TST: add test for df.where() with category dtype

[ganevgv]

• xref BUG: DataFrame.where with category dtype #16979
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[gfyoung]

@ganevgv : Good job with the tests so far! #16979 is a two-parter, and since both parts have been fixed on master, they both need tests.

If you would like, you can add (in this PR) a second test to address the .astype error.

[ganevgv]

@gfyoung, thanks for the guidance up to now! After adding my second test test_df_where_change_dtype(...) in this PR, there seems to be some inconsistencies across the different operating systems.

df = DataFrame(np.arange(2 * 3).reshape(2, 3), columns=list("ABC"))
mask = np.array([[True, False, True], [False, True, True]])

result = df.where(mask)
expected = DataFrame([[0, np.nan, 2], [np.nan, 4, 5]], columns=list("ABC"))

tm.assert_frame_equal(result, expected)

After running the code above, on some operating systems (linux -- Python 3.6.7, win32 -- Python 3.6.7, win32 -- Python 3.7.5) applying the boolean mask changes the dtype of the last column [2, 5] from int64 to int32. I cannot reproduce this behaviour locally.

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit : 35e91f9 python : 3.6.5.final.0 python-bits : 64 OS : Darwin OS-release : 17.7.0 machine : x86_64 processor : i386 byteorder : little LC_ALL : None LANG : en_GB.UTF-8 LOCALE : en_GB.UTF-8

pandas : 0.26.0.dev0+734.g0de99558b.dirty
numpy : 1.17.3
pytz : 2019.3
dateutil : 2.8.0
pip : 19.3.1
setuptools : 39.0.1
Cython : 0.29.14
pytest : 5.2.2
hypothesis : 4.42.6
sphinx : 2.2.1
blosc : 1.8.1
feather : None
xlsxwriter : 1.2.2
lxml.etree : 4.4.1
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.10.3
IPython : 7.9.0
pandas_datareader: None
bs4 : 4.7.1
bottleneck : 1.2.1
fastparquet : 0.3.2
gcsfs : None
lxml.etree : 4.4.1
matplotlib : 3.1.1
numexpr : 2.7.0
odfpy : None
openpyxl : 3.0.0
pandas_gbq : None
pyarrow : 0.15.1
pytables : None
s3fs : 0.3.5
scipy : 1.3.1
sqlalchemy : 1.3.10
tables : 3.6.1
xarray : 0.14.0
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.2

What would be your suggestion? I'm happy to remove or xfail the test_df_where_change_dtype(...) test from this PR (so we can merge it) and raise an issue. I can also force the dtype but that's not ideal.

[gfyoung]

@ganevgv : Why don't you revert your last commit, and we can focus on it in a separate PR.

[ganevgv]

@gfyoung thanks - removed the second test in this PR, will create a new issue specifying the problem

[gfyoung]

Actually, can you comment in the original issue? I plan on keeping that one open.