Change in behavior of DatetimeIndex + Offset

[TomAugspurger]

import pandas as pd

offset = pd.tseries.offsets.Week(weekday=6)
idx = pd.DatetimeIndex(['1999-12-26', '2000-05-14']) 

assert (idx[0] + offset) == (idx + offset)[0]

bisected to #18952

Before that the assertion was true, after it's false

# before
>>> idx + offset
DatetimeIndex(['2000-01-02', '2000-05-21'], dtype='datetime64[ns]', freq=None)

# after
>>> idx + offset
DatetimeIndex(['1999-12-26', '2000-05-14'], dtype='datetime64[ns]', freq=None)

cc @reidy-p is this an expected change due to #18952, or a new bug? If it's intentional, we should document that.

[TomAugspurger]

Hmm if I had to guess, https://github.com/pandas-dev/pandas/pull/18952/files#diff-63e1050a658278e30a3e7a744c4d6435R1324 is the line directly to blame, which seems pretty intentional :)

[reidy-p]

@TomAugspurger thanks for spotting this. It seems like the problem is a bit deeper than the line you're pointing to - I'll take a closer look and see if I can fix it.

[TomAugspurger]

Fails for me on master.

In [1]: import pandas as pd
   ...:
   ...: offset = pd.tseries.offsets.Week(weekday=6)
   ...: idx = pd.DatetimeIndex(['1999-12-26', '2000-05-14'])
   ...:
   ...: assert (idx[0] + offset) == (idx + offset)[0]
   ...:
   ...:
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-1-8c110b991a5b> in <module>()
      4 idx = pd.DatetimeIndex(['1999-12-26', '2000-05-14'])
      5
----> 6 assert (idx[0] + offset) == (idx + offset)[0]

AssertionError:

are your C-extensions up to date?

[reidy-p]

@TomAugspurger Sorry that was a mistake on my part. Forgot that I had made some changes that I didn't stash before testing.