BUG: Index.difference and Index.intersection doesn't preserve type of Index for some Index subclasses for corner cases #20040
Labels
Algos
Non-arithmetic algos: value_counts, factorize, sorting, isin, clip, shift, diff
Bug
Dtype Conversions
Unexpected or buggy dtype conversions
Milestone
Code Sample, a copy-pastable example if possible
Problem description
The result of taking the difference of an
Index
for variousIndex
subclasses and theIndex
produces a resultingIndex
that does not preserve the type of the subclass.From a set algebra point of view, for a set S,
S.difference(S)
should equalS.intersection(nullset)
.The output from the above is:
There is some discussion in the pull request #19849, where I discovered this bug, but at request of @jreback, I have split this into a separate issue.
Expected Output
Note that for
RangeIndex
, the result of theintersection
operation is also incorrect.Output of
pd.show_versions()
INSTALLED VERSIONS
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.22.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
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