Series.hasnans shouldn't be cached

[jbrockmendel]

Series.hasnans is as a cache_readonly inherited from IndexOpsMixin

ser = pd.Series([0, 1.0])
>>> ser.hasnans
False
ser.iloc[0] = np.nan
>>> ser.hasnans
False

[TomAugspurger]

I wonder if that's intended to be Index only? Might want to just deprecate it in favor of .isna().any().

[jbrockmendel]

I was thinking of adding an _immutable flag to Index/NDFrame(/Array) and a decorator cache_if_immutable that acts like cache_readonly for if obj._immutable and like property otherwise. There's a bunch of stuff (in datetimes/timedeltas/periods, where else) like this that could make use of it.