BUG: Join of DataFrames with MultiIndex does not keep order of index · Issue #34133 · pandas-dev/pandas (original) (raw)

Code Sample, a copy-pastable example

import numpy as np import pandas as pd

arrays = [np.array(['top', 'top', 'top', 'top', 'bottom', 'bottom', 'bottom', 'bottom']), np.array(['bar', 'bar', 'baz', 'baz', 'bar', 'bar', 'baz', 'baz']), np.array(['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two'])]

left = pd.DataFrame(np.random.randn(8, 4), index=arrays) left.index.set_names(['pos', 'vars', 'numbers'], inplace=True)

right = pd.DataFrame({'vars': ['bar', 'bar', 'baz', 'baz'], 'numbers': ['one', 'two', 'one', 'two'], 'value': [1, 2, 3, 4]}) right = right.set_index(['vars', 'numbers']) joined = left.join(right)

print(left.index.names) print(joined.index.names)

output

['pos', 'vars', 'numbers']
['vars', 'numbers', 'pos']

Problem description

When two multindexed DataFrames are joined on a subset of the left index the resulting index becomes reordered.

Expected Output

I expect the order of the left dataframe to be kept after the left join.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.5.final.0
python-bits : 64
OS : Linux
OS-release : 5.3.0-51-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.3
numpy : 1.16.2
pytz : 2019.2
dateutil : 2.7.3
pip : 18.1
setuptools : 41.1.0
Cython : None
pytest : 3.7.4
hypothesis : None
sphinx : 2.4.4
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.5.0
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.10.2
pandas_datareader: None
bs4 : 4.9.0
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : 4.5.0
matplotlib : 3.1.2
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 3.7.4
pyxlsb : None
s3fs : None
scipy : 1.1.0
sqlalchemy : None
tables : None
tabulate : None
xarray : 0.14.1
xlrd : None
xlwt : None
xlsxwriter : None
numba : None