Description
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I have confirmed this bug exists on the latest version of pandas.
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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.
Code Sample, a copy-pastable example
import pandas as pd
foo = pd.DataFrame(data={'e': 5}, index=pd.MultiIndex.from_tuples([(1, 2, 4)], names=('a', 'b', 'd')))
bar = pd.DataFrame(data={'f': 6}, index=pd.MultiIndex.from_tuples([(2, 3)], names=('b', 'c')))
joined = foo.join(bar, how='inner')
print(foo.join(bar))Expected Output
Something deterministic
Produced Output
When running the code multiple times (restart the python interpreter), the output switches (apparently randomly) between these two outputs:
e f
b d a c
2 4 1 3 5 6
e f
b a d c
2 1 4 3 5 6
Problem description
When joining with a data frame with a MultiIndex, the order of the levels in the resulting data frame is non-deterministic. This is very surprising, and resulted in a hard-to-debug issue in my code. This issue happens with both inner joins and left joins.
Possibly related issues: #36909, #25760
Past discussion on non-determinism in pandas: #32514, #32449, #12679
Output of pd.show_versions()
INSTALLED VERSIONS
------------------
commit : db08276bc116c438d3fdee492026f8223584c477
python : 3.8.6.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.18362
machine : AMD64
processor : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : English_United States.1252
pandas : 1.1.3
numpy : 1.19.2
pytz : 2020.1
dateutil : 2.8.1
pip : 20.1.1
setuptools : 50.3.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None