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I'm trying to find things to split off of this or do as bite-size deprecations. Not much optimism on those fronts. I expect we'll need to do this all-at-once in 3.0 or as a giant deprecation in 2.x with a future option.

all at once sounds fine to be honest, I'd consider it a bug that

In [44]: pd.Series([datetime(2020, 10, 25)]).dt.tz_localize('Europe/London') + pd.tseries.offsets.Day(1)
Out[44]:
0   2020-10-25 23:00:00+00:00
dtype: datetime64[ns, Europe/London]

happens

Please do ping when it's ready for review (looks like there's lots of conflicts atm)

@jorisvandenbossche can i get you to take a look here pre-merge (which is not imminent)?

Are the conversions of Day to hours needed for this change to be less breaking?

IIRC they were for either a) cases where we went DatetimeIndex->TimedeltaIndex (e.g. subtracting a scalar from DatetimeIndex) or b) to allow users to continue to pass freq="2D" to TimedeltaIndex-like functions. The latter could be disallowed either directly or via deprecation.

I'll take another pass and see if any of these usages can be removed.

On last week's dev call we discussed an option to allow TimedeltaIndex.freq to be a Day even though that isn't a Tick, the idea being that this would reduce churn for users. I've been working on a branch that does that, but still have 76 tests failing. Most of these seem to stem from the fact that Day(1) no longer compares as equal to Hour(24) (or Second(86400) or [...]).

e.g. in pandas/tests/resample/test_period_index.py::TestPeriodIndex::test_resample_with_offset (in main and the branch) result.index.freq is Day(1) while expected.index.freq is Hour(24). tm.assert_series_equal(result, expected) passes in main because they compare as equal.

One option is to patch the affected tests to say "close enough". I need to look more closely to determine if I'm comfortable with that. Update Very comfortable.

There's also a handful of resample tests (test_resample_origin_with_day_freq_on_dst) that look materially different that I don't have a good handle on.