BUG: modulo of pd.Int64Index returns negative values under some conditions

[felixpatzelt]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

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
(pd.Int64Index(list(range(0,24))*500) - 5) % 24

# returns Int64Index([-5, -4, -3, -2, -1,  0,  1,  2,  3,  4,
#            ...
#             9, 10, 11, 12, 13, 14, 15, 16, 17, 18],
#           dtype='int64', length=12000)

# note: (pd.Int64Index(list(range(0,24))*400) - 5) % 24 does not return negative values

Problem description

In Pandas 1.1.2, taking the modulo of an Int64Index can return negative values depending on the values in the index and on its length. This problem was not present in pandas 1.0.5. It is also inconsistent with the generally expected behaviour of modulo in python and numpy.

Expected Output

Int64Index([19, 20, 21, 22, 23,  0,  1,  2,  3,  4,
            ...
             9, 10, 11, 12, 13, 14, 15, 16, 17, 18],
           dtype='int64', length=9600)

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 2a7d332
python : 3.7.5.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.1.2
numpy : 1.19.2
pytz : 2020.1
dateutil : 2.8.1
pip : 19.2.3
setuptools : 41.2.0
Cython : None
pytest : 6.0.2
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : 0.10.1
psycopg2 : 2.8.6 (dt dec pq3 ext lo64)
jinja2 : 2.11.2
IPython : 7.18.1
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : 0.8.2
fastparquet : None
gcsfs : None
matplotlib : 3.2.2
numexpr : 2.7.1
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 1.0.1
pytables : None
pyxlsb : None
s3fs : 0.4.2
scipy : 1.5.2
sqlalchemy : 1.3.19
tables : 3.6.1
tabulate : None
xarray : 0.16.1
xlrd : None
xlwt : None
numba : 0.51.2

[jorisvandenbossche]

@felixpatzelt Thanks a lot for the report, that's indeed a regression and wrong result ..

It's not specific to Index, also a Series (or DataFrame) operation has the same issue:

In [43]: (pd.Series(list(range(0,24))*500) - 5) % 24
Out[43]: 
0        -5
1        -4
2        -3
3        -2
4        -1
         ..
11995    14
11996    15
11997    16
11998    17
11999    18
Length: 12000, dtype: int64

[dishak331]

I would like to work on this

[dishak331]

take

[jorisvandenbossche]

@dishak331 Thanks!

See also #35685 in case you might first want to find out which change caused the regression (sometimes that can be helpful)

[asishm]

possibly duplicate of #36047
don't have access to an environment right now

[dishak331]

@jorisvandenbossche thanks!

[jbrockmendel]

xref pydata/numexpr#365

[dishak331]

okay so i just checked, this is a bug in Python itself. When we are doing -5%24 it is outputting as 19 instead of -5

[marcelmindemann]

@dishak331 No, it's not a bug in Python. Thats the defined behaviour of the modulo operation in Python 3. It's numexpr that is wrong here by applying C conventions into Python.

[dishak331]

oh okay okay @m1nde

[simonjayhawkins]

possibly duplicate of #36047

looks like the same PR responsible, #36047 (comment)

first bad commit: [01623f8] CLN: Refactor pandas/tests/base - part3 (#30147)

https://github.com/simonjayhawkins/pandas/runs/1150901284?check_suite_focus=true

[simonjayhawkins]

I think can close this as duplicate of #36047