Merge two python pandas data frames of different length but keep all rows in output data frame

Question

I have the following problem: I have two pandas data frames of different length containing some rows and columns that have common values and some that are different, like this:

df1:                                 df2:        Column1  Column2  Column3           ColumnA  ColumnB ColumnC     0    a        x        x            0    c        y       y     1    c        x        x            1    e        z       z     2    e        x        x            2    a        s       s     3    d        x        x            3    d        f       f     4    h        x        x     5    k        x        x             

What I want to do now is merging the two dataframes so that if ColumnA and Column1 have the same value the rows from df2 are appended to the corresponding row in df1, like this:

df1:     Column1  Column2  Column3  ColumnB  ColumnC   0    a        x        x        s        s   1    c        x        x        y        y   2    e        x        x        z        z   3    d        x        x        f        f   4    h        x        x        NaN      NaN   5    k        x        x        NaN      NaN 

I know that the merge is doable through

df1.merge(df2,left_on='Column1', right_on='ColumnA') 

but this command drops all rows that are not the same in Column1 and ColumnA in both files. Instead of that I want to keep these rows in df1 and just assign NaN to them in the columns where other rows have a value from df2, as shown above. Is there a smooth way to do this in pandas?

Thanks in advance!

Answer 1

You can read the documentation here: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.merge.html

What you are looking for is a left join. The default option is an inner join. You can change this behavior by passing a different how argument:

df1.merge(df2,how='left', left_on='Column1', right_on='ColumnA')