do I create a new column to an existing data frame which is the product of other columns? I have data estimating what kind of food is used on a daily basis at school to feed kids.

Pizza   Apples  Lemonade  Lemon   Banana  Milk  Soup
 12        3       10       1      9      10     6
 14        5       8        0      13     11     7
 ....................................................


I want to create a new variable Fruits_total which is a sum of variables Apples, Lemon, and Banana, and add it to an existing data frame.

  df['Fruits_total'] = df.Apples + df.Lemon + df.Banana


However, it doesn't work. Where is my mistake?
Use the below code

df['Fruits_total'] = df[['Apples','Lemon','Banana']].sum(axis=1)


axis = 1, shows that you are summing up the rows. This is what you missed in your code.