Get previous week value Python dataframe

Question

I have the following dataframe:

Date        Time    Quantity  
20171003    5:00           2       
20171003    5:15           5  
....
20171005    5:00           1   
20171005    5:15           9  

I need to create a new column containing the quantity of the same day of the previous week, that is:

Date        Time    Quantity    Quantity-1
20171003    5:00           2         NaN
20171003    5:15           5         NaN
....
20171005    5:00           1           2
20171005    5:15           9           5

I figured out how to get the same day of the last week by using for example:

last_week = today() + relativedelta(weeks=-1, weekday= now.weekday())

How to apply this to my dataframe?

Thank you in advance!

Answer 1

Does your index have a pattern? If yes, you could use pd.shift(). The periods paramater would be the number of periods in your df. For example, assuming your Time column is always whether 5:00 or 5:15, and that you have calendar days, your period would be 7 * 2 = 14

df['Quantity-1'] = df['Quantity'].shift(14)