Snap drops name from datetimeindex

[danielwlogan]

Code Sample, a copy-pastable example if possible

import pandas as pd

dti = pd.DatetimeIndex([], name='my_dti', freq='H')
print(dti.round('D'))  # Name is preserved
print(dti.snap('D'))  # Name is lost

Problem description

The DatetimeIndex.snap method loses the name of the DatetimeIndex which is a different behaviour than DatetimeIndex.round (or floor, ceil, etc.) that preserves the name of the DatetimeIndex.

Expected Output

DatetimeIndex([], dtype='datetime64[ns]', name='my_dti', freq='D')

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-45-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.24.1
pytest: 4.0.0
pip: 19.0.3
setuptools: 40.4.3
Cython: 0.29.6
numpy: 1.16.2
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.3.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.2.1
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.18
pymysql: None
psycopg2: 2.7.7 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: 0.9.0
pandas_datareader: None
gcsfs: None

[mroeschke]

Thanks for the report. There's actually a comment in the code that notes that this is missing (as well as tz). PR's welcome!

pandas/pandas/core/indexes/datetimes.py

Lines 790 to 791 in 3e652ac

	return DatetimeIndex._simple_new(snapped, freq=freq)
	# TODO: what about self.name? tz? if so, use shallow_copy?

[danielwlogan]

How does snap actually work?

pd.date_range(start='2019-1-1', end='2019-1-2', freq='10T').snap('H')

Doesn't return what I expect. Is this a bigger bug?

[heidek]

How does snap actually work?

pd.date_range(start='2019-1-1', end='2019-1-2', freq='10T').snap('H')

Doesn't return what I expect. Is this a bigger bug?

I'm not exactly sure how snap is supposed to work or what differentiates it from round. Don't think I've seen a single working example showing how snap is intended to be used. Does anyone know what it's supposed to do beyond the one sentence description in the docstring?

[danielwlogan]

I have been poking around through trial and error. The best that I can come up with is that:

1. It is for dates only (times seem to be unaffected)
2. It seems to work when the frequencies are things like W-MON. Which means it just works on "Anchored Offsets"?

I came across snap when I was looking for a round/floor type method that can handle time changes. I still don't know if snap would work, but I know that it doesn't seem to do what I expect based on the little documentation I can find.