BUG: DatetimeIndex.is_year_start breaks on double-digit frequencies

[MarcoGorelli]

Pandas version checks

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• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

dr = pd.date_range("2017-01-01", periods=2, freq="10YS")
print(dr.is_year_start)

Issue Description

This outputs

array([False, False])

Expected Behavior

array([True, True])

this absolute hack may be to blame

pandas/pandas/_libs/tslibs/fields.pyx

Lines 256 to 257 in f2c8715

	if (freqstr[0:2] in ["MS", "QS", "YS"]) or (
	freqstr[1:3] in ["MS", "QS", "YS"]):

Installed Versions

INSTALLED VERSIONS

commit : d9cdd2e
python : 3.11.9.final.0
python-bits : 64
OS : Linux
OS-release : 5.15.146.1-microsoft-standard-WSL2
Version : #1 SMP Thu Jan 11 04:09:03 UTC 2024
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.2.2
numpy : 1.26.4
pytz : 2024.1
dateutil : 2.9.0.post0
setuptools : 65.5.0
pip : 24.0
Cython : None
pytest : 8.1.1
hypothesis : 6.100.1
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.3
IPython : 8.23.0
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : 2024.3.1
gcsfs : None
matplotlib : 3.8.4
numba : 0.59.1
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 15.0.2
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.13.0
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2024.1
qtpy : None
pyqt5 : None

[VISWESWARAN1998]

take

[VISWESWARAN1998]

Replaced absolute string slicing into split to resolve this issue

# YearBegin(), BYearBegin() use month = starting month of year.
# QuarterBegin(), BQuarterBegin() use startingMonth = starting
# month of year. Other offsets use month, startingMonth as ending
# month of year.
period_str = "".join([dt_char for dt_char in list(freqstr.split("-")[0]) if not dt_char.isdigit()])
if (period_str in ["MS", "QS", "YS"]):
    end_month = 12 if month_kw == 1 else month_kw - 1
    start_month = month_kw

Gives me

[4/4] Linking target pandas/_libs/tslibs/fields.cpython-310-x86_64-linux-gnu.so
[ True  True]

[MarcoGorelli]

I don't think this is the solution, we need to go up some levels and look for something like freq.n

@natmokval you'd done a bunch of work in this area, so this might be interesting to you to try fixing?

[VISWESWARAN1998]

I don't think this is the solution, we need to go up some levels and look for something like freq.n

@natmokval you'd done a bunch of work in this area, so this might be interesting to you to try fixing?

Can you elaborate more on this one?

[natmokval]

@natmokval you'd done a bunch of work in this area, so this might be interesting to you to try fixing?

Yeah, sure, I would like to work on this.

[VISWESWARAN1998]

period_str = "".join([
    dt_char for dt_char in list(freqstr.split("-")[0]) if not dt_char.isdigit()
])

Splits the freqstr and removes the digits. So it can properly parse 10MS, 10YS-JAN, 2B etc., I believe it is more viable option to absolute slicing