Pandas copy values from another column only upto a certain date

Question

I have a column Date of type datetime64[ns] consisting of one row per date . And two more columns A and B . Suppose the dates start from 1st Dec 2019 and goes on till 29th Feb 2020, I wish to copy all the values from column A to column B upto a certain date x.

For example, if x = 15th Feb 2020, I need to copy values of A from 1st Dec 2019 to 14th Feb 2020 into column B.

Any help appreciated!

Answer 1

I think you need Series.mask for copy data less like x , so mask is created by Series.lt :

np.random.seed(2020)

rng = pd.date_range('2019-12-01', '2020-02-29')
df = pd.DataFrame({'Date': rng, 
                   'A': np.random.randint(10, size=91),
                   'B': np.random.randint(10, size=91)})  
print (df.head(20))
         Date  A  B
0  2019-12-01  0  5
1  2019-12-02  8  2
2  2019-12-03  3  4
3  2019-12-04  6  3
4  2019-12-05  3  0
5  2019-12-06  3  9
6  2019-12-07  7  8
7  2019-12-08  8  2
8  2019-12-09  0  0
9  2019-12-10  0  6
10 2019-12-11  8  7
11 2019-12-12  9  1
12 2019-12-13  3  7
13 2019-12-14  7  2
14 2019-12-15  2  5
15 2019-12-16  3  9
16 2019-12-17  6  6
17 2019-12-18  5  7
18 2019-12-19  0  9
19 2019-12-20  4  3

---

x = '2019-12-10'

df['B'] = df['B'].mask(df["Date"].lt(x), df['A'])
print (df.head(20))
         Date  A  B
0  2019-12-01  0  0
1  2019-12-02  8  8
2  2019-12-03  3  3
3  2019-12-04  6  6
4  2019-12-05  3  3
5  2019-12-06  3  3
6  2019-12-07  7  7
7  2019-12-08  8  8
8  2019-12-09  0  0
9  2019-12-10  0  6
10 2019-12-11  8  7
11 2019-12-12  9  1
12 2019-12-13  3  7
13 2019-12-14  7  2
14 2019-12-15  2  5
15 2019-12-16  3  9
16 2019-12-17  6  6
17 2019-12-18  5  7
18 2019-12-19  0  9
19 2019-12-20  4  3