CODEWITHSUNDEEP
pythonpandasmergedataframe
Anthony
Anthony

Reputation: 1543

Pandas dataframe left merge without reindexing

Wondering if there's a more intuitive way to merge dataframes

In[140]: df1 = pd.DataFrame(data=[[1,2],[3,4],[10,4],[5,6]], columns=['A','B'], index=[1,3,5,7])
In[141]: df1
Out[141]: 
    A  B
1   1  2
3   3  4
5  10  4
7   5  6

In[142]: df2 = pd.DataFrame(data=[[1,5],[3,4],[10,3],[5,2]], columns=['A','C'], index=[0,2,4,6])
In[143]: df2
Out[143]: 
    A  C
0   1  5
2   3  4
4  10  3
6   5  2

My desired merged should look like this

    A  B  C
1   1  2  5
3   3  4  4
5  10  4  3
7   5  6  2

The key is to retain the origin left dataframe index.
Left merge does not work because it reindexes

In[150]: pd.merge(df1, df2, how='left', on='A')
Out[150]: 
    A  B  C
0   1  2  5
1   3  4  4
2  10  4  3
3   5  6  2

After some trial and error, figured out this way that works but wonder if there's a more intuitive way to achieve the same.

In[151]: pd.merge(df1, df2, how='outer', on=['A'], right_index=True)
Out[151]: 
    A  B  C
1   1  2  5
3   3  4  4
5  10  4  3
7   5  6  2

Upvotes: 4

Views: 1371

Answers (1)

Yang
Yang

Reputation: 114

pd.merge(df1, df2, how='outer', on=['A'], right_index=True)

looks a little weird to me. It says let's join two tables on column A and also the index of the right table with nothing on the left table. I wonder why this works.

I would do something like this:

In [27]: df1['index'] = df1.index
In [28]: df2['index'] = df2.index
In [33]: df_merge = pd.merge(df1, df2, how='left', on=['A'])
In [34]: df_merge
Out[34]:
    A  B  index_x  C  index_y
0   1  2        1  5        1
1   3  4        3  4        2
2  10  4        5  3        3
3   5  6        7  2        4

In [35]: df_merge = df_merge[['A', 'B', 'C', 'index_x']]

In [36]: df_merge
Out[36]:
    A  B  C  index_x
0   1  2  5        1
1   3  4  4        3
2  10  4  3        5
3   5  6  2        7

[4 rows x 4 columns]

In [37]: df_merge.set_index(['index_x'])
Out[37]:
          A  B  C
index_x
1         1  2  5
3         3  4  4
5        10  4  3
7         5  6  2

Upvotes: 3

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