BUG: ordered categorical comparison with missing values evaluates to True

[mzwiessele]

Code Sample, a copy-pastable example if possible

import pandas as pd
pd.Categorical(["1", "2", "3", None], categories=["1", "2", "3"], ordered=True) <= "2"
# => array([ True,  True, False,  True])

Problem description

Here a missing entry is being evaluated as None <= "2" == True. Shouldn't missing values always be evaluate to False in any comparison?
I think this is related to #4537

Expected Output

import pandas as pd
pd.Categorical(["1", "2", "3", None], categories=["1", "2", "3"], ordered=True) <= "2"
# => array([ True,  True, False,  False])

Output of pd.show_versions()

Details

INSTALLED VERSIONS ------------------ commit: None python: 3.6.8.final.0 python-bits: 64 OS: Darwin OS-release: 17.5.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8

pandas: 0.24.2
pytest: 3.6.2
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.3
numpy: 1.16.3
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.1.1
sphinx: 1.7.5
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.5.4
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.0.5
lxml.etree: 4.2.2
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.2.8
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

[TomAugspurger]

Yep, should probably be False. It looks like we correctly mask for categorical : categorical comparisons, but not categorical : scalar.

In [16]: c
Out[16]:
[1, 2, 3, NaN]
Categories (3, object): [1 < 2 < 3]

In [17]: c <= c
Out[17]: array([ True,  True,  True, False])

This fix will be somewhere in

pandas/pandas/core/arrays/categorical.py

Line 100 in 65eac80

if is_scalar(other):

, similar to what we do above.

[shreyateeza]

@TomAugspurger I am new here. I would like to contribute to this issue. Can you tell me what needs to be done?

[shantanu-gontia]

The Categorical object has the _codes property, which is used for comparison.

In [1]: import pandas as pd

In [2]: a = pd.Categorical([1, 2, 3, 4], categories=[1, 2, 3], ordered=Tr
   ...: ue)

In [3]: a._codes
Out[3]: array([ 0,  1,  2, -1], dtype=int8)

In [4]: a._codes < 2
Out[4]: array([ True,  True, False,  True])

The _codes takes NaN to be -1 leading to incorrect comparison

---

Perhaps the fix lies in correct construction of the Categorical object and not in the comparison itself.