BUG: nunique not ignoring both None and np.nan

[Tomer-Eliahu]

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Code Sample, a copy-pastable example

import pandas as pd
import numpy as np
pd.Series(['yes','yes', None, np.nan]).nunique(dropna = True)

Problem description

The output was 2.
It seems that nunique can only ignore None or np.nan but not both together. This seems odd to me and I think it
should either be changed or the docs should reflect this.

Expected Output

I expected to get 1 as the result but I got 2.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : db08276
python : 3.7.6.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.49+
Version : #1 SMP Sun Oct 18 19:43:35 PDT 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : C.UTF-8
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.1.3
numpy : 1.18.5
pytz : 2019.3
dateutil : 2.8.1
pip : 20.2.4
setuptools : 46.1.3.post20200325
Cython : 0.29.21
pytest : 5.4.1
hypothesis : 5.10.0
sphinx : 3.0.2
blosc : None
feather : 0.4.1
xlsxwriter : 1.2.8
lxml.etree : 4.5.0
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.13.0
pandas_datareader: 0.9.0
bs4 : 4.9.0
bottleneck : 1.3.2
fsspec : 0.8.4
fastparquet : None
gcsfs : 0.6.1
matplotlib : 3.2.1
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.3
pandas_gbq : None
pyarrow : 0.16.0
pytables : None
pyxlsb : None
s3fs : 0.5.1
scipy : 1.4.1
sqlalchemy : 1.3.16
tables : 3.6.1
tabulate : 0.8.7
xarray : 0.16.1
xlrd : 1.2.0
xlwt : 1.3.0
numba : 0.48.0

[jorisvandenbossche]

@tomereliahu Thanks for the report!

That's indeed clearly a bug, and looking at the code of nunique, it's also clear where it is coming from:

pandas/pandas/core/base.py

Lines 1035 to 1039 in 337bf20

	uniqs = self.unique()
	n = len(uniqs)
	if dropna and isna(uniqs).any():
	n -= 1
	return n

So it always subtracts 1, instead of checking how many NA-like values are present in the uniques. So we should be subtracting isna(uniqs).sum() instead, I think.

Pull requests welcome!