If I have a dataframe similar to this one
Apples Bananas Grapes Kiwis 2 3 nan 1 1 3 7 nan nan nan 2
You can first select by iloc and then sum:
df['Fruit Total']= df.iloc[:, -4:-1].sum(axis=1) print (df) Apples Bananas Grapes Kiwis Fruit Total 0 2.0 3.0 NaN 1.0 5.0 1 1.0 3.0 7.0 NaN 11.0 2 NaN NaN 2.0 3.0 2.0
For sum all columns use:
df['Fruit Total']= df.sum(axis=1)