Right/outer merge behaviour on left column and right index is unexpected

[ianepreston]

Code Sample, a copy-pastable example if possible

import pandas as pd
big_index = [123, 124, 125, 126, 127, 128, 129, 130]
big_dat = {'year': pd.Series([2000, 2000, 2000, 2001, 2002, 2002, 2002, 2004], index=big_index),
          'other': pd.Series(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'], index=big_index)}
big_df = pd.DataFrame(big_dat)

year_index = [2003, 2000, 2001, 2002]
year_dat = {'a': pd.Series([1, 2, 3, 4], index=year_index),
            'b': pd.Series([5, 6, 7, 8], index=year_index)}
year_df = pd.DataFrame(year_dat)

merged_right = pd.merge(
        big_df,
        year_df,
        how='right',
        left_on='year',
        right_index=True
        )
merged_outer = pd.merge(
        big_df,
        year_df,
        how='outer',
        left_on='year',
        right_index=True
        )

merged_right
Out[5]: 
    other  year  a  b
123     a  2000  2  6
124     b  2000  2  6
125     c  2000  2  6
126     d  2001  3  7
127     e  2002  4  8
128     f  2002  4  8
129     g  2002  4  8
130   NaN  2003  1  5

merged_outer
Out[6]: 
    other  year    a    b
123     a  2000  2.0  6.0
124     b  2000  2.0  6.0
125     c  2000  2.0  6.0
126     d  2001  3.0  7.0
127     e  2002  4.0  8.0
128     f  2002  4.0  8.0
129     g  2002  4.0  8.0
130     h  2004  NaN  NaN
130   NaN  2003  1.0  5.0

Problem description

merged outer and merged right both return year 2003, a 1, b 5 associated with index 130.

Expected Output

Either a NaN index entry or an error/warning. There's no obvious reason to associate that data with 130 index

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.20.2
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

[gfyoung]

@cornucrapia : Thanks for reporting this! Could you instead print out the repr version of the DataFrame that you are getting from the merge? That would be easier for us to read.

[ianepreston]

@gfyoung absolutely. See below.

repr(merged_right)
Out[3]: '    other  year  a  b\n123     a  2000  2  6\n124     b  2000  2  6\n125     c  2000  2  6\n126     d  2001  3  7\n127     e  2002  4  8\n128     f  2002  4  8\n129     g  2002  4  8\n130   NaN  2003  1  5'

repr(merged_outer)
Out[4]: '    other  year    a    b\n123     a  2000  2.0  6.0\n124     b  2000  2.0  6.0\n125     c  2000  2.0  6.0\n126     d  2001  3.0  7.0\n127     e  2002  4.0  8.0\n128     f  2002  4.0  8.0\n129     g  2002  4.0  8.0\n130     h  2004  NaN  NaN\n130   NaN  2003  1.0  5.0'

[gfyoung]

@cornucrapia : Sorry, I misspoke 😄. Can you just print out the DataFrame i.e.:

In[5]: merged_right
Out[5]: <DataFrame>

[ianepreston]

merged_right
Out[5]: 
    other  year  a  b
123     a  2000  2  6
124     b  2000  2  6
125     c  2000  2  6
126     d  2001  3  7
127     e  2002  4  8
128     f  2002  4  8
129     g  2002  4  8
130   NaN  2003  1  5

merged_outer
Out[6]: 
    other  year    a    b
123     a  2000  2.0  6.0
124     b  2000  2.0  6.0
125     c  2000  2.0  6.0
126     d  2001  3.0  7.0
127     e  2002  4.0  8.0
128     f  2002  4.0  8.0
129     g  2002  4.0  8.0
130     h  2004  NaN  NaN
130   NaN  2003  1.0  5.0

[gfyoung]

Ah, that's a little better to read now! I do agree: I too am a little perplexed why there would be such a row, as I don't know where that would show up in either such join.

[TomAugspurger]

I believe this is the same example

import pandas as pd
left = pd.DataFrame({'a': [1, 2, 3], 'key': [0, 1, 1]})
right = pd.DataFrame({'b': [1, 2, 3]})
result = left.merge(right, left_on='key', right_index=True, how='right')

output

In [5]: left
Out[5]:
   a  key
0  1    0
1  2    1
2  3    1

In [6]: right
Out[6]:
   b
0  1
1  2
2  3

In [7]: result
Out[7]:
     a  key  b
0  1.0    0  1
1  2.0    1  2
2  3.0    1  2
2  NaN    2  3

I may have expected something like

In [11]: pd.merge(left, right.rename_axis('key').reset_index(), on='key', how='right').set_index('key', drop=False)
Out[11]:
       a  key  b
key
0    1.0    0  1
1    2.0    1  2
1    3.0    1  2
2    NaN    2  3

https://github.com/pandas-dev/pandas/pull/24904/files is adding a test with that. I'll add a note to that test that it's probably buggy, and to refer here.