根据条件转换特定列值，并使用该转换后的值更新另一行

Question

I have a dataframe that looks something like:

Obj_1   Obj_2   Obj_3   Num_1    Num_2     Num_3   Month

A       BCD      QW      111       222     3456    2018-12
B       Bdfh     AQW     1114     1222     23456    2018-12
A       BCD      QW      22222     67      3463    2019-01
B       Bdfh     AQW     15511    2422     13456    2019-01
A       BCD      QW      257      457867    34663   2019-02
B       Bdfh     AQW     11551     27722    53456    2019-02
.....
.....
....

I would like to apply some basic math computation such that if Obj_3 == 'AQW' and Month == '2018-12', then pick the corresponding Num_3 value and multiply by 2, and assign that to the row that corresponds to 2019-02 with the same Obj columns combinations.

So the output would look something like :

Obj_1   Obj_2   Obj_3   Num_1    Num_2     Num_3_adj   Month

A       BCD      QW      111       222     3456    2018-12
B       Bdfh     AQW     1114     1222     23456    2018-12
A       BCD      QW      22222     67      3463    2019-01
B       Bdfh     AQW     15511    2422     13456    2019-01
A       BCD      QW      257      457867    34663   2019-02
B       Bdfh     AQW     11551     27722    46912    2019-02
.....
.....
....

I am thinking of first ordering the dataframe by object columns, then assign a flag or counter to unique object row combinations. Post that filter for the condition and assign it to a row where the counter matches and the Month matches the desired type.

Answer 1

You can achieve the expected output with the following code,

>>> df.loc[df.query('Obj_3 == "AQW" and Month == "2019-02"').index,'Num_3']=df.query('Obj_3 == "AQW" and Month == "2018-12"')['Num_3'].mul(2).values

>>> df

  Obj_1 Obj_2 Obj_3  Num_1   Num_2  Num_3    Month
0     A   BCD    QW    111     222   3456  2018-12
1     B  Bdfh   AQW   1114    1222  23456  2018-12
2     A   BCD    QW  22222      67   3463  2019-01
3     B  Bdfh   AQW  15511    2422  13456  2019-01
4     A   BCD    QW    257  457867  34663  2019-02
5     B  Bdfh   AQW  11551   27722  46912  2019-02
>>>