BUG: resampling error when date_range includes a single DST

[Flix6x]

• I have checked that this issue has not already been reported.

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

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

>>> import pandas as pd

>>> # works as expected (note the daylight savings transitions in the head and tail)
>>> pd.Series(1., pd.date_range('2020-03-28','2020-10-27', freq='D', tz="Europe/Amsterdam")).resample('24H').pad()

2020-03-28 00:00:00+01:00    1.0
2020-03-29 00:00:00+01:00    1.0
2020-03-30 01:00:00+02:00    1.0
2020-03-31 01:00:00+02:00    1.0
2020-04-01 01:00:00+02:00    1.0
                            ... 
2020-10-23 01:00:00+02:00    1.0
2020-10-24 01:00:00+02:00    1.0
2020-10-25 01:00:00+02:00    1.0
2020-10-26 00:00:00+01:00    1.0
2020-10-27 00:00:00+01:00    1.0
Freq: 24H, Length: 214, dtype: float64

>>> # fails unexpectedly
>>> pd.Series(1., pd.date_range('2020-03-28','2020-03-31', freq='D', tz="Europe/Amsterdam")).resample('24H').pad()

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/felix/anaconda3/envs/bvp-venv/lib/python3.6/site-packages/pandas/core/resample.py", line 453, in pad
    return self._upsample("pad", limit=limit)
  File "/home/felix/anaconda3/envs/bvp-venv/lib/python3.6/site-packages/pandas/core/resample.py", line 1092, in _upsample
    result.index = res_index
  File "/home/felix/anaconda3/envs/bvp-venv/lib/python3.6/site-packages/pandas/core/generic.py", line 5287, in __setattr__
    return object.__setattr__(self, name, value)
  File "pandas/_libs/properties.pyx", line 67, in pandas._libs.properties.AxisProperty.__set__
  File "/home/felix/anaconda3/envs/bvp-venv/lib/python3.6/site-packages/pandas/core/series.py", line 401, in _set_axis
    self._data.set_axis(axis, labels)
  File "/home/felix/anaconda3/envs/bvp-venv/lib/python3.6/site-packages/pandas/core/internals/managers.py", line 178, in set_axis
    f"Length mismatch: Expected axis has {old_len} elements, new "
ValueError: Length mismatch: Expected axis has 4 elements, new values have 3 elements

Problem description

In my first example, resampling from an offset of 1 day to an offset of 24 hours works as expected, but only when the start and end of the DatetimeIndex share the same timezone. In my second example the date range start and ends in a different timezone due to a daylight savings transition on 29 March 2020, for which resampling to 24 hours fails.

Possibly related issue:

• BUG: Resampling a Series with a DST to 24 hours gives a different result depending on whether timedelta(hours=24) or "24H" was used. #35248

Expected Output

>>> pd.Series(1., pd.date_range('2020-03-28','2020-03-31', freq='D', tz="Europe/Amsterdam")).resample('24H').pad()

2020-03-28 00:00:00+01:00    1.0
2020-03-29 00:00:00+01:00    1.0
2020-03-30 01:00:00+02:00    1.0
Freq: 24H, Length: 3, dtype: float64

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.7.final.0
python-bits : 64
OS : Linux
OS-release : 4.9.0-11-amd64
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8
pandas : 1.0.5
numpy : 1.19.0
pytz : 2020.1
dateutil : 2.8.1
pip : 20.1.1
setuptools : 47.3.1.post20200622
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[TomAugspurger]

The issue is possibly around here

   1090         # if we have the same frequency as our axis, then we are equal sampling
   1091         if limit is None and to_offset(ax.inferred_freq) == self.freq:
   1092             result = obj.copy()
-> 1093             result.index = res_index
   1094         else:
   1095             result = obj.reindex(
   1096                 res_index, method=method, limit=limit, fill_value=fill_value
   1097             )

Do we also need a length check in that if condition? Can you investigate @Flix6x?

[TomAugspurger]

And given the discussion in #35248, is this a duplicate or distinct?

[Flix6x]

Adding the length check and len(obj) == len(res_index) to line 1091 indeed resolves the problem.

I would have actually expected the preceding offset comparison to fail:

>>> pd.offsets.Day(1) == pd.offsets.Hour(24)
True

To me, adding the length check remedies a symptom of maintaining this equality. Both date_range() and resample() treat pd.offsets.Day(1) as a calendar day, in which case this equality shouldn't hold.

This issue is distinct from #35248, and both are symptoms of #22864. Your suggested fix would resolve this issue (which deals with offsets only), but not #35248 (which deals with pandas offsets versus datetime timedeltas).

Would you like me to make a pull request (with or without test)?

[TomAugspurger]

That'd be great (including a test).

[jreback]

Adding the length check and len(obj) == len(res_index) to line 1091 indeed resolves the problem.

I would have actually expected the preceding offset comparison to fail:

>>> pd.offsets.Day(1) == pd.offsets.Hour(24)
True

To me, adding the length check remedies a symptom of maintaining this equality. Both date_range() and resample() treat pd.offsets.Day(1) as a calendar day, in which case this equality shouldn't hold.

This issue is distinct from #35248, and both are symptoms of #22864. Your suggested fix would resolve this issue (which deals with offsets only), but not #35248 (which deals with pandas offsets versus datetime timedeltas).

Would you like me to make a pull request (with or without test)?

yeah we had quite some discussion about #22864 and actually @mroeschke pushed a patch, but its too big of a change to do in a minor release; we would have to wait till 2.0 for this.