BUG: Intersection of RangeIndexes fails when the step sign is negative · Issue #17296 · pandas-dev/pandas (original) (raw)

Code Sample, a copy-pastable example if possible

Setup

In [1]: import pandas as pd ...: idx1 = pd.RangeIndex(10) ...: idx2 = pd.RangeIndex(9, -1, -1) ...: idx3 = pd.RangeIndex(9, 5, -1) ...:

Issue 1: Mixed sign steps

In [2]: idx1.intersection(idx2) Out[2]: RangeIndex(start=0, stop=10, step=1)

In [3]: idx2.intersection(idx1) Out[3]: RangeIndex(start=0, stop=10, step=-1)

Issue 2: Both steps negative

In [4]: idx2.intersection(idx3) Out[4]: RangeIndex(start=6, stop=10, step=-1)

Problem description

Issue 1: The output is inconsistent depending on the order of the intersection; one returns an index identical to idx1, the other an empty index.

Issue 2: The returned index is empty, even though idx3 is a subset of idx2.

Expected Output

Issue 1: I'm not 100% sure what the output for Issue 1 should be. To obey the properties of set intersections, all ten values should be returned, but what should the sign of the returned step be?

Seems like the options are:

Always default to positive step?
Default to the step sign of the first index in the intersection?
Or does the step sign not matter as long as the values within the index are correct?

Issue 2: An index identical to idx3 should be returned. Should the step sign be negative in this case, since both inputs have a negative step? Or should we always default to positive?

Seems like this issue could be lumped in with #13432. However, this issue seems restricted to just RangeIndex (all code modifications for a fix would be contained to range.py), whereas the other issue seems to be broader than a specific type of index. Regardless, I'm happy to work on a PR to fix this specific bug.

Output of `pd.show_versions()`

INSTALLED VERSIONS

commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None