Inconsistent keyword argument name for the 'periods' in pandas.Index.shift for a DatetimeIndex

[SaberStrat]

The actual implementation of pandas.Index.shift for a datetime-like index takes a differently named parameter for the number of shifts than the base method.

It should be named periods, which would also be consistent with the pandas.DataFrame.shift or pandas.Series.shift methods. But for a DatetimeIndex it's n.

See:
https://github.com/pandas-dev/pandas/blob/v0.23.4/pandas/core/indexes/base.py#L2586
vs.
https://github.com/pandas-dev/pandas/blob/v0.23.4/pandas/core/indexes/datetimelike.py#L1021

With a DatetimeIndex, following the documentation leads to a

TypeError: shift() got an unexpected keyword argument 'periods'

If this is somehow intended, then it should be stated so in the docstring for pandas.Index.shift.

pandas version is 0.23.4

[TomAugspurger]

They probably used to be separate classes, and then were merged. We could deprecate n and replace it with periods.

[TomAugspurger]

@arminv do you have any interest in doing the same for PeriodIndex? I think we should be consistent everywhere if possible.

pandas/pandas/core/arrays/period.py

Line 322 in 7343fd3

def shift(self, n):

[arminv]

@TomAugspurger yeah sure! I’ll start working on it