COMPAT: Scalar types immutable

My impression (discussion somewhere on gitter) is that pd.Period is supposed to be immutable. That would certainly make sense for objects going into a PeriodIndex.

per = pd.Period('2014Q1')
>>> per
Period('2014Q1', 'Q-DEC')

freq = per.freq.copy()
freq.n *= 2
per.freq = freq
>>> per
Period('2014Q1', '2Q-DEC')

per.freq is a DateOffset object*, which is defined in tseries.offsets. There is a comment in tseries.offsets.BusinessHourMixin.apply saying “# calculate here because offset is not immutable”.

So a couple of questions:

1. Are we sure pd.Period should be immutable? If so, is there a preferred way of imposing that? Note that the class is defined in a .pyx file and some methods explicitly call __new__.
2. Does the comment about offset not being immutable mean that it can’t or shouldn’t be? Outside of __init__ and __setstate__, the only place in tseries.offsets where I see any self.foo attributes being set is self.daytime = False in BusinessHourMixin._get_business_hours_by_sec. grepping for “daytime”, this attribute does not appear to be referenced anywhere else.

* BTW, I think DateOffset is one of a small number of commonly-used classes for which there is not a ABCDateOffset equivalent. Is that by design? If not, I’ll be happy to make one.

[jbrockmendel]

Possibly closed by #21341

[jbrockmendel]

Closed by #17239 and #21341