BUG: Index._searchsorted_monotonic(..., side='right') returns the left side position for monotonic decreasing indexes

[jschendel]

Code Sample, a copy-pastable example if possible

In [1]: import pandas as pd
   ...: idx = pd.Index([3, 2, 1])
   ...: idx
   ...:
Out[1]: Int64Index([3, 2, 1], dtype='int64')

In [2]: idx._searchsorted_monotonic(3, side='right')
Out[2]: 0

In [3]: idx._searchsorted_monotonic(2, side='right')
Out[3]: 1

In [4]: idx._searchsorted_monotonic(1, side='right')
Out[4]: 2

Problem description

The left side positions are returned even though side='right' is specified.

Note that this is also the root cause of #16417

Expected Output

I'd expect the output to be 1, 2, 3 instead of 0, 1, 2.

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

pandas: 0.20.3
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.26
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

[jschendel]

This issue appears to be due to an invalid ternary statement in pandas/core/indexes/base.py:

elif self.is_monotonic_decreasing:
    # np.searchsorted expects ascending sort order, have to reverse
    # everything for it to work (element ordering, search side and
    # resulting value).
    pos = self[::-1].searchsorted(label, side='right' if side == 'left'
                                  else 'right')
    return len(self) - pos

Note that the ternary statement side='right' if side == 'left' else 'right' always evaluates to 'right'. The fix is to end it with else 'left'. I'll submit a PR shortly.