How do you add a value to a float index of a dataframe for every other row?

df = pd.DataFrame({'A': [1,2,3,4,5,6,7,8,9],
                   'B': [1,2,3,4,5,6,7,8,9]})

df.iloc[1::2, 1] = df.iloc[1::2, :].eval('B + 0.005')

    A     B
0   1   1.000
1   2   2.005
2   3   3.000
3   4   4.005
4   5   5.000
5   6   6.005
6   7   7.000
7   8   8.005
8   9   9.000

Just have to make sure that your picking the correct column with the initial iloc. [1::2] is every other starting from index 1 (so 1,3 ect). You need to select all the columns in the second iloc due to eval only working with df's and not series. Then you can set that column to index as you did in your code.