Wrong char in Polish datetime month_name function

[CezaryPukownik]

Code Sample, a copy-pastable example if possible

# using this:
df['date'].dt.month_name('PL')
# should return this:
df['date'].dt.month_name('PL').str.replace('ñ','ń')

Problem description

[this should explain why the current behaviour is a problem and why the expected output is a better solution.]

Wrong char was used for Polish month name.
Pandas returns 'Styczeñ', should be 'Styczeń'.

ñ -> ń

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Expected Output

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]
INSTALLED VERSIONS

commit : None
python : 3.7.4.final.0
python-bits : 64
OS : Windows
OS-release : 10
machine : AMD64
processor : Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder : little
LC_ALL : None
LANG : en
LOCALE : None.None

pandas : 0.25.1
numpy : 1.16.5
pytz : 2019.3
dateutil : 2.8.0
pip : 19.2.3
setuptools : 41.4.0
Cython : 0.29.13
pytest : 5.2.1
hypothesis : None
sphinx : 2.2.0
blosc : None
feather : 0.4.0
xlsxwriter : 1.2.1
lxml.etree : 4.4.1
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.10.3
IPython : 7.8.0
pandas_datareader: None
bs4 : 4.8.0
bottleneck : 1.2.1
fastparquet : None
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 : None
scipy : 1.3.1
sqlalchemy : 1.3.9
tables : 3.5.2
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.1

[TomAugspurger]

Does this differ from whats available from calendar?

In [9]: import calendar

In [10]: list(calendar.month_name)
Out[10]:
['',
 'January',
 'February',
 'March',
 'April',
 'May',
 'June',
 'July',
 'August',
 'September',
 'October',
 'November',
 'December']

If they're the same, then there's likely an issue with your locale settings, or an issue in Python itself.

[CezaryPukownik]

Yes it differ.

import calendar
calendar.month_name[1]
'styczeń'
list(calendar.month_name)
['', 'styczeń', 'luty', 'marzec', 'kwiecień', 'maj', 'czerwiec', 'lipiec', 'sierpień', 'wrzesień', 'październik', 'listopad', 'grudzień']

I think this is just a cosmetic issue.
The spanish "ñ" (with tylda) sign was used, that is more common than polish "ń" (with ' )

[TomAugspurger]

I think our calendar stuff is all in https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/ccalendar.pyx, if you're interested in debugging this.

[lodagro]

pandas generates the month names using get_locale_names from the mentioned ccalendar, using a locale context manager changing the setting for LC_TIME. However on windows (can`t reproduce this on linux) there is seems to be a difference when using locale.LC_TIME or ```locale.LC_ALL```. Using ```LC_ALL``` the output is ok.

In [1]: from pandas._libs.tslibs.ccalendar import get_locale_names

In [2]: get_locale_names('f_month', 'pl_PL')
Out[2]:
['',
 'styczeñ',
 'luty',
 'marzec',
 'kwiecieñ',
 'maj',
 'czerwiec',
 'lipiec',
 'sierpieñ',
 'wrzesieñ',
 'pa\x9fdziernik',
 'listopad',
 'grudzieñ']

In [1]: import locale

In [2]: import calendar

In [3]: locale.setlocale(locale.LC_TIME, 'PL')
Out[3]: 'PL'

In [4]: calendar.month_name[1]
Out[4]: 'styczeñ'

In [5]: locale.setlocale(locale.LC_ALL, 'PL')
Out[5]: 'PL'

In [6]: calendar.month_name[1]
Out[6]: 'styczeń'