Adding/inserting values in pandas dataframe based on 1 or more columns

Question

I have 2 dataframes

date       sitename  Auto_name                  AutoCount                         
2012-05-01 chess.com Autobiographer               8
2012-05-05 chess.com Autobiographer               1
2012-05-15 chess.com Autobiographer               3

And

date       sitename  Stu_name      Student count
2012-05-01 chess.com Student        4
2012-05-02 chess.com Student        2

How the output should be

date       sitename    Autoname                 AutoCount     Stu_name    Stu_count                     
2012-05-01 chess.com Autobiographer               8            Student       4
2012-05-02 chess.com Autobiographer               0            Student       2
2012-05-05 chess.com Autobiographer               1            Student       0
2012-05-15 chess.com Autobiographer               3            Student       0

I want to insert the name and student count from second into first but based on date column. It doesn't look that difficult, but I am not able to figure out this one.

Answer 1

You can eg use the merge function (see the docs on merging dataframes: http://pandas.pydata.org/pandas-docs/stable/merging.html ). Assuming your dataframes are called df1 and df2 :

In [13]: df = pd.merge(df1, df2, how='outer')

In [14]: df
Out[14]: 
         date   sitename       Auto_name  AutoCount Stu_name  StudentCount
0  2012-05-01  chess.com  Autobiographer          8  Student             4
1  2012-05-05  chess.com  Autobiographer          1      NaN           NaN
2  2012-05-15  chess.com  Autobiographer          3      NaN           NaN
3  2012-05-02  chess.com             NaN        NaN  Student             2

Above it uses the common columns to merge on (in this case date and sitename ), but you can also specify the columns with the on keyword (see docs ).

In a next step you can fill the NaN values as you like. Following your example output, this can be:

In [15]: df.fillna({'Auto_name':'Autobiographer', 'AutoCount':0, 'Stu_name':'Student', 'StudentCount':0})
Out[15]: 
         date   sitename       Auto_name  AutoCount Stu_name  StudentCount
0  2012-05-01  chess.com  Autobiographer          8  Student             4
1  2012-05-05  chess.com  Autobiographer          1  Student             0
2  2012-05-15  chess.com  Autobiographer          3  Student             0
3  2012-05-02  chess.com  Autobiographer          0  Student             2