BUG: round() on tz-aware DateTimeIndex (spanning DST switch) throws AmbiguousTimeError as if tz-unaware

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• 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.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

import pandas as pd
dt = pd.Timestamp("2020-10-25 02:00:00").tz_localize("Europe/Brussels", ambiguous=False).round('min')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslibs/timestamps.pyx", line 489, in pandas._libs.tslibs.timestamps.Timestamp.round
  File "pandas/_libs/tslibs/timestamps.pyx", line 443, in pandas._libs.tslibs.timestamps.Timestamp._round
  File "pandas/_libs/tslibs/timestamps.pyx", line 826, in pandas._libs.tslibs.timestamps.Timestamp.tz_localize
  File "pandas/_libs/tslibs/tzconversion.pyx", line 232, in pandas._libs.tslibs.tzconversion.tz_localize_to_utc
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from 2020-10-25 02:00:00, try using the 'ambiguous' argument

import pandas as pd
dt = pd.Timestamp("2020-10-25 02:00:00").tz_localize("Europe/Brussels", ambiguous=True).round('min')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslibs/timestamps.pyx", line 489, in pandas._libs.tslibs.timestamps.Timestamp.round
  File "pandas/_libs/tslibs/timestamps.pyx", line 443, in pandas._libs.tslibs.timestamps.Timestamp._round
  File "pandas/_libs/tslibs/timestamps.pyx", line 826, in pandas._libs.tslibs.timestamps.Timestamp.tz_localize
  File "pandas/_libs/tslibs/tzconversion.pyx", line 232, in pandas._libs.tslibs.tzconversion.tz_localize_to_utc
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from 2020-10-25 02:00:00, try using the 'ambiguous' argument

Problem description

A localized Timestamp does not give an error when rounding. A Series of a localized DateTimeIndex shouldn’t either.

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

Expected Output

Output is expected to be identical as for

pandas.Timestamp("2020-10-25 02:00:00+01:00").round('min')

respectively

pandas.Timestamp("2020-10-25 02:00:0+02:00").round('min')

i.e. without AmbiguousTimeError

Output of pd.show_versions()

[paste the output of pd.show_versions() here leaving a blank line after the details tag]

INSTALLED VERSIONS

commit : db08276bc116c438d3fdee492026f8223584c477 python : 3.8.5.final.0 python-bits : 64 OS : Linux OS-release : 5.9.1-050901-generic Version : #202010170731 SMP Sat Oct 17 07:42:59 UTC 2020 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : en_US.UTF-8 LOCALE : en_US.UTF-8

pandas : 1.1.3 numpy : 1.19.3 pytz : 2020.1 dateutil : 2.8.1 pip : 20.0.2 setuptools : 44.0.0

rest is not installed

[mroeschke]

Yes, and the documentation will be updated in https://github.com/pandas-dev/pandas/pull/44357 (available in version 1.4) to reflect this. Closing.

[bluppfisk]

Note that a workaround exists by converting to UTC first:

>>> pd.Timestamp("2020-10-25 02:00:00").tz_localize("Europe/Brussels", ambiguous=True).astimezone("UTC").round('min').astimezone("Europe/Brussels")