BUG: mask with alignment

[Redoute]

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Reproducible Example

import pandas as pd
s = pd.Series(range(5))
t = pd.Series([True, False])
s.mask(t, 99)

Issue Description

Series.mask replaces values if index is missing in condition. Example output:

0 99
1 1
2 99
3 99
4 99
dtype: int64

Expected Behavior

"Replace values where the condition is True." Values should not be touched if index is missing in condition.

Installed Versions

INSTALLED VERSIONS

commit : 66e3805
python : 3.8.12.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19042
machine : AMD64
processor : Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : de_DE.cp1252

pandas : 1.3.5
numpy : 1.22.0
pytz : 2021.3
dateutil : 2.8.2
pip : 21.3.1
setuptools : 60.2.0
Cython : None
pytest : None
hypothesis : None
sphinx : 4.3.2
blosc : None
feather : None
xlsxwriter : 3.0.2
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.9.3 (dt dec pq3 ext lo64)
jinja2 : 3.0.3
IPython : 7.19.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.5.1
numexpr : None
odfpy : None
openpyxl : 3.0.9
pandas_gbq : None
pyarrow : 6.0.1
pyxlsb : None
s3fs : None
scipy : 1.7.3
sqlalchemy : 1.4.29
tables : None
tabulate : 0.8.9
xarray : None
xlrd : 2.0.1
xlwt : None
numba : None

[phofl]

This behaves as documented. See:

The mask method is an application of the if-then idiom. For each element in the calling DataFrame, if cond is False the element is used; otherwise the corresponding element from the DataFrame other is used.

But could probably be clearer

[rhshadrach]

I find this a bit surprising myself - I'd expect ser.mask(cond, other) to default to ser rather than other when values are missing. In other words not changing values should be the status-quo, modifications should be opt-in.

That said, I can't really justify this, and if others don't share in my surprise it should be left as-is.